Everything on this page is © Ilkka Yrjölä,
1999...2015. All rights reserved.
The other reason was perhaps the lack of documents containing information on more, than just one kind of setup and software. I was missing a document giving the key features of different types of operational observing systems. Perhaps the comparison table I have made, will make the picture clearer and more objective to all. See what is on the table and think for your self! I do not list all methods I've been proposed, or heard used, to do the job - though some of them were quite hilarious, as are some amazing explanations on some documents... So excuse me - I am not focusing here on a five minute science projects, but on something more.
At the latter part of this page there are descriptions of receiving hardware and free software making it a complete description of a home made receiving setup. Some technical skills are needed to duplicate these systems, so if you feel you do not have them, find some local friends to help you out.
This document does not go through all the basics of the phenomena, they are found in the MS-Soft's documents, which you can download from this website download area, but concentrates on a few important factors which I feel, have not been properly presented elsewhere. Some additional material is recommended to be read along, as basic reference, but I hope this document is useful, even, or just, for a beginner.
I hope you eventually come to the conclusion that just listening, occasional
or periodic, is a big waste of time in many senses. Most of the false alarms
of unusual meteor activity has been put out by the persons who occasionally
listen to meteor reflections with a less than ideal setups. (Bye the way,
how do you find out if the receiving setup works right when you don't have
any 24/7 long term data available to analyze it?). If you look at the visual
observations, you are bound to find similarities with this phenomena. Broadcasting
out those alerts a few times/month is great? publicity, but it backfires
as they all, or at least 99% prove to be false. It you spend 10 hours of
listening to the radio, you get 10 hours of data that is not easily comparable
to anything, but if you would have used that 10 hours to work on an automated
meteor counting system, you maybe would have a working setup and you could
use the spare time in the future to analyze the data, instead of just taking
those random listening samples.
Are these "Radio Meteors" often reported, somehow different from ordinary visible meteors? Do they perhaps produce radio signals on VHF? If someone is doing "Radio Meteor Observing", then obviously they are? Or are they just trying to fool you?
Actually, this is active remote sensing of the upper atmosphere by use of bipolar VHF radar. The same event causes the visible meteor and the bouncing of the radio signal, and though the physical reasons for those is different, if we ignore differences in limiting magnitude and geometrical aspects, we can generally say there no are such things as just "Radio Meteors". No HF or VHF radio emission emanating form the meteor or the trail can be found, only heat in the infra red region, but those are not radio wavelengths.
I've been told: the ionized plasma trail of a meteor, or the ionization cone around the head of the meteor, is capable to reflect, or scatter, radio waves. Perhaps everyone who has observed the phenomena, knows the reflected radio signals carry modulation from a more, or less distant, ordinary radio transmission and not "programs?" from the "meteor"!
O.G. Villard (Mike, W6QYT) at Stanford University was the first to propose
to non-professionals to observe meteor reflections with radio receiver
on his article in QST Magazine in January 1946, though the first such observations
reported by the professionals date around the mid 1930's. Some strip chart
meteor recordings were taken by non-professionals in the 1960's and 1970's,
but the coming of home computers in the 1980's, some 40 years later, made
practical the long duration observing projects for amateurs.
The software or rather the complexity and price of the interface sets certain limits to what receiving mode can be employed. Therefore it plays a major role and reflects on the whole system hardware.
The comparison table was made only with the objective to present systems producing relative hourly meteor counts in numeric format, though a system could also produce other data on meteors.
The setups can be divided in three different types according to what output they sample from the receiving equipment:
1. Systems sampling signal strength (F/S) with A/D converter at more,
or less faster rate.
2. Systems sampling SSB receiver's audio output and making time/frequency/intensity images.
3. Systems detecting the existence of signal with simple hardware, squelch, etc.
4. Hybrids of the above using at least 1. and 2. to enable better detection of only true meteoric signals on-line.
|ID||OS, appl. written for||Sampling rate, save intervals||Required computer min, recomm||Mass storage data format||Bytes of data /month||# of receiver inputs||Computer - RX connection||Need for manual work: h/month||Recommended recv mode ( & receiver), principle||List of HW||Misc.|
|DOS, for m-obs||<33 ms, 10 min||PC, 9 MHz, XT||ASCII||255 kB||2 (+1)
X vers =1
|direct, squelch relay to PC||0.5 to 1||narrow band FM, squelch detection||RX, PC||MCT5X: Dowload from this page|
|EA3BTZ||DOS, for data capturing||18 ms||PC, 486||ASCII||~700 kB?||1||F/S via A/DC||4 to 6||SSB with F/S output||RX, A/DC, PC|
|OH2AYP M-Analayzer||WIN9x, NT, for m-obs||continuous save (one dib/day)||PC, Pentiums||ASCII and DIB images||~90 MB||1||direct, SBcard to RX audio||SSB audio analyzed with FFT||RX, Sound card, PC||Download! Link is on the last chapter on this page!|
|HROFFT||WIN95, m-obs||continuous save (GIF/10 min)||PC, Pentium >166MHz||GIF images||43 to 180 MB||1 or 2||direct, SBcard to RX audio||8 to 15||SSB audio analyzed with FFT||RX, Sound card, PC||crashes with WIN98|
|DOS, m-obs||100 ms, each meteor?||286,386,486, Pentium||FITS||70/80 MB||1||detects audio (yes/no) via special interface that mimics squelch||wide band FM, audio HW squelch detection||RX, interface, PC||$40 for the interface|
|AMS/M||DOS, m-obs||0.1 ms & 100 ms,
|PC, 75 MHz Pentium||ASCII||125 kB||1||audio + F/S via A/DC||2||SSB, IC-R8500, IC-R7100-2,audio analyzed with FFT and sig.level with A/D conv.||RX, A/DC, PC||source code is on the net, hardware is commercially available|
|AMS/R||Apple OS, m-obs||Apple 2e only||1(+1)||direct F/S to Apple game port||SSB with F/S output||2*RX, Apple||availability:
|Urania||DOS, m-obs||5 ms, each meteor||PC, 286||Special?||150 MB||1||F/S via A/DC||wide band FM with F/S output||RX, A/DC, PC||System no longer operational|
|Ghent||4 ms||DAT tapes...||1 (+1)||F/S via A/DC||availability unknown|
(Table 1.) Main features of MS radio observing systems as they were
around year 2002.
|ID||Interference handling (Noise and non meteoric propagation)|
|MTC5X||Pulse noise: FM mode rejects. Prop: Off-line: manual+automatic (with 2 receivers and MCT7 reporting software).|
|EA3BTZ||Pulse noise: no rejection. Prop: Off-line: manual.|
|M-Analyzer||Automatic noise level based QRM registration, Manual off-line review of DIB images and ASCII data files.|
|HROFFT||Manual off-line review of GIF images.|
|Terrier||Pulse noise: FM mode rejects. Prop: Off-line: manual.|
|AMS/M||Pulse noise: On-line: F/S+audio based software rejection. Prop: On-line:software based, automatic.|
|AMS/R||Pulse noise: On-line: Second receiver (HF) for interference. Prop: On-line: software based - automatic?|
|Urania||Pulse noise: On-line: F/S based automatic software rejection. Prop: On-line: F/S based automatic software rejection.|
|Ghent||Pulse noise: Second receiver for interference. Prop: unknown.|
(Table 2.) Interference handling methods.
Information is based on material available on Internet sites and on
information acquired directly via e-mail in Jan. and Feb. 1999 an in Dec.
2000. Missing information was not given by the user and I am reluctant
to guess. Some of the software have new Windows versions requiring typically
a 200 MHz Pentium.
Corrections are of course welcome!
Acronyms used on this list:
F/S: Field strength, signal strength
N/A: Not available
A/DC: Analog/digital converter
Let me give you the worst case example: you have a tabletop PC and a synthesized receiver, both running from AC power and taking 90 W. If the energy price would be double to what we have here (it is in some countries), in 10 years time you would have to pay $ 1400 US for the electricity. But if the receiver would be a type with low power consumption and the PC would be a laptop, the power drain could be as low as some 25 W and the price tag for electricity only $ 400 US / 10 years. If comparable results could be achieved with different hardware, by paying some attention to hardware's power consumption, the wasted $1000 could have been used for something else, than just heating the shack. A setup of 3 receivers, magnetometer and a laptop took 62 W which means it adds about $70 to my annual utility bill. Fortunately the generated heat is wasted only during one or two months in summer, the rest of the year the house needs heating anyway.
I have made a short list of power consumption on some equipment types
Forward scattering of signals from meteor trails enables scattered radio signals to be received from low density trails, raising the underdense echo ceiling. Signals observed using long baseline forward scatter are also of longer duration, than back scattered ones.
Back scatter and forward scatter are just ways to express, if the signal has traveled away from the transmitter after becoming reflected back from the trail. The reflection properties change gradually, as the reflection angle from the trail changes. The scattering angle of received reflection is much affected, or even limited by the observing setup: If the signal emanates from great distances, the scattering angle is always low because of the geometry.
(Figure 1.) Sky (end) view from the receiving side towards transmitter.
If the signal emanates from relatively nearby source, the scattering angle tends to remain very sharp.
The distance has clear effect on both signal strength and duration. The overall result of this aspect of the phenomena seems to be a compromise between these two.
(Figure 2.) Signal to Noise ratio (S/N) and T-wait computed with MS-Soft and duration vs. distance on for certain VHF frequency and for certain assumed trail density. (These are merely calculated single case examples - not fixed due to altering meteoric and technical aspects.)
Meteor Scatter exhibits some problems in the form of large illumination area necessary to cover the full sky at distances much under 800 km, but works well beyond 1000 km and begins a slow taper off at about 1500 km. The reflections are strongest from distance of 500 km, but they are short and occur on large area of the sky disabling use of directional antennas having some gain. Signals sent from a distance of 1400 km are some 5 dB weaker, but their duration is almost three fold.
(Figure 3.) A 400 km baseline - a Turkish sauna?
A test was performed on long and short baseline system in as equal conditions,
as possible. Both setups used low gain antenna (a dipole), same polarization
and sensitivity was matched with an attenuator to compensate the difference
(4 dB) in their ERPs.
The long baseline system signals originated from
and the short baseline signal originated from
both operating on same E4 channel, but on offset frequencies not used elsewhere. The receiver bandwidth was 7 kHz.
(Figure 4.) Long baseline meteor counts.
(Figure 5.) Short baseline meteor counts.
(Figure 6.) Duration of meteor reflections, long baseline.
(Figure 7.) Duration of meteor reflections, short baseline.
Meteoric conditions during the consecutive two test periods of two days each, were very stable. Variation of daily counts on previous years had been under 1%.
Data from Lopik's off the air hours was not used from either station for the numerical analysis.
The average counts / h and duration / h were:
giving the long baseline system a marginal advantage in counts by +5.6% and in duration slightly more: +15.7%.
In comparison; on much higher (FM band) frequency with multiple transmitters received, a setup which is 6 dB less sensitive, the counts were in the 4500 / h range and no zero detection periods ever occurred.
Both baselines tested gave several (10 and 6) zero detections in sampled 10 minute periods.
The "statistical noise" in the form of peaks in the data are approximately the same in quantity on both setups and are close also in quality, so there appears to be no major difference there either.
This does not mean that the setups, could not be made to work better. One way is to optimize the antenna's illumination pattern (gain). A dipole is hardly optimal for a distance of 1600 km, which might explain the small difference in reflections duration. An array of eight 7-element stacked Yagis, while requiring a huge 40 meter high tower, would certainly have immense effect on meteor counts on the long baseline system, compared to the results with the dipole. On the other hand, the dipole's orientation may have caused some polarization coupling losses for the short baseline system, but it was necessary to eliminate tropospherically propagated signals from being received.
Some drawbacks exists with short baseline: airplane reflections may occur, tropospheric signals may be received time to time, unless terrain blocks them.
(Figure 8.) This panel shows scaled hourly counts for 24 h for 5 different systems.
The high variances, zero detections and abnormal daily variation pattern are some obvious deficiencies seen on the MS counts. The red curve have least noise, no zero count hours and have clear diurnal variation. suffers from low counts and zero hours, poor diurnal shape, green shows an odd pattern compared to others and cyan bounces up and down from small number effect. Numerical analysis is difficult because of lack of base curve to compare to.
(Figure 9.) Variation from the shape on figure 10. Data: green, red, gray.
These are not the wildest sets of counts I have seen, but they show there is some room for improvement in most cases, though they are sometimes impossible or difficult to accomplish due to local circumstances.
The shape of daily curve can be used to verify, if the data is of meteoric origin and useful for further use. A curve for a single day is never as smooth, as the curve below (Fig. 10), but the closer it gets, the better.
(Figure 10.) Daily variation of meteor counts. Average of long baseline
counts from several months of data when no major showers are active. The
helion and antihelion components around the apex effect are clearly visible
and on right hour like the evening minima.
(Figure 10b.) Spectrogram from 62.213 MHz from 15. Feb. 2001
0000...2400 UTC. Slices are 100 Hz wide (y), x axis is time left to right
and up to down. Colorful speck indicates short duration, white long
and intensity is signal strength. Activity consists mostly sporadic meteors
since no major shower is active. The long and strong overdense trails are
observed in the morning hours, and rarely in the evening when the overall
amount of reflected signals is lower (dark horizontal area on the lower
part of the image).
The meteor trail footprint is large enough to cover an oval some 50 by 100 km, where almost same reflections on a long baseline setup can be registered.
When we use transmitter separated by a few hundred km, only some of the brightest meteors causing overdense trails cause simultaneous registration at both stations. Use of two long baseline receiving setups located even tens of km away using the same transmitter and frequency will not bring anything new to the data, therefore if two or more receivers are used, they preferably should be set to receive signals from different stations (usually also on different frequencies). This will also be most beneficial in eliminating non-meteoric propagation events by cross-correlating the data from two or more receivers.
(Figure 10c.) Spectrogram from two receiving stations separated by
120 km and same transmitter. Most meteoric signals do occur on both receiving
(Figure 10d.) Spectrogram using two receiving stations separated
by 120 km and with separate transmitters; Innbsruck, Austria and Flensburg,
Germany. Just some of the overdense trails still are simultaneous at both
receiving sites. (Sorry about the frequency drift problems on the lower
It seems the best way to avoid small number statistics and zero detections,
is to use a frequency, on which there are at least two, or three high power
transmitters operating. This is one of the implementations of diversity
- meteor reflection's footprint usually is not large enough to include
more than one transmitter site.
(Figure 11.) Side view of a VHF band I or II transmitting antenna's vertical radiation pattern.
Broadcasting signals are beamed to form a doughnut ring like pattern
(vertical HPBW 20° to 40°) towards the horizon, leaving the ionosphere
above the tower poorly radiated compared to the horizon around it. Only
a short baseline system using broadcasting stations can suffer from this,
while a long baseline system sees only the part of the sky radiated with
(Figure 11b.) Transmitting antenna's horizontal radiation patterns. Examples of three different basic types, omni and two directive versions.
The horizontal radiation pattern may be omnidirectional, or more or
less directional. The ERP of 100 kW may be down to 50 or even 15 kW towards
the rear side of a directional transmitting antenna.
Since we now got into antennas and antenna patterns, let me show you two different type of arrays for short and long baseline systems. Neither of them are optimized to ultimate gain, or any specific pattern feature, but merely to 50 ohm low reactive feed point impedance for easy feed directly with coaxial cable. If you do not wish to buy a commercial antenna, that you actually know nothing about, and are willing to use a little of your time, you can make the antenna by your self.
(Figure 12.) Short baseline antenna with deep null towards horizon.
Remember, the short baseline system should be able to see much of the sky, for which a (crossed) dipole would be fine, but in some cases the direct, or tropospherically propagated signal has to be attenuated. This could be done by aiming the dipole's end minima towards the station, if it is not a crossed dipole. An array of two 2-element Yagis pointed upwards and phased normally, will have a deep minima not only towards the ends of the dipoles, but also towards the side(s), because of the anti phasing effect nulling out that direction also. The attenuation at the minima can be as high as 35 dB. This configuration may reduce the polarization coupling loss with horizontal polarization compared to a simple dipole with it's end towards transmitter.
(Figure 13.) A VHF low band log periodic dipole array (LPDA) for 50..115 MHz.
LPDA is an antenna type which is directional, but as it is usually designed to cover a wide frequency range, it does not have so much gain, but may be useful when searching for applicable frequencies on VHF for reception of MS signals, or just as a general tool for checking various issues over a broad frequency range. The large size of the antenna may fool you to believe it has much gain, even up to the same level that can be had from a Yagi, but this is not the case because of it's large bandwidth. For a wider frequency coverage miminum boomlenght LPDA only one or two dipole's are active on any single frequency at time, while the rest are just wind load. The little gain and side minimas the LPDA provide, may still proof to be quite useful for many purposes and even that lowish ~5 dBd gain is better than nothing.
The double boom structure of LPDA is used to feed the elements, while
the coaxial line is connected to the front of the antenna. The feed is
done crudely with a 50 ohm or even a 75 ohm coax cable, that is ran through
inside the lower boom from the rear end to the front where it's braid is
connected to the lower boom. This arrangement acts as a balun. The coaxial's
center conductor is clamped to the upper boom section. Feed point is protected
from moisture by a plastic cap and some RTV silicon. The booms are supported
and separated by plastic insulators and the mast mounting clamp structure
is also fully insulated from the booms. A shorting coil is used to connect
the upper and lower boom at the rear of the antenna to boost the lower
end gain and drain off static charge. I do not think there is anything
critical on this antenna type, so if you have the space and need for one,
by all means construct it.
(Figure 14.) Mechanical dimensions and construction of the 50 to 115 MHz 5-element LPDA.
The two graphs below are taken from the receiver end of a 75 ohm feed line with a 50 ohm instrument and 50/75 ohm transformer. The boom spacing (gap) in this model was 25 mm.
Resistance and reactance as seen at the end of the feed line from 40
to 120 MHz.
If you know exactly on which frequency need the antenna for, make a
Yagi, which when constructed to cover 5 to 10% bandwidth, will give more
gain with smaller size and has more simple mechanical construction.
(Figure 15.) A 4-element Yagi for long baseline system.
For a long baseline system, a working antenna arrangement is often less troublesome to set up. The image shows a 4-element Yagi, which can be aimed with a slight, 10 degree elevation angle, towards to transmitter horizon. This antenna should not be mounted near the ground, or roof, but to a height of about 6 meters, or more, to reduce possible adverse effects of ground reflection raising the radiation angle too high. If you wish to double the gain (~+3dB), stack two of these vertically at 1.66 m spacing and use the same 75 ohm 3-quarter wavelength matching/phasing cables, as with the antenna in Fig.12. Of course, this antenna's polarization has to match the one used by the transmitter.
This antenna works well with in a bandwidth of few megahertz, so if you use it on other frequency than about 90 MHz, please scale the dimensions, except the element diameters, to desired frequency. It is easy: 9 MHz up means 10% smaller antennas and shorter phasing cables. Antenna engineering is a little bit magic and even though these antennas are CADed, access to expensive measuring equipment and proper testing at antenna test range may reveal they are not just right "on the money", but small drop in gain won't ruin your reception.
If you think the so called 'antenna gain' is something you get for free, you are mistaken. This 'gain' is achieved from squeezing the antenna's main lobe to be even narrower and directing radiation as much as possible, off from other directions to main lobe. A reduction in gain usually means the main lobe is just slightly wider. If the antenna is not located in free space, the radiation pattern will not be what you might expect from a computer simulation, but something quite unpredictable and hard to measure, specially if it is located near the ground or roof and you are considering the vertical radiation pattern.
Where is the balun? OK, roll a few turns of the coaxial cable around your fist at the feed point(s) and tape the roll on the boom - that's all, if it makes you sleep better. The element lengths are expressed without metallic boom. If you mount them through an aluminum boom and they make contact to it, increase the element's length by 0.66 * the boom's diameter to compensate. If you use plastic insulators when mounting elements through the boom, increase the element's length by 0.33 * the boom's diameter. Shield all coaxial connections carefully from moisture and use proper electrical contact grease to reduce corrosion!
(Figure 16.) Antenna's azimuth and elevation minimum beam widths (HPBW) and the optimum size of a Yagi (array) vs. distance on MS.
If you wish to optimize the illumination of common scattering area by
the rule "half of the volume should be illuminated", Table 2 gives
you a rough approximation of the maximum applicable gain and configuration
of the Yagi antenna array for various distances listed, providing only
one station is being received. If there are several stations on different
headings and different distances, the necessary maximum spread in azimuth
and elevation should be calculated and an antenna solution with wider pattern
selected with the aid of Figure 16 and Table 2. For practical reasons,
stacks of with more than 2 or 4 Yagis are rarely used because of the tower
height needed. The number of elements of a Yagi here refers to a narrow
band (B.W.~ 2..5 MHz) design an not on some commercial FM band antenna
that covers 87 to 108 MHz with much less gain and broader main lobe.
(Table 2.) Selection of Yagi (narrow band design) antenna for different distances. For example: for 1800 km you would preferably "need" a 16 * 11-element antenna array. Listed gain value is the maximum usable gain - anything more causes under-illumination of the common scattering area.
The horizontal beam width is first selected with the E-field pattern from a list of antennas and then the required vertical beam width is achieved, by stacking up two or more of these Yagis, if necessary.
The expressed size of the antenna is the maximum size and gain. A more even illumination can be achieved by using an antenna with 3 dB less gain. The number of elements is for a Yagi with <1:2 VSWR on bandwidth of ~8%. Wide band TV and FM aerials and log periodic (LPDA) wide band arrays exhibit lower gain and wider main lobe. The whole issue of illumination and antenna gain is not very critical, but on the other hand there is no reason for doing it all wrong.
Because of the lack of high power transmissions above (but in the vicinity of) 108 MHz, higher frequencies are rarely used for the purpose. The long duration of MS signals on frequencies on the lower part of the VHF makes that part of the spectrum tempting, but all the typical short-wave band propagation modes and other adverse ionospheric effects, create a bundle of serious problems - ones that don't have to be taken! Specially sporadic-E (Es), will occur often causing massive losses of data, as summer time Es signals swamp the receiver for half a day on 50 MHz, while visiting only perhaps for an hour on 90 MHz. The duration of Es has a dependence of wavelength raised to power of 2, or even 3 and because of this effect, there is a lot less sporadic-E observed higher up in frequency, making the 90 MHz broadcast band less affected by this and other ionospheric propagation modes. Since the broadcasting stations around the continent are (re-)using the same frequencies, or channels, at every 200 to 600 km or so depending on their power and thus their coverage. The more transmitters are operating on the particular frequency within 2500 km range, the more often there will be signals via sporadic-E. Es has slightly longer maximum range and can on some cases make multiple hops than MS bringing Es propagated signals from distant stations thousands of km away that you never pick up on MS.
The biggest problem on the CCIR FM band in most countries, is it's just crowded with local and national station on almost every frequency - up to the point where no clear spots are found. I have one idea to try out, if you live within practical MS radio range from Italy: try the 50 kHz offset stations from Italy with a narrow band FM receiver! For all of them I do not know the polarisation for and some are directional, but there are:
87.850 MHz 100 kW, Italy region : PD
90.950 30 kW, vertical, PV
94.750 30 kW, VA
95.550 30 kW vertical, PV
96.250 30 kW, SV
99.550 60 kW, PV
104.250 20 kW, PN
104.450 30 kW, VI
106.750 100 kW, vertical, GR
107.850 MHz 100 kW, vertical, PV
There should not be any other stations on these frequencies in the whole
Europe and they can be used if only the nearby channels +- 200 kHz are
not locally occupied. You have to try out the polarisation if not listed.
(Figure 18.) Local frequency spectrum between 88.050 MHz to 88.550 MHz. Weak tropospheric signals from distant Swedish transmitter on 88.3 MHz surrounded by strong local transmissions. Taken with 3-element Yagi beaming SW.
Tropospheric propagation has inverse dependence of wavelength, but careful selection of locally and nationally free frequency can save from the interference of tropospheric propagation. Tropospheric propagation may bring signals even from distances of 400 to 600 km away from time to time. BBC has a nice VHF propagation page you could take a look at. The same precautions help to avoid airplane reflections, which though, soon loose their strength as the distance from receiver to transmitter increases beyond some 500 km.
Noise level reduces towards higher frequencies (-10 to -15 dB
from 40 to 100 MHz). Signal strength drops slightly faster (-15 dB from
40 MHz to 100 MHz), than noise level, as frequency is increased. This difference
is not significant (0 to 5 dB) and can be mostly ignored on VHF MS propagation.
The below graphs shows you the difference between the VHF Band 3 data
and 89 MHz FM band data.
By listening, the meteor reflections are weak, very short and overdense trails are rare.
First, make a note on the count and duration scales: they are 10 times smaller on 191 MHz graph!
Second, there does not seem to be clear trace of daily variation of counts like there is on 89 MHz. This may also be due to the low counts causing statistical noise and preventing seeing such variations. Surely the major showers will be indicated by this system. The 10 minute data has several zero detections and the counts usually trickled from 0 to 5, which to my opinion, makes it useless for such small time scale (10') analysis.
There could be more reflections if the receiver would be more sensitive
and with narrower bandwidth, and more antenna gain and if the transmitter
would have more power. Since the data is far from the quality and quantity
of 89 MHz data and there are similar difficulties in finding a clear frequency
on the crowded TV Band 3, where the stations I can receive unfortunately
shut down for the night, this is not the direction I would go, though it
can provide some meteoric data and has no sporadic-E propagation.
Please note the 191 MHz sample is edited (noise-only parts removed), the 144, 88, 85 and 62 MHz are unedited.
The 191 MHz reflections during the shower are mostly of underdense
and specially of head reflections. Since there is an abundance of bright
meteors during the peak, four overdense long enduring reflections from
the Perseids were observed in 10' . R7000, SSB, 10-el Yagi.
The MS CW and SSB signals from ham-radio stations making random contacts
on 144.100 and 144.200 MHz are
shown here as a noise reduced graph, but unedited. Two overdense reflections and several underdense were observed in 1' 5 s with
Yeasu FT-736R, SSB mode, and 4*17-element Yagi array with GaAs pre amplifier.
The 88 MHz signals are taken on narrow band FM mode with squelch.
Three overdense reflections and
some 20 underdense ones were observed in 1' 15 s with Icom R-7000, n-FM mode, LPDA antenna.
The 85 MHz TV carriers displayed as noise reduced graph. Two
overdense and several underdense were observed in 28 s with
Icom R-7000, SSB, LPDA.
The 62 MHz TV carrier as a noise reduced graph from Lopik, the Netherlands.
Three overdense and three underdense reflections were observed in 1' 15
s. R-7000, n-FM, LPDA.
By listening the audio samples and possibly viewing them with some FFT software such as Spectran, or Gram, you are able to see the head echoes becoming more dominant on higher frequencies, while the number and duration of overdense echoes is reduced, their duration is dramatically shortened as well as the signal strength is weakened.
Since this data is taken during the peak of a major meteor shower, the
conditions normally are much worse, less than 1/10 reflections are observed
outside major meteor showers and the number of overdense long enduring
echoes is reduced even much more!
The 50 MHz ham beacons and a R1 49.75 MHz carrier offset as reveiced
in southestern central Finland on July 15th late afternoon with a 5-element
Yagi at 16 m above ground level, is shown here for comparison. Note: the
TV stations run tens of kW ERP while those ham beacons run just tens of
Station, ERP, polarization, frequency, antenna direction(s) on-the-air-time comments.
Note: Analog TV is almost fully phased out in Europe, only Iceland is still on air.
62 MHz short baseline echoes from Vuokatti on FFT spectrogram display:
LPDA is beaming west. The short specular echoes are above the carrier frequency
(The ionized air and the trails are moving with the high (~ 50...150 m/s)
mesospheric W-E winds giving about +30 Hz of Doppler-shif. Exact speed
can not be calculated due to unknown trail position). Ticks: 10 seconds,
slant traces are aeroplane echoes.
62 MHz short baseline echoes from Vuokatti on FFT display: LPDA is
beaming east. The short specular echoes are mostly found below the carrier.
Ticks: 10 seconds, slant traces are aeroplane echoes.
Yes and no. The noise floor on VHF plays some role in this application and receiver performance should be matched to it. To make things more complicated, different parts of the sky are seen by the antenna as the Earth rotates giving a 2 to 5 dB daily variation of noise power at the antenna feed point with small antennas. A high gain antenna may give even larger daily variation.
A receiving system has certain noise figure, which can not be lower than 0 dB (usually 0.5 dB to 5 dB). The MS signals must rise a little (3...10 dB) above the noise floor for the detection method to discern them (see the signals on Fig. 22 at 0.3 and 8.2 seconds). The less ERP power the station(s) have, the weaker signals you have to be able to detect.
Because of high (compared to microwave) background noise floor on the
lower half of VHF band, you can get some advantage by not using the lowest
possible detection level a state-of-the-art ~0.5 dB noise figure would
yield. You can make the noise figure worse with attenuator at the antenna
input, which reduces meteor data bias effects caused by noise floor variation.
Whether you can 'afford' to do this, depends on the MS link budget:
what band width you are using and how much ERP power the station(s)
have and how low electron density trails you wish to detect, etc. just
to mention few factors. If the (minimum) detection signal level is set
too high, you cannot count any meteors, because in practice, the MS signal
strength, or more precicely - the propagation path attenuation, has a "hard"
limit, which means certain maximum levels are not exeeded due to taper-off
effect in the process of reflection attenuation vs. trail density.
The bandwidth is inversely proportional to system's sensitivity to detect meteors. Wider bandwidth means the detection level rises to higher signal level (voltage from the antenna). The broadest bandwidths found on commercial receivers are about 160 kHz. That is what you get with a standard broadcast FM receiver, such as home HIFI or car radios. More narrow bandwidths are found on communication receivers, radio amateur receivers and so called "scanners". The mode (method of demodulation) is not necessarily important, whether receiver can be set to FM or SSB, but if you wish to use FFT software to analyze the audio, you need SSB (CW) receiver. Unfortunately FFT analysis with narrow bandwidths (in the order of just a few Hertz tp 100 Hz or so) can NOT be used with wide band FM transmissions (FM broadcast signals). The FFT method suits only for those signals that are stable like most TV video carriers. On broadcast FM band you can use narrow band FM, though it also works with video carriers.
SSB receivers have typical bandwidths of 2.4 kHz and taking a 100 Hz slice from the audio further decreases the bandwidth allowing very low detection level to be achieved and providing the link is optimal by other means, the faintest meteors observed may well be fainter than +8...+10 visual magnitudes. This extreme sensitivity brings some problems along: a propagation called "D layer scatter" or "ionoscatter" yields signals that are detectable with such a system and sets a lower usable level below which the meteors are buried to ionoscatter signals. Naturally this also increases the total amount of time when Sporadic E and radio Aurora or FAI or whatever non-meteoric propagation interrupts meteor data gathering.
I list here some typical RF signal detection levels you get with various
providing your method of detection senses a >6 dB noise quieting:
In practice I could get maybe one meteor detected / week with the JVC and several in one minute with Icom R7000. Of course there are differences between models and units, so you never know, unless you can measure the sensitivity with proper equipment. Those home receivers are designed with a philosophy of making them somewhat less sensitive and at the same time less prone to go haywire from the S-meter needle pegging FM broadcast station(s) located in the neighborhood.
Even if I would use R7000 on wide FM mode, I could get some meteors detected / hour. Is that enough? NO, it is lottery machine that gives you random sets of numbers from 0 to about 5 for most of the year. Data from such setup can only be useful for analysis of unusual meteor showers with lots of bright meteors in the mag. +2.0 or 0 range, or brighter. Then there is that problem of typical overdense trail fading which the system may or may not count as several counts (it can be eliminated by suspending detection for - say 30 seconds, but then you loose observing time). How the overdense trail fades, is again rolling a dice and this makes the counts vary quite irrationally, specially with shorter observing periods.
If you settle for this, fine, but by large, the data is often useless. The vast geographic sampling area, hundreds of thousands of square km, suggests you should be able to count more than just a few meteors/hour, which is what you get at certain times of day and year with a wide band FM receiver. Specially if you wish to take comparable data to visual meteor magnitude range, you need to detect meteors down to mag +5 or +6, which on VHF means you also now pick up many underdense trails also. For this, you need a powerful enough transmitter, proper distance to keep the reflections long enough to be detectable, and sensitive enough receiving equipment of proper bandwidth and noise figure (and strong signal handling characteristic to prevent potential IMD problems). You can play with these parameters with the Power-si.exe calculator that comes with MS-Soft v5.1 available on this web site's download area.
There however is some limit to how narrow bandwidths you can use. If the transmission is using wide band FM modulation, you can not use FFT analyzers with bandwidths of 100 Hz or less, because the carrier is so unstable it hardly ever stays long enough with in your receiver's bandwidth to be detected. The 15 kHz-class narrow band FM mode, is still useful for FM broadcast signals, though even at peak modulation, the signal power within the 15 kHz window may be so low momentarily so that some meteors are being missed because of it. It though does not bias the data unless counts are taken in minute periods or less. With TV carriers this is no problem, since they are stable (except for some of the warbling russian TV carriers, which shut down for the night time anyway).
Reducing the receiver bandwidth to one tenth (1/10) will reduce the detection threshold by 10 dB - what it means for the counts you get, see the figure 19 below.
(Figure 19.) X: visual magnitude of meteor, Y: typical signal level
[dBm] on 90 MHz with 0 dB antenna from ~100 kW ERP class distant station
and number of meteors counted / hour for different detection levels. Figures
are approximate and not to be taken literally. These values are reached
when trail geometry is close to ideal. Trail to propagation plane angle:
90 °. If the station has only 10 kW ERP, re-draw the curve 10 dB lower,
if ERP is 1 kW, re-draw 20 dB lower, etc. -120 dBm equals to 0.225 uV (PD)
(micro volts potential difference) or 0.45 uV EMF, -130 dBm = 0.07 uV (PD),
I have created a calculator
that produces much like the above picture and some numerical data. The
system parameters such as ERP, distance, frequency and RX antenna's gain
can be set, and then the maximum signal level in dBm estimated for each
class of meteor brightness (or rather electron line density of trail) etc.
to see how the observing system (or MS / MB link) could perform. Please
download MS-Soft v5.1 from my web site's index page. You find there a DOS application called POWER-SI.EXE.
(Figure 20.) Peak signal strengths, 9 separate scans of 100 seconds each with max. hold and 10 second sweep time.
The forward scattered signals the 100 kW ERP produces from marginally overdense (critical) trails, have peak levels in the vicinity of -113 dBm +-5 dB (0.7 uV to 50 ohms) on ~90 MHz and dipole antenna. A system unable to detect signals with a 0 dB antenna below that level, can not register underdense trails, setting the corresponding visual Lm close to mag. +5. A raise of some 5 to 10 dB in the detection level will make the counts diminish to unacceptably low in numbers - not much room to raise the detection threshold level any higher.
(Figure 21 and 22.) Time sweeps: Left: strong signal from underdense (but not far from being overdense) trail profile from 85.260 MHz TV carrier (peak 105 dBm). Right: overdense trail with some oscillation, same frequency (peak -103 dBm).
(Figure 23.) Time sweep: Non specular overdense trail from 85.260 MHz TV carrier (peak -114 dBm). The level is typically lower because the trail has already dissipated for some while, before being able to reflect signals to directions unfavorable to trail's initial orientation.
(Figure 24.) Time sweep: "Maximum hold" mode display after 166.5 minutes of sweeping on 88.3 MHz (peak level: -95.2 dBm). Ignore the wave forms; analyzer BW < transmission's BW . Compare the counts on different signal levels with theoretical graph in figure 19. Levels are referenced to 0 dBd antenna with no feed line loss.
(Figure 25.) A longer max. hold scan bringing even more higher peak signal levels which occur less often.
The highest signal level I have ever recorded on ~90 MHz, was close
to -90 dBm (7 uV to 50 ohms) during Leonids 1997 with a 0 dB antenna from
a 100 kW ERP station located in Germany. The difference between the -90
dBm level during Leonids and curve on figure 19 leaves a gap of some 5
dB, but this can be overcome by trail orientation along the plane of propagation,
which gives up to 10 dB more, or perhaps by a bright negative magnitude
meteor- conditions that occur rarely and therefore the usual measured peak
levels remain lower. On 85.260 MHz there are TV stations with even more
ERP (>170 kW) and highest measured peak signal strength was -89 dBm during
one hour scan.
(Figure 25b.) Duration vs. number of meteor reflections on 87 MHz.
The duration of <0.2 seconds are not
shown, because the detection system begins to taper off from this point due to it's technical construction.
Systems with ultimate sensitivity and low detection level, counting normally from hundreds to few thousand(s) of meteors / hour, become saturated during major meteor showers and when ever the conditions are exceptional, for instance, in the form of abundance of long duration overdense trails from bright meteors encountered in some meteor outbursts.
Merely by using a pre-amplifier for low noise figure and narrower bandwidth,
the corresponding visual Lm of the system could be pushed down to mag +8-range
and with some antenna gain, even a bit lower - but this may cause those
noise floor variation problems, that would need additional SW and possibly
HW to correct.
(Figure 26.) Noise level vs. frequency on VHF from different sources.
Depending on location, antenna's pointing, it's radiation pattern (and
gain), the antenna's noise temperature (amount of noise power) varies
during the day, as discussed before, because of radio noise sources,
such as the galactic plane and sometimes the Sun passing
through the antenna's main lobe. The spectrum of extra terrestrial noise
is of broad band, with the exception of time to time occurring radio
sweep events from Solar flares, which fortunately are of short duration.
Such events cause only less than a day or so of lost data annually mostly
during maxima of the solar activity cycle and never give a false alarm
of high meteor activity, quite the opposite. The most sensitive receiving
systems suffer the most form these noise level variation, as mentioned
Man-made noise variation follows to some extent the daily cycle of human activities, and power line noise to local weather conditions. Also man-made noise level may be high (urban areas), to non-existent (remote sites), from which the latter one is preferred. The spectrum of man-made noise is complex and almost random on lower VHF band. High noise level prevents from reaching low detection signal levels, making a perfectly good system useless in a noisy location. What is worse, the interfering signals may be counted as meteors.
As explained before, a very low signal detection level may cause the system to observe better the noise variations (in the form of fluctuation of actual detection level), which biases the data, but it also increases the sensitivity to propagational interference (tropo, Es, etc.) for obvious reasons.
A compromise of detection level must be found, or clever ways to work
around the problems presented. It could be beneficial to set the system's
limiting magnitude close to the same value, as used in visual observing
TV carrier's power (with RX BW < 2 times horizontal scanning frequency) varies with the picture content. The variation is picture content modulated and therefore random. With 10 minute sampling periods the meteor counts are only a little effected by this on average. The usual variation of carrier power is in the 3 to 4 dB bracket, peaking <9 dB's from white to black picture content. During test chart the carrier power remains stable.
Receiving wide band FM with a narrow band receiver the same kind of variation of power within receiver's IF pass band occurs, as modulation content spreads the power out side of receiver's pass band. This variation is neither of any concern with longer sampling periods as long, as there are no extended periods of no modulation.
A pure 24 h / day carrier wave with stable power would be perfect for many reasons, but often the radio transmitters are meant to be used for transmitting information and they have to be modulated to accomplish this task. Some test projects have used a constant tone with low modulation as a signature that enables the receiver to recognize the signal's origin. Unfortunately RDS requires to high signal level and the slow data rate needs too long time to pass station ID.
The local radio interference environment is a subject, that requires some thought, as some of the setups do not have any on-line interference rejection and merely doing manual off-line rejection is not very time efficient and in some cases, leads to extensive loss of data. In a quiet location all reasonably sensible setups will work equally well (or bad).
Another subject to consider, is the amount of hardware and money spent to achieve the desired performance level. It is often a virtue in this world, to do a job with minimum amount of money spent. If money is of no concern, one may design and construct as complex, sophisticated and expensive setup, as one likes. The greater number of hardware increases number of equipment failures causing downtime, need for funding for spare parts and need of time to make repairs in a hurry, to get back on-line again (large gaps in data are annoying) - time that one may not have just at that moment. If the system consumes vast amounts of data space , the price of data storage media might also become a factor in the long run. I wonder, if anyone has ever thought who is going to analyze those gigabytes of data and for what reason? Such analysis take time and often time is money - who pays it? Since it is virtually impossible to retain absolutely stable system performance in the long run, some minor meteoric, or ionospheric effect become difficult to prove. Individual meteor reflections come in limited number of types and outside special projects, such as some particular showers, I do not see why terabytes of these few kinds of received power profiles should be stored over, and over again. If you have nothing else to do, by all means do it, but I try to stick to what makes sense to me and does not cost a fortune to run.
The last issue to pay some attention to, is the amount of work and time the system needs to post process the raw data and to keep the data storage from becoming full. It may sound trivial at first, if one has to use 10 hours monthly for processing the data manually and doing other related household work, but as this same demand persists over several years and the excitement to do all this wanes, it may become a factor in making the decision of closing down the system, due to "lack of time" to run it. Short runs of meteoric data are mostly of low value and long series of full time coverage data, is just what one should aim at.
Since you might be considering to setting up something of your own design, perhaps you should go through this list and make comparison to solutions already used to do the job:
Back to the issue of what observing setup works best at the location:
It is a complex issue that can not be answered easily. The factors discussed
above may be of help. A newcomer can not know what could have a fair chance
to work, simply because he does not have the equipment and/or the knowledge
to do some pre testing, but what is most sad, is that they often become
"victims" of some silly information on how to do radio observations they
just happen to stumble on. I do not see how such docs covering just one
particular way to do it helps the newcomers to succeed, and I dislike the
tendency of commercializing the hobby in the form of software licensing
fees. The few voluntaries on this planet, who are willing to use their
time and resources, should merely be directed towards efficient gathering
of scientifically good meteor data using whatever proper setup possible,
not for setting up mediocre systems producing low quality data.
All you need is a car radio which has a synthesizer tuner (recognized from the digital frequency display), a surplus (VHF) FM two-way radio's (radio telephone) receiver unit or a surplus 10.7 MHz IF - FM demodulator&squelch unit and a few components, wires and tools. Please note: this is not just to reduce IF bandwidth, the Squelch detection method uses the narrow band IF unit's SQ amplifier and gate systems. SQ detection does not measure signal power within RX bandwidth, it actually makes S/N comparison and does not detect pulse noises, just carriers with good enough S/N ratio. It is less affected by unwanted transient signals.
You may find discarded surplus FM radiotelephone units just lying around and do not have to spend your money on new equipment if you are lucky, or store radio junk like I do... The IF bandwidths you can find in these two-way radios depend mostly on their age, the real old stuff may have a filter with +-20 kHz band width, while there even may be some newer units with +-4 kHz filter. All of them should work OK, it is just a compromise between sensitivity vs. bandwidth.
The complete narrow-band modification to a car radio includes these
Removing of the cassette player hardware is usually easy and involves unplugging the motor's DC plug and the audio output plug, removing a few screws and lifting out the cassette chassis. The additional receiver IF unit or receiver board about to be installed should not be larger than the space made vacant by the cassette player. The IF unit must have 10.7 MHz as first IF (21.4 MHz units can not be used!) which can usually be quickly verified from the narrow band unit's crystal filter's type/frequency marking. Also the narrow band IF board should include the squelch circuit in order to pass the signal-present information to the PC for automatic meteor data logging. If a complete receiver board is used, the orignal signal path from the narrow band unit's RF front-end should be cut and the possible RF amplifier's DC power also cut off. The 10.7 MHz signal from the car radio's receiver is brought in to the narrow band receiver to either to the first mixer input or output depending on the type (active or DBM...) simply by stealing some 10.7 MHz signal from the car radio's IF chain to the narrow band receiver unit.
This is done by coupling some of the car radio's IF signal from the last 10.7 MHz ceramic filter via 5 to 50 pF series coupling capacitor and feeding IF signal through a short piece of coaxial (RG-174) to the narrow band unit. If the narrow band unit is just a 10.7 MHz IF unit and not a complete receiver with RF and mixer stage, it already has terminals for connecting the IF input, so use them. Having schematic diagrams of both receivers is of help, but if you have experience, you don't need them.
The necessary wiring of the narrow band unit must be performed before its installation. This include wires for ground, +12V if that is the voltage it works with (if lower - add a voltage regulator, 7809, 7808, 7805...), IF input coax from the car radio to the narrow band unit, audio output to car radio's volume potentiometer's hot end, from which the original audio path on the car radio's PCB cut open, the two wires from the opto coupler, or if it is externally located, one wire from the squelch circuit to drive the opto coupler (see figure 26).
The squelch control voltage supplied by the narrow band unit may either be low when squelch is open and high when shut, or other way around depending on the unit design. A simple transistor (PNP or NPN or a FET) with some series resistors may be used as buffer to drive the 10 mA current the opto coupler requires without loading much the squelch circuit.
(Figure 27.) Topside of the double RF pre amp. equipped receiver with battery back up.
The receiver is tunable to any frequency within the FM band tuning range
(87.5 to 108 MHz in Europe) in 50 kHz steps. Modifications to make it tune
outside that range are not feasible, except perhaps with the use of external
(Figure 28.) Principal block/schematic diagram of the narrow band car radio receiver and the squelch - opto drivers.
On Fig. 28 are two different opto driver circuits. The upper is suitable
for those squelches where the DC voltage goes low when squelch is open.
The lower is for those where DC voltage goes high.
R1 resistor may range from 10 k ohms to 470 k ohms and in some cases there has to be added a 10 k ohm resistor between the driver transistor's Base and Emitter to make the tranistor fully non-conductive when not intentionally driven. Transistors may be BC558 (PNP) or BC548 (NPN) or any similar type (2N2222...). Opto's outputs are connected to the computer's COM port for the MCT5X software to read (see the last chapter!). It would be wise to add small ferrite chokes such as VK-200 on the new wires that come out of the car radio's chassis to reduce the risk of EMC problems.
|First: if you did not know, an (external) RF pre amplifier is a device connected between antenna and receiver, depending on feed line losses, close the antenna or at the receiver end of the feed line (coaxial cable). The pre amplifier needs (DC) power, so that must be arranged for it as well, which is not a problem, even if it is located close the antenna. Locating the amplifier near the antenna is not very practical or necessary on frequencies below 100 MHz with proper coaxial cables of normal lengths with insignificant losses.|
|I do not recommend the use of commercial wide band "aerial amplifiers" or "boosters" marketed for consumers TV's & Radio's for two reasons: they amplify wide range of frequencies increasing the chances of receiver overloading and their noise figure is not what you get from a narrow band device. The gain of the amplifier is not the most important factor! Too much gain makes things only worse. This pre amplifier is conventional (state of the art in 1985) and not based on recent gigahertz-range performing expensive and tricky to use (they can oscillate on 5 GHz and you may not notice it) GaAs technology, but on cheap and widely used semiconductor device that is only little noisier than those newer devices are.|
The lack of strong signal handling capability may appear in different forms of problems. The usual harm is the intermodulaton (IMD) where off-frequency, usually same band, strong local signal's amplitude (received power) exceeds the level that the receiver's RF and mixer stages can handle without distorting (peak clipping) the RF signals. The distortion of one or more such signals means everything that comes in to the receiver's RF stage or mixer is mixed with these distorted strong signals producing numerous mixing products which appear as spurious ghost stations on vacant frequencies. The other usual phenomena is desensing. The sensitivity of the receiver on a clear frequency is reduced by off-frequency strong signals. Both these phenomena makes the reception of relatively weak signals reflected via meteor trails to be missed. The cure for these is either to filter off the strong signals and/or rebuild the receiver's RF and mixer stages, which is not so easy job than just screwing filters on the antenna feed line. The farther off in frequency the problem causing strong signal(s) is, the easier it is to filter it out. If it is closer than about one or two MHz on FM band, the task needs more and more cavities. If the interfering strong signal is far off the band, it is relatively easy to filter it from receiver's input.
To avoid overloading the receiver from strong signals on the FM band, specially if extra RF pre amplifier is used, you may need some band pass filtering. While the large cavity type resonator may produce impressive sounding high unloaded Q values, they must be carefully designed and used, or they might be only scrap metal on your table. This RF pre amplifier with two pole BPF has over 20 dB gain, low noise figure (around 1 dB) if properly tuned and 3 dB bandwidth of ~5 MHz. For even more narrow pre amplifier band widths, add one more similar L/C tuned circuits at the output. The unit can be constructed in a small metal enclosure (Eddystone type) on double sided glass fiber (sc.. G-10) PCB sized about 80 mm by 30 mm.
This device will be tunable tens of megahertzes and by altering the number of turns of L1, L2, L3, and the maximum value of trimmer capacitors it can be made to work on any single VHF frequency (30...300 MHz).
(Figure 29.) Frequency sweep of the pre amplifier with tracking generator.
Fc = 90 MHz, x= 5 MHz/div, y= 10 dB/div. Marker @ 95 MHz.
(Figure 30.) Frequency sweep of the FM band signals at the antenna
Fc=90 MHz, x= 5 MHz/div, y= 10 dB/div.
(Figure 31.) Frequency sweep of the FM band signals at the pre amp
output after 4 dB hybrid splitter.
Fc=90 MHz, x= 5 MHz/div, y= 10 dB/div.
(Figure 32.) Schematic diagram of the FM Band RF pre amplifier with BPF.
L1 8 turns of 0.6 mm Cu wire, ID 8 mm, tap at ~ 1.5 turns from ground.
L2 9 turns of 0.6 mm Cu wire, ID 6 mm, tap at 4 turns from hot end.
L2 9 turns of 0.6 mm Cu wire, ID 6 mm, tap at 1.5 turns from ground.
All trimmer capacitors ...22 pF (plastic trimmers are cheap but not reliable...prefer high quality tubular types). All unmarked capacitors are 1000 pF SMD (or chips). 0.1 uF is SMD and 1 uF is a 16 V Tantalum. DC is fed in to the metal enclosure via 1000 pF feed through capacitor.
The difficulty level of constructing this unit requires the constructor to have some experience of constructing RF equipment. If you have little or no experience in building electronic equipment, I recommend you don't try to make this unit. The difficulties might make you frustrated and the resulting equipment not working as it should. Find someone with some experience to help you.
You can view PDF data sheets of BF961 with the Acroreader.
(Figure 33.) PCB layout and component locations. Size 80 x 30 mm.
If you have no access to signal generator or other RF testing gear, use a weak station for tweaking the device. Tuning to maximum gain will be OK at the output BPF filter, at the input circuit there may be necessary to find the best signal to noise ratio (noise figure). Best match for the input is found by adjusting the input tapping, which is though a bit difficult. While tuning, also adjust the length of the coils and adjust the spacing between L2 and L3 for desired (slightly loose) coupling for desired bandwidth. Tight coupling gives a twin peak frequency response, which sometimes can be helpful if more than just one frequency is used. Loose coupling means narrower bandwidth and more filter losses. The bandwidth of the LC circuits depends a lot on the tappings - too low, and the peak is narrow, too high and the pass band becomes wide, while the pre amplifier overall gain (because of the filter losses) on the extreme cases suffers form the value achieved by the optimal coupling.
There is only one combination of L1 inductance, input tap position and trimmer capacitor setting that provides the best noise match. The change of tapping or length of the coil requires re-tuning of the capacitor. At the output BPF, the tuning of other LC circuit interacts to the other and re tuning of both circuits must be repeated a few times.
Once complete and the preliminary tuning is done, connect the pre amp input to antenna and output to receiver and adjust supply voltage from zero to 13 V and listen for any 'blobs' on the receiver audio. You could also monitor carefully for any abrupt changes on the DC current the amplifier is drawing. The DC current should change at all and transient sounds should not be heard from the receiver while adjusting the DC voltage. There should neither be any of those while tuning the trimmers near their proper settings, or from any mechanical causes: if any such signs are observed, the amplifier is oscillating or suffers from parasitic oscillations on UHF (~900 MHz) and needs some more work to make them go away. (Tip 1: a ferrite bead (FB43) on Drain lead. Tip 2: increase the 10 ohm Drain resistor's resistance.) Don't use the unit before it is stable or you might cause interference to local radio and TV reception! Use whatever coaxial connectors you prefer.
A resistive pi-attenuator with 3 resistors of 10 dB (6 ...15 dB) of attenuation is incorporated since the pre amplifier may produce gain up to 32 dB, which really is too much and the attenuator also increases the amplifier stability. The gain is useful in masking out the poorer noise figure of the receiver (if it is not any worse than the pre amplifier's noise figure, the pre amp will not be of any help). The poorer the receiver noise figure is, the more gain has to be used to get the system's overall noise figure closer to the pre amp's noise figure. If the receiver noise figure is about 5 dB, you still don't need more than 15 to 20 dB of pre amp gain, so practically, if you don't split the pre amp's output to several receivers, gain in much excess of 20 dB does only harm in the form of increased possibility of receiver overloading ("ghost" stations appearing on top of each other on frequencies that should be clear).
If the pre amplifier (not the receiver!) is overloaded by strong signals,
you may try to reduce the resistance at the Source lead (82 ohms) to some
lower value and thus increasing the FET's current. You can resolve the
cause of overload by trying attenuators at proper places and making logical
decisions on what is the cause of the overloading, unless you have a spectrum
analyzer to trace down the cause. There is a limit to the signal levels
these FETs can handle, so you may need to seek for other types that can
put out more power without signal compression, but also the receiver has
to be made to handle these harsh conditions! Cavity filters at the input
are one option, but their pass band may not be narrow enough and if they
are, they usually exhibit loss that far exceeds the tolerable 1..2 dB value
at the system input and thus make the system's overall noise figure
too high (>4 dB). So, while a pre amp may help, it may also cause
problems and solving them may give you the same performance or even worse
what you had before the pre amplifier. This is a problem on bands that
have high signals levels from high power transmitters.
There are no major differences in sensitivity between PCR1000 and R7000, but the strong signal handling is not as good on the PCR1000. In fact in my location, where the strongest FM band stations peak to S9+35 dB (true level -40 dBm, 2 uV+) range with Icom R7000 causing only a few FM broadcast IMD products below 87 MHz and some desensing in the middle section of the FM band on the R7000
The PCR1000 indicates the strongest FM band signals as being nearly +60 dB over S9, in which conditions the receiver suffers from severe IMD on VHF. There are some IMD products between 75 and 80 MHz, but from 80 to 112 MHz the needle never falls below S3 and there are no clear gaps anywhere making it impossible to use PCR1000 with the LPD antenna for any meteor observing from about 80 MHz to 112 MHz in here, without some extra technical gimmicks such as band pass and/or notch filters. PCR1000 has a switchable 20 dB attenuator which takes away all the FM band trash, but then the receiver is too deaf for any meteor reflections, so an external step or 10 dB fixed attenuator on the antenna input may be bettter if you ned one. To make the most out of it, I would use the pre amplifier-BPF described above, possibly with an attenuator in between, to achieve good sensitivity.
Below 75 MHz I have no strong signals as TV band 1 is not used here locally and both receivers did the job fine on any VHF frequency below 75 MHz.
The SSB filters on these receivers are a bit wide, not what is used on ham radio transceivers, which may cause some trouble if one is trying to receive a TV carrier with an interfering carrier just below zero beat on SSB mode or on a nearby offset.
PCR1000 has two AGC positions: slow and fast (light on) in spite of the manual saying it is an OFF position - it is not.
The narrow FM mode squelch seems to work well on PCR1000 too.
The old R7000 is notorious for it's power supply induced heat problem which makes it drift for days after startup (solution: use external PSU). The PCR1000 does not seem to drift any and warms up just a little.
The PCR1000 has more birdies than R7000, which is not a surprise, but may be annoying if a birdie is on frequency of interest.
The PCR1000 polls regularly the PC via the COM port, so if the PC is down, disconnected or crashed, PCR1000 goes mute within a minute.
Use 12 V sealed Lead Acid batteries only.
(Figure 34.) Schematic diagram of the Power Supply. 7815 is
a 1 A +15 volt regulator IC.
It does not only creates spectrograms, but automatically also groups the reflections into 4 categories by duration and to 4 categories strength and makes 10 minute and 1 hour interval numerical listings of both parameters as text file without any need to count the reflections manually from images counting work. It also has some other useful features and we are now giving out version 0.,9. One of the special features includes a selectable notch to eliminate one weak interfering carrier.
With a 133 MHz Pentium based laptop with WIN95/98 there exists a clock drift problem. This software as it is now, takes all the time the system can allocate with no idle left and obviously some housekeeping tasks such as keeping up the Windows time is not performed properly as of now. Note: this should not concern Windows NT-4 or its successors that have a clock daemon.
M-Analyzer's operating screen.
View of MCT5X operating screen.
I really hope you read everything on this chapter, it answers to all main questions, but not to those which I consider self explanatory or part of skills using a PC computer. This is not meant for computer & radio illiterates. The revised version (Dec. 2001) is for PCs with VGA and hard disk, therefore no old .ARC files are any longer deleted automatically from the default drive..
Consider it a free demo with no support what so ever, though it is fully functional. It has a 10 minute time resolution and can be used to do real meteor counting with radio. It works under DOS and any kind of PC (286 or better) capable to VGA graphics mode. Software reads one binary input from one receiver. Don't download it, unless you have a receiver to connect it to. It alone does nothing exiting - it has no bells, or whistles to play with (this is not a Nintendo), since it is meant merely for gathering data unattended and undisturbed and it does just that. The software is simply a counter, but the terminology used here, refers to it's usage in meteor counting with radio!
The MCT5X reads a closing contact type of information via the COM-port (don't panic - there really is no A/D converter!). I have used narrow band FM receivers, or scanners on that mode, to produce the 'signal present' contact information from their squelch circuit, or tape recorder control output relay, such as Icom IC-R7000 or AOR 3500+ has. The contact sampling rate depends on the computer, but on obsolete 9 MHz XT the sampling interval was 33 ms. Nothing prevents you from using a machine with the latest CPU version, but in this job, nothing beats a quiet laptop and the cheaper second hand ones are usually 386s and 486s which suit just fine! I really recommend you run the PC under true DOS, otherwise things might not work well.
The software saves the counts (one line of data is generated every 10 minutes, saving 6 lines of data every hour to default drive) into ASCII data file, each covering the period of one month, have
There will be 144 such lines / day and then, a new date & time stamp marks the next day and so on...
The data generated on the default drive is copied automatically at the end of each month to A:> drive. By hitting the 'A' key, the data up until the last full hour is copied from the default drive to A:> disk drive, from where it can be exported to another PC for analysis simply by removing the diskette momentarily and copying the file to another PC's hard disk. It is up to you to know what to do with the data, whether to import it to a spreadsheet for viewing or whatever.
The option to convert the .ARC file to .MAI file is used to sum the 10' counts to 1 h counts, thus making the data reduced by 1/6th. Such process makes the data less noisy and clearer to view, though with lower time resolution.
I have these comments on things that have caused some confusion:
(Figure 35.) The connection of the closing contact from the receiver to the PC. The connection must be isolated from the receiver by relay or optocoupler. There should be adequate RF filtering installed at least on the Com-port connector to prevent interfering signals from being conducted to the receiver from the PC.
Pins for 25 and 9 pin connectors:
25p D-conn. 9p D-conn.
COM PORT COM PORT
1 = shield = 5
4 = set high = 7
5 = input 1 = 8
The software can be tested easily by closing and releasing the shown Com-port input pins and verifying the screen counters are reacting accordingly.
There are no other documents involved, or no additional advice attached.
If the software for some odd reason would not happen
to read the Com-port, try another PC - don't send me complaints.
It is up to the user - NOT ME - to find and arrange the closing contact information from the receiver, or create it with additional electronic circuit. I can not respond to inquiries on this connection, since all receivers are built in different ways and I can not know how your receiver is built - I am not a clairvoyant. If you can't sort it out by your self, get a scanner that has a tape control relay... (Hint: opto couplers are much faster than relays and more reliable too.) Making the connection is not difficult, if someone knows about receivers.
This software can also be used to count other things:
General disclaimer on MCT5.EXE:
MCT5 is just a counting software. If you connect it to a hardware that does something you did not want it to do, MCT5X can not count what you originally desired. The application to meteor detection as explained above will work, if the conditions are good. In bad conditions and with receiver problems (IMD, wrong mode) things will not work out right. This is NOT a failure within MCT5, but in your hardware or the application. It is up to you to improve the hardware and use it right (find a clear channel, achieve good enough sensitivity, get rid of IMD, use a mode that is not sensitive to man made noise) in an environment where the goals can be achieved.
If you have doubts you are not receiving (only) meteoric signals,
take a look at Figure 25b, to get the picture how long and how many of
those reflections are heard from meteors. If the quantities vs. duration
sounds like right, make the daily plot on activity and compare your counts
to Figures 5 and 10 and see if the shape of daily variation is the same!?
If not, you are not picking signals bounced off meteor trails! VERY SIMPLE!