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To cure the problem without causing other new problems by too radical changes, the Radio Club of Kouvola, OH5AG kindly bought a new crystal for the 28 MHz beacon's TX for 28.2285 MHz. The PIM  has not been heard since on the repeater's input, after the QRG change to 28.2285 MHz was implemented in Aug. 2008. The antennas of 5 beacon's and FM repeater are stacked on the same 6 m tall mast in the top-4 m section.
 
 

Beaming SW towards Central Europe from 65 mAAT

OH5SHF's 28 MHz  beacon's ground wave coverage. Computed signal strengths (S-units) with 6 dBd RX antenna at 25 m AGL, S 1 = 0.2 uV (-121 dBm). Note:  the noise level is higher on 28 MHz and HF receivers are not  typically built to be as sensitive as 50 MHz radios, and unfortunately are most insensitive in particular on 10-meter's band.; expect signal/noise ratios much lower on 28 MHz than on 50 MHz

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OH5SHF's 50 MHz  beacon's ground wave coverage. Computed signal strengths (S-units) with 8 dBd RX antenna at 25 m AGL, S 1 = 0.2 uV (-121 dBm)

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"Corrected" Tropospheric Path Loss = TX ERP [in dBm] + RX Ant. Gain [dB]  -  RX feed line loss [in dB, often negligible]  - antenna height loss  [in dB, see graph below]  -  -145 dBm [CW RX sensitivity level] - band noise above 290 K [in dB, from upper graph per  your location's man-made noise level]

It is expected your RX system Noise Figure is substantially below 3 dB! (RX k curve)

Using the calculated Corrected Tropospheric Path Loss value, you are able to determine the approximate maximum range (CW mode) from upper graph (signal received 50% of time).

Note: the "Corrected" Path loss here is used include effects that reduce RX S/N ratio specially on 50 and 28 MHz due to higher noise levels on those bands to get the true range and not just pure path loss. If you need only the path loss for shorter range to estimate signal level when S/N is not an issue, do not correct for band noise!

Note: Range can be reduced by lower TX and/or RX antenna's height,  terrain and other obstructions and by factors such as poor RX system Noise Figure, higher local noise level which to some degree can be corrected using the graphs.
 
 

The CAD model used 1.4 mm enameled wire as elements and 4 mm brass welding rods as phasing line. Prototype antenna's element wires were stuffed inside four pcs of 3 m long telescopic fiber-glass fishing poles (2.50 € a piece), but inspite of being a prototype, it has survived the elements since September 2003 at the beacon site.

Computed 50 MHz gain was 2 dBd and F/B ratio 4 dB. On 28 MHz, the gain was 1 dBd and F/B- ratio 2 dB on the current site at 6 m above the roof.

Mechanical construction for such antenna includes stand-off insulator-supports for the 200-ohm open-wire phasing line, long aluminum element brackets to alleviate torque of fragile (reinforced with short PVC tube sections) fishing poles and finally, shielding the feed-connections from moisture and making sure the poles can not get filled with water - open ends - waterhole at center. The feed uses a ferite sleeve balun, many small toroids over RG-58 inside a heat shrinking tube installed inside the boom. This would call for a DC-shorting arrangement (perhaps a shorted "quarter-wave" stub after the ferrite sleeve, 1.24 m of RG-58 - but I have not verified how it affects SWR) so I recommed using a proper balun-transformer that works for 28 and 50 MHz both, instead. Don't forget to connect (grounding) the feed line's shield to the boom, or mast.

The excess un-vacated fiber-glass poles' tips were cut to reduce overall antenna size, wind and snow loads. The elements could be made of aluminum tube - the element lengths should be shortened a little. Making it even more light and small it could also be mounted pointing upwards by bolting the element brackets directly to the side of a mast (with no boom) - price: about 2 dB of ERP lost on 50 MHz..

Unless traps and loading coils are use, this may be the smallest "full-size" antenna that works both on 28 and 50 MHz since only 8.6 m (incl. boom) of tubing material is used. In comparison, a GP with trap and proper (3) radials for both bands needs some 14...15 m of element material and a GP can only deliver vertical polarization. This dual-band antenna can be used as vertically, or horizontally polarized. If set for vertical polarization, it is practically omnidirectional. Two such crossed antennas could be phased for circular polarisation. The same basic LPDA-style idea could be adopted for a set of any multiband HF, VHF or even UHF ham beacons fed with just single coaxial.
 
 

OH5RBG's 70 MHz  beacon's ground wave coverage. Computed signal strengths (S-units) with 8.5 dBd RX antenna at 25 m above average ground level  with unobstructed take-off, S 1 = 0.2 uV (-121 dBm) @ RX input.
 

The 70 MHz beacon was built in fall 2009 with latest technology: USB-controlled synthesizer, PIC-based PC configurable keyer and FET PA. The scheme works for all bands from 28 to 70 MHz with just modifications to driver LPF and PA stage coils and capacitors - the single ended FET PA is narrow-band type. Using the 175 MHz application sheet for RD06HVF1 FET, the beacon can be built for 144 MHz operation. 432 MHz MHz version would need different version for the synth. chip - a faster LVDS reaching up to 910 MHz.

The first built beacon module was for 40 MHz, but uncertain and troublesome licencing caused rebuild it to 70 MHz (took some 2 h)  which has been a ham band on Finland since 4th Nov. 2009. The enclosure holds the 70 MHz beacon TX module with keyer and a 4.5 A SMPSU. FET PA feeds some 5W to a horizontally polarized low-gain antenna (AV1462-70?) beaming Az. 200 degrees (SSW) mounted on a cellular tower at hilltop and delivering 10 W of ERP (once installed).

In detail, the synthesizer, which is configurable for any QRG from 3.5 to 260 MHz in 1 Hz steps (and for exact FSK shift) via the USB connection, delivers a 0 dBm drive (square wave) via via 2 section LPF to ERA5 SM MMIC amplifier. The +16 dBm drive feeds the cheap SWR tolerant Mitsubishi PA FET. The 5W RF-output is filtered with 3 section pi-LPF filter for good harmonic rejection. G4JNT RS-232 configurable PIC keyer keys the SDR-Kits syntesizer. Total currrent is monitored with 2 A analog panel meter. The antenna output has a 230 V gas-disharge tube, 2 kV blocking capacitor and antenna connector & case is bonded to station ground bar.

OH5SHF 2320 MHz  beacon's ground-wave coverage. Computed signal strengths (S-units) with 24 dBd RX antenna at 25 m above average ground level and take-off clear of trees, S 1 = 0.2 uV (-121 dBm) @ RX input (needs a pre-amp.).
 
 

OH5SHF 10 GHz  beacon's ground-wave coverage. Computed signal strengths (S-units) with 30 dBd RX antenna (0.8 m dish) at 23 m above average ground level and take-off clear of trees, S 1 = 0.2 uV (-121 dBm) @ RX input.
 
 

Aerial's AV1525, two of them used in OH2UHF, is basically a directional aerial with gain of 7.3 dBd and high F/B-ratio. We wanted to get more omnidirectional-style coverage but little ERP towards 130..180 degree zone. The solution was to abandon the two-wavelength ominidirectional Slot aerial and use a rugged commercially made second-hand (abt. 35 years in-use, but in pristine condition) dipole array and point the main lobe with most ERP towards WSW and SM7/SM1/SM0/SM5- area, where most of the long-haul cluster spots come and which iluminates the path of Baltic Sea tropo openings to DL/OZ etc. The illumination of southern OH5 and UA1/ES border is reduced and St. Petersburg is just on the edge of the lobe as is OH6 and SM2 areas, but that is unavoidable without using more hardware in the form of extra gap-filler aerial.

The omission of the hefty bolt-on refector-grid reduced wind area (0.45 to 0.2 m²) and decreased the F/B from 20 dB down to 5 dB which gives enough ERP (about 5 W) towards NE sector for adequate illumination OH7/OH4 and eastern OH5 areas. The AV1525 support frame acts as long reflector element and produces the 5 dB F/B and gives about 2 dB of more gain than the closely spaced dipoles alone would. There are no stations beyond 200 km towards NE so we do not spend more of the limited ERP for that area. The SWR of AV1525 ("P") is better than 1:1.2 on 432 MHz without the reflector and this antenna is insensitive to sleet & snow. The AV1525 is fed with 12 m of Ecoflex 15 cable.

The 1296 MHz Slot's radome is PE pipe. The 1296 MHz slot with 7 dBd gain, is center-fed with 7 m of Heliax LDF-4-50A through a 1/4-wavelength semi-rigid balun. The 1296 MHz cylinder is made of rigid brass tube of 1 mm wall thickness. It radiates horizontal polarization and is practically omnidirectional (F/B: 3 dB).

The 2320 MHz radome is model G378 ABS plastic enclosure which holds the IW3HNP-designed 4-loop reflector panel antenna. Behind it are bolted a bias-T with 96 MHz BPF, the x24 multiplier, 3-section 2320 MHz interdigital BPF and 1 W WiMo PA (modified for CW BCN use) with home-made 2-section low-pass filter and a harmonic stub. The antenna illuminates a sector from SE to SW to NW with +-60°-wide (HPBW) broad beam centered at QTF 240° towards OH2 and further out - propagation condtions permitting. Reception in NE-sector is possible at shorter ranges by reflections from nearby main lobe objects.
 
 

The OH5SHF 10 368.935 MHz beacon uses A1A keying with RF-power output of 0.5 W to horizontally polarized omnidirectional waveguide slot antenna yielding 4 W of ERP.

Mechanical and temperature issues with ordinary XO oscillators caused the need to find easier-to-adjust oscillator - the Si-570 LVDS 20 ppm version was the only available, but with serious doubts about the temperature stabilty, while the QRG is easily adjusted with USB cable and laptop. After 2 months of aging the SDR-Kits unit and after the increase of chip temperature to +40 °C the drift from changes of ambient temperature reduced to a fraction of what it was unheated.

The DG8SAQ Si-570- based USB synthesizer generates 108 MHz which is multiplied to 1296 MHz. This unit is located indoors in a partially insulated enclosure at constant +35 °C(+- 1 °C) temperature for stability and the Si-570 is heated to +40 °C with QH40A heater chip. This "double ovening" procedure achieves +-2.5 kHz stability at 10 GHz with IDU room temperature between +10 to +20 °C. Without heaters the stabilty would be in the order of several tens of kHz +-. The synthesizer output is low pass filtered, boosted to +18 dBm with keyed ERA5SM MMIC amplifier stage, filterd with 108 MHz LPF and BPF and delivers +14 dBm drive to ODU via CATV75 coaxial cable. The keying with message: "OH5SHF  KP30HV and a long carrier is cycled every 1 minute, is generated by a G4JNT RS232- programmable PIC-keyer installed inside the indoor unit.

The outdoor unit includes 108 MHz input BPF, DB6NT's MKU10G beacon unit with multiplier stages from 108 MHz to 10.368 GHz. 10 GHz 200 mW output is filtered with a narrow band-pass 10.368 GHz cavity filter and drives a RATS sponsored  FLX202MH GaAs-FET power amplifier producing about 0.5 W RF-output to a RATS sponsored 9dBd omni waveguide slot antenna at 144 m ASL with free take-off. The output spectra is exceptionally clean with filters. RF power is indicated with LED-bar display located on the bottom of outer ODU enclosure. However, the indication depends strongly on ambient temperature. The outer polycarbonate enclosure is EMC screened with spray and EMC-gasket material (RF adsorber foam sheet and ferrite sleeves on cables), has a top-hole mounted radome tube of Polypropylen sealed with rubber O-rings, MS Polymer and G.P. type hot melt glue painted over with rim spray paint for UV protection. While the Polypropylen material is easy to find, has very good electrical properties at 10 GHz, it is hard to glue, only adhesives with Polyolefins (w. PP surface flame treatment) really work with PP material. The radome also increases the 4 dips of the Az-pattern severely.

The 28.8 MHz GPS-locked QRP beacon for frequency calibration, is co-located with the 10 GHz beacon, but sharing only PSU. The PIC keyer is G4JNT design which keys (A1A)  the MMIC in the TX amplifier chain. RD06HHF1 FET PA feeds 2 W RF to a slanted-dipole with horizontal polarisation. The ID is "OH5TEN" every 90 seconds. Description of the unit was presented at OH5AG's summer meeting in June 2009 and in July 2009 issue of Radioamatööri-lehti (available at SRAL website in PDF format for SRAL members).

The frequency stability is typically better than 2*10^-12 on a minute basis and better for longer periods of time and can be only affeted by temporary LOS of adequate number of GPS satellites, which should not occur and during short periods of high electron content of the ionopshere - typical during strong auroral displays. The other notable benefit is absolute correctness of the center frequency, which does not rely on any local frequency source prone to unknown offset.

28.8 MHz is in harmonic relationship with 144, 432, 1296 and 10368 MHz ham bands. Within the range (of about 30...50 km), anyone having a 28 MHz horizontal antenna and a receiver, can zero-beat his own signal source (HF rig's TX or signal generator) against OH5TEN 28.8 MHz beacon and by using local TX's signal's harmonics, get accurate frequency signal for 2 m, 70 cm, 23 cm and 3 cm bands. Since HF rig's harmonics are non-existent above 500 MHz, for 23 cm and 3 cm bands' a comb generator is usually needed - a simple device which creates harmonic signals from low-power (ie. 0.5 mW) RF, which can be fed to U/SHF- receiver directly with cable or a coupler. This comb generator can be fed on 28.8 MHz with signal generator or from HF TX via ~ 45 dB power attenuator of sufficient power handling capability.

OH5TEN's 28.8 MHz  beacon's ground wave coverage. Computed signal strengths (S-units) as received with a 6 dBd antenna at 25 m AGL.
 
 

I would like to thank Ari, OH5KFP and Aki, OH5MWZ for their help re-installing the OH5SHF beacons, it's antennas and OH5RAC's antennas and our radio club OH5AG for sponsoring crystals for all of the beacons from 10 meters to 3 cm.
 
 

OH5RAC 145 MHz FM repeater's mobile coverage with different mobile (green 10 W, blue 50 W) & portable (yellow) equipment.
The repeater has been accessed 7000 times / year. TX and RX frequencies were adjusted in April 2006: deviation  <200 Hz from nominal.

50 and 432 MHz beacons were 7 dB above the noise level with 2.4 kHz B.W. and 2*6-el.  (CC 6176B) & 2*19-el. (CC 719B) Yagis 30 m AGL w. mast-head SP-7000 preamp in KP31PU.