Update: May 27. 2011. Hits:  since 20060709.

Grounding, bonding and transient protection

Note: this diagram contains main things that should be taken care of but does not unfortunately fully follow or show the principle of single-point-grounding!
Protecting personnel from injury and equipment and property against damage and fire from lightning-related incidences is beneficial and required

The Finnish law on safety of electric installations says "all electric equipment and systems must be built and operated in a way that does not cause danger to anyone's life, health or property."

To comply with the above statement and general principles of lightning protection (as in SFS-manual 33, SFS-manual 118), Digita's Guide of Home's Antenna Installations gives detailed instructions regarding outdoor antennas: "All outdoor antennas reaching higher than 2 m below roof-top or 1.5 m away from the building, must be grounded with a fixed, short and straight stranded copper-wire having minimum cross-area of 6 mm2 to house potential balancing bus bar (or main ground bar). Antennas that can not be grounded, must be equipped with static discharging devices." I see no exception for any back-yard antenna tower - we must assume SFS-33 includes the concept of grounding outdoor antenna towers. In case of a Cellular sites, Ficora has issued rules on electrical safety of communication networks, which also says: the equipment room grounding must be connected to tower grounding and the tower grounding system has to be designed and built by standards. The standard gives an upper limit of 10 ohms to grounding resistance in normal soil.

IEC 62305-2 standard considers tolerable risk to human life one fatality every 10 000 years, for loss of cultural heritage and public services one event every 1000 years. Tolerable economic losses are left for the site/system designer or owner to consider.

How would you rate your home - lost every 100 years? How about your electronic equipment - destroyed fully every 7 years?

If your 30 m tall tower is located in area with lowish 0.5 ground strokes /km2/year, it gets direct lightning hit every 30 years - will damage your radios, but may also burn down your house. If your station is built as "termination point of lightning-downconductor" and you use radios 2.5 h every day regardless of lightning, risk to your radios from direct strikes is perhaps tolerable, but risk to your house or your life is not tolerable. In practice you radios and computers will be damaged in a year or two by other than direct-hit related lightning effects which are more frequent (make that: from few summer weeks.....to years, depending on AC and TELCO line lengths). This situation  surely can not be considered tolerable. Nearly all those risk can be reduced to tolerable levels and beyond, by proper station and tower grounding and equipotential arrangements.

Minimal grounding means minimal protection against fire and injury.

Extended equipment survivability can be achieved by adding proper transient protection devices, which don't work unless the grounding and bonding system is good, not just piece of buried wire or a rod.

From International Standard IEC 1024-1, Protection of Structures Against Lightning, International ElectroTechnical Commission, Geneva, 1991:

"In order to disperse the lightning current into the earth without causing dangerous overvoltages, the shape and dimensions of the earth-termination system are more important than a specific value of the resistance of the earth electrode. However, in general, a low earth resistance is recommended.

Earth termination systems which must be separated for other reasons should be connected to the integrated one by equipotential bonding"

  We all know fellow hams who scramble to unplug power supplies, disconnect antennas, open circuit breakers when they hear the sound of thunder, or spend the summer monitoring lightning radar maps and explain they are doing useful work in preventing the equipment from being damaged by lightning surges, etc.   Really - isn't there any other way?!

If you don't include tropical areas like Florida where lightning is very intense and ham antennas really do get direct hits more frequently,  direct hits on ham towers elsewhere occur maybe only once in 30 years or so in normal locations at temperate latitudes. Wherever we live we  must build a proper grounding network for the tower and assume it dissipates all the electric surges picked up by the antennas. So, how can one suffer equipment damage once or twice every summer, if your antenna tower only gets hit every 30 years? It's same lightning everywhere around the county - we see no attractive ground targets so far on lightning locator maps, except for the tall broadcast towers.

The AC&TELCO-line surges (mostly inductively coupled) can be destructive to electronics from distances of up to 1.5 km away down the line even if the lightning ground-hit occurs up to 200 m away from the line - that creates a risk area of  0.6-to-2.5 km2. Lets bravely assume ground hits around your tower farther than 150 m away are harmless. The 150 m radius around tower covers only 0.05 km2 - that means the surge damage risk from AC&TELCO lines is at least 10...50-times greater than from your tower. The conclusion can only be: most of the surges causing damages to electronics primarily arrive from the AC&TELCO lines.  The longer the lines and fewer grounding points along the line are, the more surges your equipment gets.

What does unplugging of antenna cables from radios have to do with this? Very little... but I agree your radio equipment are absolutely safe if you place them inside a Faraday's cage with no cables attached or ship them away to your cousin... very inconvenient. If you do not have a grounded antenna entrance panel for all incoming antenna cables, the loose cable-ends become effectively lightning down-conductors terminated to you shack floor lying amongst other wiring which might get zapped, or produce even more devastating effects. How clever is this habit of unplugging incoming antenna feeder cables when done like this?

How about unplugging all TELCO equipment? The surge finds the end of the line at sockets and with the sudden increase of impedance,  power converts to very high voltage and appears as violent loud visible aerial discharges up to a meter long - what a way to dissipate it - ozone and noise! Anything near the socket will get zapped any even unplugged computers, etc. nearby are damaged and the visiting relatives get scared out of their wits.  Would a proper surge protection be useful?

The remaining overhead AC lines in many countries are nowadays bundled cables so AC line's transient surges tend to be of lower voltage, but still sometimes "damage equipment" in rural areas. The common reaction to deal with this, specially in the countryside, is opening the main circuit breakers during thunderstorms (as dis- and re-connecting all AC sockets is practically impossible). Opening breakers may actually be of some help: when the flash-over occurs in unpowered equipment, the AC current does not resume to flow through the failed insulation path and cause more destruction. Fuses on unpowered AC circuit don't get blown away by the surge, but if the AC is there, the surge's arcing to ground causes a short-circuit and the resuming AC current flow over the fault blows the fuse. If this occurs inside in a freezer's compressor's motor windings,  the excessive AC current flowing over the compromised insulation will form a permanent copper or carbon bridge short-circuit or melt the coil wire creating an open circuit fault. Opening breakers is not exactly a perfect solution because it needs a vigil operator who trips the circuit breakers just before lightning occurs and resets them afterwards - meanwhile no electrical appliances work, which is a second nuisance.

We are talking about risk management, in case of ham radio, minimizing personal and economical risks by having Lightning Protection (LP). Each country has it's own standards on LP, some good, some not. The new standard is called IEC 62035 published in Jan. 2006 and gives guidelines how to do these things. There will be no inspector paying a visit (unless you cause TVI, but he's only interested on "grounding") to verify your station's lightning safety or transient protection - it's up to you to build it right. The radio and antenna manuals all say you should take care of it, but don't include any supplies or show any details how to do the job.

Lightning protections of ham station is not the spark-gap adapter with ground lug, nor merely talk about excellent tower grounding with that low-ohmic uncorrosive connection to the single copper-wire bolted on tower's leg (but I have seen copper wire wrapped around rusty tower base held "tight" by rusty loose hose-clamp - 10 milli-ohms?!) LP is more about limiting potential differences by bonding, followed by proper grounding where it's possible. Bonding reduces also those more-frequently occurring non-direct lightning's effects sometimes destructive to electronics.

Some fellow hams think this whole issue has almost nothing to do with their ham radio hobby, do little or nothing about it and just take their chances. On some parts of the world it may be successful strategy, on some it's a looser - depends on lightning density.  Being about equal height to tallest surrounding trees, ham radio tower is no more desirable target to lightning than those trees are. The notable difference is;  trees have no metallic conductors attached on running in to you house, but your tower does!

Ham radio station lightning protection should always include limiting voltage potential differences by bonding and almost always creating good low-impedance path to ground from antenna installations spreading the lightning surge-current for the soil to absorb in controlled fashion via something else, than your radios. It will get there anyway, you can stop it, but you radios and household's AC wiring can't withstand it. A proper current path is 16 mm2 cross-area or thicker copper wire, which will not melt from a 100 kA surge. Commercial tower grounding uses 35 mm2 copper or 50 mm2 steel cable.

Any conductor has self-inductance (and surge impedance, Zs), which is far more more dominant factor than any of the more easily measurable low-current DC resistances in lightning ground network.

Besides grounding everything, another less frequently used approach is "floating" the equipment, applied mostly in locations where proper grounding isn't available anywhere near (soil conductivity around very poor, rocky hill-top sites). It can be equally successful, but since it has to be built with great consideration and expertise and involves personnel safety issues, I would use it only at unmanned sites.

The concept of single-point grounding means in practice: the house wall has a heavily grounded Entrance Panel, or so-called "ground window", where all grounds, cable braids, surge protection grounds are connected to and via which every single cable entering the shack is routed. This arrangement leaves no ground through-loop for surge currents to pass and damage equipment inside the house this way.

Consideration should be used when planning to erect new masts. Locating the tower close to the house causes many negative aspects from lightning protection viewpoint to arise:
  • just 180° opening angle for buried radials instead of 360° - doubling tower base grounding impedance
  • strike voltage potentials and currents higher on short feeder cables entering the house
  • magnetic fields strong inside the house - will damage equiment when tower gets direct hit
  • house perimeter ground has to dissipate considerable amount of energy, increases GPR
  • side flashes to house structures possible
The tower should be space away from the house by 10 m or more to reduce magnetic fields inside the house which causes  equpiment damage. Don't run any of guy-wires (or ground radials) towards the house. It would also be good to have the radial network clearly inside the property by a margin. Where soil conductivity is good, a tower can be grounded effectively on a smaller piece of land, but on bare rock, the poor grounding situation call for longer radials and causes issues such as step-voltages, surface arcing, etc. - not a safe place to be during lightning.

 This station does have ground electrodes for the house and tower - many fellow hams see nothing wrong with this (their) construction. To me it looks downright dangerous! I was hoping you know why.... bye the way they all have reported on equipment damage - bad luck or bad engineering!? Is there something missing in ham training?

 The above military comm. site lay-out has: single-point-grounding (SPG), tower GND radials, perimeter GND - bonded to tower base, cable braids bonded to tower base GND, buried metallic cable duct, grounded guy-wires (not shown..), transient protection with main GND bar bonded to perimeter GND, no overhead lines entering the building.

By spacing the tower away the magnetic fields during strike are lower inside the equiment building and the longer ground bonding cable's and coaxial cable's self-inductance reduce building perimeter potential rise (GPR). 

I see no substantian increase of material or labor costs compared to the site on the left image.

Can we control lightning?

 The corona dischargers are nonsense and can not prevent a lightning strike, but they do behave in different way than blunt surfaces. Scientists say the ion-current dischargers can generate, is insignificantly small to have any dissipating effect on the charge of the cloud and this ineffectiveness has been proven by field-testing. Only thing these brushes have done when hit, is spraying molten metal on the grass which has caught fire! Comparison between blunt rods and sharp rods have shown the sharp rods being poorer capturers of lightning strikes. The blunt rod tip don't willingly emit corona, untill the E-field increases to 3-times higher values compared to sharp tip and it then suddently puts out larger charge - presumably initiating the strike. If your tower has VHF-UHF-Yagis, you allready have a lot of sharp points - what ever the overall effect is. If you tell your neighbour your tower attracts lightning, he may not like it, if you tell him your tower rejects lightning, he'll ask will it divert the strike to his property... Perhaps it would be safest to to say the tower picks up (rarely) strikes and affect's little outside the radius of it's height.

Some claim the so-called Early Streamer Emission (ESE) rods can catch strikes better than Franklin Rods - no proven advantage in catching strikes either. Conventional Franklin Rods are the only sensible way to catch the hits and are used in tall buildings, but are not applicable to ham towers, since they already in a way are "Franklin Rods". Those rods also need proper downconductors to pass the currents to the ground electrode to dissipate.
Trivia: a 30-meter long run of hefty 35 mm2 copper wire has 0.0032 ohms of resistance. 30 kA DC current would cause 96 volt drop over the length of the cable - not that much? What does cable's self-inductance cause with a 30 kA lightning surge that rises in 1 microsecond? 

Answer: almost 1.5 megavolt end-to-end voltage drop, ~ 50 kV/meter!       Did you expect this?

Ground conductors are usually circular in cross-section, but same amount of metal formed as a wide strip gives lower impedance (inductance) and on certain places it should be used. One of these places is grounding of the Entrance Panel to perimeter ground, two 30...150 mm wide grounding copper strips does good job.

Flat-shaped conductors give lower surge impedance (Zs) as long as their width is not equal or greater than a pole or whatever they are mounted near or on. A 90 mm diam. circular conductor has same Zs as 100 mm wide flat conductor. Two parallel ground wires spaced by 60 mm gives same surge impedance than a single 120 mm diameter wire (wire radius = wire spacing). But parallel flat conductors have only 5% lower Zs, compared to Zs of parallel circular wires.
Perhaps you could take a look at Ericore down-conductor to get an idea of low inductance conductor's structure and how it affects on inductive voltage drop/meter. This cable has only 1.3% of the inductance a 35 mm2 copper wire has.

Never route any cables from half-height of the tower into the house in mid-air via the shortest route. Why? If tower-top potential in a lightning strike peaks to 400 kV, there will be 200 kV at mid-tower - just like center-tapping to a HV transformer! A feet up from the ground level there is only some 10 kV left - at least the larger diameter coaxial's braids' should be bonded there to tower leg's ground as they carry substantial portion of surge currents (20...60%).
Underground cable ductOur effort is trying to lead as much surge current to ground as early as possible and as little current as possible to the ham shack; we ground the tower and the cables near tower's leg and at the Entrance Panel. As we do not unplug the incoming cables from there, some of the surge-current gets in to the house via the feed-lines and control-cables and there may be several kilovolts of potential difference left. Grounding (bonding) of coaxial braids at multiple points is necessary and useful, but alone is unlikely to save your equipment, though it reduces the common-mode (longitudinal) surge current.

Damages can be reduced by adding transient protective devices to reduce differential-mode (copper) surges, but they need good grounding and bonding to work. Those differential-mode voltages at the equipment-side coaxial terminals are partially caused by  the faster sharp-rising and larger surge-current traveling on the braid. The propagation delay (cable Vf) (and are high-frequency roll-off'd) of center-conductor surge (the longer the cable, the more delay) is less significant. The early-arriving braid surge flows to the center conductor (at radio terminals) and later center-conductor surge flows out to the braid via the radio, unless you have a grounded arrester in between. There are other physical factors (inter-cable, braid-duct, etc. capacitance) causing some roll-off of braid-surge's energy spectra with longer coaxial runs (specially in metal ducts).

All cables should be routed from tower to the house via underground metal duct (it also has some surge choking effect). Electronic equipment installed to the tower-top needs to have transient damping filters up there. All (coaxials') braids should be "grounded" (bonded with tower) at tower-top. All the tower's guy-wires should be used as part of tower grounding unless they are used as HF aerials. The surge impedance (Zs) of  three guy-wires, about 170 ohms, is in parallel with tower's Zs and we get tower top to ground-level Zs of about 90 ohms.

The coaxials too have Zs and when spaced closely and fastened between tower legs, their Zs is highest - that would reduce the surge currents on the coax braids a little, but is that good or not? At least when you route the cables inside from the Entrance Panel, tying them up to a tight bundle gives higher Zs than spreading them apart loosely - increase of cables' Zs between Entrance Panel and radio shack is beneficial to buffer surge current flow towards the radios.

Grounding electrode construction - Grids - Rods - Radials - merely a boring economical issue?

An almost ideal lightning ground electrode for a tower would be a circular 5 mm thick solid copper plate of 200 m in diameter with the tower at the center and possibly having n pcs of long ground rods under it - but we can't afford it and perhaps we do not even need it, any more than making our houses as Faraday-cages? So we deal with it using a lot less copper, but how to make the most out of the material we use? Digging a hole and throwing the roll of copper wire there is waste of good electrode material.

AC power grounding has different goals from lightning grounding: reliability and low resistance (50/60 Hz impedance ~ DC resistance, with almost no reactance involved), typically up to 10...25 ohms are accepted though in some (HV) systems 2 or 5 ohms are the goal. Vertical rod electrode(s) alone are accepted in USA for customer AC grounding, but being inductive (a few uH), is inferior as lightning ground. A rod electrode may work well in good soil, but sometimes it would have to be 10..20 m long to make it's resistance fall below 10 ohms. Conductors have a typical inductance of 1 uH/m and a long rod may well show 10 uH of inductance, which transfers to 63 ohms of reactance for 1 us surge rise time. You can't add more rods closely spaced in parallel - spreading apart calls for bonds which have inductance, so it is kind a hard to get a long rod electrode to show low transient impedance. Further, the voltage gradient is steep near the rod and causes high step and touch voltages contrary to grid or radial grounds. It is preferential to make the horizontal ground network to form a shallow cone - the outer ends should be dug deeper to the ground. The ring-electrode(s) around the tower base do not dissipate much current, but reduces step voltages and increases reliability since it bonds the radials together.

In Finland the customer utility AC grounding standard calls for a (semi-)perimeter-loop around the building or one or two 10 m long buried radials - those underground metal water pipes are no longer installed. Lightning grounding aims for low surge impedance at higher frequencies. Utility power company ground systems aim for low grounding resistance. Bigger untility installations use underground square grids, which are a lot more efficient also in absorbing surges than rods, but the grid-wires transmit the sharp-rising current surge efficiently only four ways and the mesh-grid takes a lot of copper wire to build. With it you could make a real dense radial electrode. The center-point's grounding surge impedance of a 4-square grid, about 20 ohms, converts to 8 ohms for 10-radial electrode of same area and total length of wire spent.

Efficient lightning ground electrode is usually build of buried copper wire radials with additional rods driven about 2 m to the ground. Rods do not bring much advantage if the soil is homogenous. If the soil is stratified and underneath it exist a low-resistivity soil (water table) layer to which at least 20% of the rod's tips can be driven in to, using the rods along the radials makes sense. If the soil has no such layer, rods are useful just near the tower's base, where about 20% of the current gets disspated in 1.5 m radius. The 80% travels outwards on the buried radials (and your coaxial and control cables) with approximate disspation rate of 1 % / m (depending heavily on soil).
Spacing ground-rods too close to each other does not improve the grounding - the ground saturates around the rod  - depending on soil quality, anything from 1...10 meters away. Good rod spacing is 2-times it's length, but in poor conductivity soil the spacing can be decreased.
In excellent soil (10 ohm*m) you can get 6 ohms with a 1.5 m rod, but when driven in to granite, 10 000 ohms grounding resistance may be all you get.

The best use of buried copper wire is to spread it radially away from the tower-base towards guy-anchors (bonded to them) and beyond, not to form circles around the tower - with the exception of touch-potential reducer ring 1 m away from the base. There seems to be a charge-related effect that drives 85% of the strike charges to spread out radially by (top-soil) - sometimes arcing of top soil occurs even if the pole it hit, was grounded "efficiently" with vertical ground rod. On an open field 75% of the charges are estimated to be in a circle of 8 m radius just before the strike and high percentage of strikes to ground-rods with current exceeding 15 kA, have caused top-soil arcing. This also implies ground  radials should be longer than 8 m a piece, but a decade-fold increase in radial lengths would bring less advatage than expected and beacause of the inductive nature of a long radial, it does have a maximal length.

Lightning ground radials have a practical maximum length that depends on soil conductivity because of velocity factor differences by it's permeability (conductivity of soil roughly matches with it's given typical permeability). The empirical formula by Farag et al. ("Grounding Terminations of Lightning Protective Systems", IEEE transactions..., 1998) gives little different radial lengths compared to other methods (see graph above). Soil's resistivity depends heavily on moisture; for clay the cut-off point is at 18% moisture content, below which the resistivity sky-rockets and reaches comparable resistivity of rocky soil when at 10% moisture. The soil's annual moisture variation is typically +- 30%, but depends on electrode depth and amount of rain. Since the grounds should work as planned also when it's dry, add 30% for your typical soil resistivity to get a corrected radial length. Concrete is hygroscopic and has fairly good conductivity but if it dries out, it's as bad as rocky soil.

Surge impedance is significant and valid during the rising slope of the surge and modeling the lightning radial system is somewhat like modeling AM station's GP-antenna's radials. The lowest radial surge impedance is observed when the radial wire is 1/4-wavelengths long (the reflected wave is in anti-phase with the source at tower-base) (note: Vf values vary from 0.05 to 0.4 by soil properties). Relax: this is just a theoretical single-frequency example as in reality, the "big broad-band spark-transmitter's" emission covers kilometric to decametric wavelength bands.
For soil with 500 ohm*m conductivity and permeability of (k) 8, a 50 m long buried (6 mm diam. Cu-wire at 30 cm depth) wire is 1/4-wave long at 280 kHz, which is within peak lightning energy spectra. The wire has attenuation of 13 dB along it's length for that frequency and the reflected wave gets attenuated by 26 dB (the soil has absorbed 99.7% of it) when it gets back to tower base. This means the the idea of having low impedance at tower base with resonating radial does not really apply and it shouldn't. At double frequency the line's 2-way attenuation increases by 9 dB killing the higher frequency resonance and anti-resonances even more efficiently - a buried radial is just a lossy transmission line. If the current would be DC, a 3.3 km wire would be optimal. The early surges' rising slope's power spectra peaks around few MHz, rolling-off towards lower frequencies (20 kHz - DC resistances may be applied). We make a compromise between rise- time of the surge and the setting slope and limit the radial length to a value where the radial's absorption losses are high enough and save money, as extending the radial would not be as effective, as adding another radial at tower base (if there is nothing much left to dissipate at the end of the radial, why make it any longer?). Radials shorter than about 0.5 m in good soil or <4 m in poor conducting soils show almost no Zs behavior and in practice can be accounted by their grounding resistance minus ground ionisation effects. Google-up the RADIOETH.exe radial calculator by G4FGQ and see what it says.

A 100 m roll of copper wire can be used cost-effectively with best surge performance to form a 4-radial buried-wire system in a soil with conductivity of 200 ohm*m or 5 radials in soil with 100 ohm*m or just two radials for soil with 500 ohm*m. It appears the poorer conductivity the soil has, the poorer you get when buying all the copper wire it takes. A 6-branch double-Y radial electrode of 3 longer buried radials bonded to guy-wires and 3 shorter buried wires (at 30 cm below ground) spread evenly, would perhaps be more efficient: the shorter set of (5 m each arm) buried wires act as capacitive electrode with Zs(peak) around 70...80 ohms (Rdc 5...450 ohms). These would sink more of the sharp-rising slope of surge-current and the inductive electrode set of longer buried wires about 20...100 m each (depending on soil conductivity) having Zs(peak) from 80 to 110 ohms  (Rdc 2...35 ohms),  would sink the smoothly setting longer tail-current of the surge (if the use of different length radials work for multi-band GPs, it should work for lightning grounds too...). Those Zs figures are given for buried radials without ionisation-effect and with no rods, but adding rods would improve the system, if the soil is not too rocky to install them. The grounding impedance could be further reduced by doubling or tripling the number of those radials (Zstotal = Zssingle/N). Good tower lightning grounding system reduces the current-absorbing stress from the house perimeter grounding and reduces GPR. Radials should not be located pointing towards the house, but to all other directions.

The resistance of ground electrode is measured by 3-rod system with current rod taken to distance 5-times greater than the diameter of the electrode and the potential electrode is driven at 62%-distance. This arrangement should produce a plateau near the meter's potential rod where it's outside the strong influence of the current rod and the ground electrode. Too close and the results vary a lot when moving the potential rod. So, if you have a radial ground with 20 m radials, that means you need to take the meter's current rod 200 m away and potential rod 124 m away. This is impractical except on farms in the country but there is also a pitfall by variance in soil condutivity at that great distance. The metering works fine on homogenous clay-fields but think how bad it works if the tower is on a lonely patch of gravel and dirt on a large granite slab. The conductivity - when measured across a path of poor conductivity (such as bare rock), means your current and potential rods are on a separate "island" with almost no conductivity to your radial grounds - the ground resistance meter's readings are very high, even if you have a perfectly good "local" radial ground and there is nothing much you can do about it. If you think this situation from the viewpoint of local nature of charges just before the strike, is the soil conductivity 200 m away meaningful?

Low depth of buried wires adds to greater variance of performance between dry and wet seasons. Where lightning can occur when the ground is still frozen, the radials should be buried deeper (0.7 m... or more, below permafrost) or deep driven ground rods added to radials. Radial wire's or grounding rod's diameter have insignificantly small influence on grounding resistance and impedance - but wires under 5 mm and rods under 10 mm diameter should not be used -  a question of mechanical reliability and some overhead must be left of corrosion. For buried radials use only bare copper or stainless or galvanized steel wires with proper (bi-metal) connectors (as few as possible, no brass or aluminum). Avoid sharp bends - inductance chokes current flow. Buried wire electrodes will work fine at any reasonable wire length and number (but no less than 3 wires and no shorter than 8 m), in a soil having excellent conductivity.

Impedance of shack AC PE conductor to main ground bar: R 2 ohms +Xj 250 ohms at 1 MHz.

Tower ground electrode data of radials buried shallow (10...30 cm) in lawn dirt layer above thin sheet of clay and about 20...60 cm of gravel and rocks above granite slab:
(2-lead meth. w. Amprobe Ultratest)
Z 1 kHz
R +Xj
(w. MT 4080A)
Z 10 kHz
R +Xj
(w. MT 4080A)
Z 100 kHz
R +Xj
(w. MT 4080A)
Z 1 MHz
R +Xj
(w. MFJ-259B)
Z 5 MHz
R +Xj
(w. MFJ-259B)
 % share of 100 kHz current
 Z sweep 0...50 MHz
Approx. Z and notes
0° Radial 19 m
(p.o. second perim. leg bond)
 50 (est.)
 (50 est.)
(47 est.)
 abt. 47 ohms
Cable duct
 20 (est.)
20 (est.)
(19 est.)
(27 +30
bonded to guy-wire)
(35 +78
bonded to guy-wire)
 abt. 25 ohms
Perimeter leg bond
 40 (est.)
40 (est.)
(38 est.)
(52 +50
bonded to guy-wire)
(120 +63
bonded to guy-wire)
 abt. 45 ohms
160° Radial
7 m
116 -4
109 -5.7
100 -7
82 -13
122 +65
 down slope to 6 ohms at 50 MHz
 abt. 95 ohms
220° Radial
15 m
 (50 est.)
(50 est.)
(47 est.)
(68 +15
bonded to guy-wire)
(106 +39
bonded to guy-wire)
down slope to 7 ohms at 50 MHz
abt. 80 ohms
290° Radial
11 m (in the best soil)
48 -1.7
46 -1.3
43 +3
52 +19
94 +44
down slope to 4.5 ohms at 50 MHz
abt. 47 ohms
Attenuation at radial's end: 1 MHz -5 dB, 2 MHz -11 dB, 4 MHz -15 dB, 7 MHz -20 dB, 10 MHz -34 dB
330° Radial
25 m
56 -2
(0.8 +0.2 bonded to guy-wire)
53 -2.6
(1.7 +1.6 bonded to guy-wire)
47 -0.7
(14 +14 bonded to guy-wire)
52 +32
(59 +53 bonded to guy-wire)
100 +66
(105 +56 bonded to guy-wire)
down slope to 15 ohms at 50 MHz
abt. 70 ohms
Base Rod
1 m long
442 -12
424 -21
377 -61
262 -95
94 -50
 down slope to 4 ohms at 50 MHz
abt. 350 ohms
two 12 m long radials
 50 (est.)
50 (est.)
50 (est.)
45 (est.)
100 (est.)
100 (est.)
Tower to water pipe (unbonded):
 88 -3deg
67 -10deg
87 +21 deg
27% through
water pipe, 4% to AC line, rest 69% via soil
3-lead tower base GND resistance 72 ohms.

Soil arcing due to strong E-fields can decrease single electrode's surge impedance down to 7% on average (from 2...50%) of the original value. This effect is not well understood, but is significant. The circuit model on the picture assumes all electrodes benefit 50% from ionisation. Estimates concerning site grounding potential reduction by ionisation at sites with proper grounding usually quote values around 40...80%, since large ground radial network shares the current and no 95 %-class reduction can be expected. Still, it is comforting to know this soil's varistor-kind of behavior acts as a potential limiting equalizer for high current strokes and for undersized electrode systems. Soil requires an E-field of 3.5 kV/cm to arc. Low-conductivity soil has tendency to produce surface arcing near the electrodes when dissipating a direct hit. Arcing over to other buried cables may extend up to distance of
      li [m] = SQR( I [kA]) * Rs [kohm*m].

When evaluating the ground radial lengths and numbers, etc., the accuracy of the models is far greater, than the knowledge of soil material's electric properties in most cases and they alter significantly by season (soil moisture). It would be useful to leave some engineering overhead and make the grounding network slightly "oversized" due to the unpredictable nature of soil properties.

Besides just having adequate lightning ground electrode for the tower, copper wire should (must) be used to bond tower-base to house perimeter grounding  connected to main potential balancing bus-bar, or main ground bar (MGB), where incoming TELCO and cable TV are bonded to where existing AC power PE-bar is connected to. If you use single-point grounding (SPG), the AC power PE bar may be connected to Entrance Panel (or ground window). The general idea to prevent potential differences from rising too much.

Separate unbonded ground electrodes for sub-systems are dangerous and cause damage to equipment in case a near-by ground-strike develops step-voltage gradients on the ground which would allow surge-current flow through the equipment installed in between two unbonded grounds (example; TV aerial mast in the garden with it's own ground rod).

To estimate the DC resistance of your ground electrode (unfortunately, not the surge impedance) you may download a handy Ground Electrode Calculator, which works with rods or wires or both combined (and Ground Enhancement material).
From passive bemoaning to corrective actions
 I have monitored equipment damage-rates at a ham station having 30-m high towers in the countryside. The story first went like this; the mast-mounted 144  MHz pre-amp blew at least once every summer. Same with ham transceivers, computers, TV & HiFi. The tower and adjacent shack were on a small rocky hill with single steel ground wire ran down the hillside buried 20 m away to a field. Tower guy-wires were not grounded. AC and TELCO lines came in alongside the single ground wire. The mast-head pre-amp had 12 V DC-feed  with separate unshielded single pair-cables. Two step-up projects to improve system shielding has been done after damages over the years. At first step-up, the tower guy-wires were properly grounded with buried-wire and rods. Later two more grounding wire radials were ran down from the tower legs 120° apart. The pre-amp was rebuild with RF-output coax also DC feeding the pre-amps and T/R relays via bias-Ts, with a 3-series resistor-MOV-TAZ-Zener chain at preamp-side bias-T. TAZ diodes were added to radio side bias-Ts. The commercially-built 432 pre-amp is identically fed with the coaxial. The owner prefers to unplug most of the cables, now from the entrance panel (there now is one) in the summer, just for safety (the cables are not bonded to tower's leg!). Unused cables entering the shack lying loose on the floor were eventually thrown out and tied to tower ground. The AC supply line, which later got 40 kA transient suppressor bonded to potential balancing bus-bar, feeds the computer and radio DC power supplies via stabilizer-transformer. The TELCO line is cut and no longer used  (socket-shields blew up) and was a known source of problems. The pre-amps have survived for 15 years now and all other damages have become random and no longer annual or total as they used to be. The measures taken, though satisfactory to the site owner, are far from comprehensive, but are a good indication, that the situation is never hopeless and even small efforts reduce lightning-related damages and electrical safety.

Poor soil conductivity near tower base is a problem: a shack alongside a tall tower with non-grounded guy-wires standing on a rocky hill with just one long ground wire running down the hillside to a distant ground-electrode, can resonate around about 1 MHz and create high voltages between true ground and the shack. A long inductive ground-wire and the capacitor formed by shack-tower-base structures vs. distant soil, acts as parallel LC resonant-circuit, which - when resonating at LF, can store and bounce the surge energy from the "coil" to the "capacitor" and back... (ringing) with very high voltages across the "capacitor". A TELCO or AC line running out of the shack will get jolted severely. Adding radially more ground wires (or strips) running down the hill and grounding tower guy-wires lowers the grounding inductance and that alone lowers the voltages and also raises the resonance frequency of the structure higher (in a way...detuning away for higher resonance). If the ground wires are very long, a ground radial electrode network with conductive concrete laid on the rocky hill may be much more efficient solution. Also rock-well electrodes are used (often filled with conductive concrete which has resistivty less than 0.5 ohm*m). Ground enhancement material (GEMs) are claimed to reduce the electrode's surge-voltage potential by 25 to 45 %.

Entrance Panel - the fortress at your doorstep

Duct, Entry Panel w. braid bonding and GDTs+MOVsThe important second stage of protection includes an Entrance Panel (EP, some times also called the bulk-head panel) by the house wall, which can have varistors (MOV), gas-discharge tubes (GDT), semiconductor devices and DC shorted coaxial stubs. The Entrance Panel is also heavily grounded and all coaxial's braids get bonded there to house perimeter ground. The majority of incoming surge currents should be diverted to ground at EP and as little as possible let in to the house. If the Entrance Panel fails to do this job well, the "game" is lost.

The third stage of protection is indoors by the radios and includes a second ground-bus tying up all coaxial braids again and may have faster, but lower power-handling transient filtering. Some of the last-stage filtering of AC and DC control-lines from the tower, may also be installed inside the equipment or in grounded enclosures. Whenever possible, use same RF-feeding coaxial (if impossible, shielded cables) to carry DC power to tower-top electronic equipment and ground all shields and never use un-shielded cables.

Inductances in the form of cable loops and ferrite-toroids and sleeves or ferrite-core coils as in solving HF TVI problems, can be used to dampen  the common-mode surge to your radios to some degree, but long lasting (30 to 400 ms) tail-surge-currents tend to saturate ferrites, which loose inductance after storing all the energy they can.

RX GND bus

Transient suppressors

Adding a single transient suppressor component across the line is not considered as a perfect surge filter. Surge filtering is build of several different electronic components:

Please select proper GDT voltage rating per power and highest possible SWR.. The RF+DC filter's antenna-side center-conductor is clamped by the low-impedance DC feed by nature - with low inductance L, a gas-discharge tube (GDT - not drawn) may never fire. If you don't' have GDT, what happens to the MOV and how high will the voltage rise across series-capacitor? It would call for higher impedance tap (L) for DC injection to the center conductor to get GDT across the coax to fire. The DC path's optional series-resistor's maximum tolerable resistance (tolerable DC voltage drop across R) depends on the amount of DC current needed to be fed through the filter. The resistor should be 1-watt type or more and must handle the normal DC current through it of course. MOV's and TAZ's voltage and types depend on the DC voltage passed through. For +13.8 V feed, use 16 to 20 V TAZ and 20...30 V MOV. De-coupling inductances and bypass capacitor values depend on the operating frequency. Capacitors across MOV and TAZ causes some voltage rise-time delays for switched DC (like 1 nF caps on VHF). Series capacitors for the RF path need to be Mica or some good door-knob (5 kV Centralab) RF-type to pass TX RF current (depends on how much power you run through them!) , but must also be able to handle at least 1 kV DC of lightning surge. Coaxialilty and 50 ohm impedance should be preserved, RF losses and radiation minimized by shielding and good general construction.

The selection of GTD (Gas-Discharge-Tube) voltage depends up on RF power you will run through the protector.
There should be some room left for occational bad SWR:
Pmax [W], low SWR
Pmax [W], SWR >1:3

When hit with a surge, a single GDT when inserted between inner and outer coaxial conductor, gives 16...22 dB of voltage attenuation (for 5 kV 10/700 us surge) with let-through voltages from 370...830 V. A transmitter may survive with this, receivers most likely not.

A GDT across, with a series-capacitor for passing frequency bands from 50 MHz & up, gives 48 dB of attenuation for similar surge with let-through voltage of 20 V. A receiver might survive with this shield.

A narrow-band shield with GDT across, capacitor in-series, followed by shorted 1/4-wl. stub, gives about 90 dB of attenuation. The power and voltages after this shied in ideal grounding environment are too low to cause any receiver damage, but can not be applied for wide-band feed-lines.

Quarter-wave DC shorted stubs

Since most of the surge energy is RF and centered broadly around 100 kHz to 2 MHz, a high-pass LC T-filter on VHF coaxial lines and above frequencies may also be of some help, but is not enough alone - the input side capacitor gets subjected to very high voltages and may flash-over if no GDTt is used before it.

These single-stage equipment-level filters don't have any common-mode rejection for (230 V) AC line. The LEDs are for monitoring the fuse has not been blown - needed specially on the simple AC plug shield.

This filter can be used on coaxial video cables and HF (VHF) receiving antenna cables with 1.5 dB (2 dB in 50 ohm system) insertion losses and maximum DC or AC peak working voltages of 5 V. The TranSil defines the cut-off voltage and for cables that carry for instance 12 V DC, 1.5KE18CA TranSil should be used. When built in metal enclosure, Fc is 150 MHz. Insertion loss increases to 12 dB at 500 MHz .

Multi-stage filtering of non-RF lines

While line's self-inductance may reduce surge-currents, also low-ohmic (fraction of line's impedance) (medium power dissipation) series resistors on the lines carrying a low power, may in some cases be applied to absorb surge energy. The surge protection device roofs the rising voltage on the line and the low-ohmic series-resistor(s) added before it on the line(s) sweats out part of the energy, like in a resistor-zener voltage regulator.

In some applications the signal line can be galvanically cut by isolating transformer, but transformers have limited across-the-terminals voltage after which they arc-over. Secondly they unfortunately have usually plenty of primary-to-secondary capacitance (audio&video) and therefore can not stop lightning surge RF currents.

 Three different type of components are needed to build a transient protective block since each component has different responce-times and power-handling properties and please note part of the work is done by those cabling series impedances. The above image and U/t graphs for a commercially produced 24 V DC line  DIN-rail-block shield is shown for further reference. Observe the order of firings and voltage levels: TAZ conducts first at 0.1 us, MOV next at 0.2 us, then the GDT finally wakes up at 0.6 us and takes the burden away from other components. DIN-rail-mounted transient shield blocks are manufactured by various companies such as Weidmuller, Phoenix-Contact, Dehn, etc., for virtually any power line-, antenna- and data lines and buses and purposes, but carry a price tag that does not always match the handful of passive components the enclosure holds - so you could buy the cheap components and enclosure for 10€ and build the 150€ shield  based on available information and figuring out the rest. On AC power lines you have to use approved commercial equipment for safety. Their pricing is reasonable (at least here at local Dehn source...) and you only loose money, if you buy MOVs and make AC line varistor modules from components. See this Trabtech application document.

Ham radio systems and equipment does have some rules concerning safety issues (electrical, fire,...), but hams generally can design and build ham radio equipment and systems for them self, but of course they must be safe - the site owner is in charge of station safety issues and you do these things at your own risk! If you do not know what you are doing, better let someone else...

10/100 MHz LAN data-line surge filters call for low capacitance. The data-lines are isolated from the capacitance of the TVS by the diodes which handle differential mode surges. Common mode filtering seems to be left for the GDT. Overall this circuit gives quite limited surge protection. If the capacitances of 1N4007 and TVS are too big for 100 Mbit LAN, try MUR1100E diodes and P0640SA SIDACtor.

Inter-equipment RS-232 data-line protectors usually have just series-resistors on all, except data ground conductors and on the protected side there are bipolar TVS-diodes from every pin to data GND and a GDT from data GND to shield GND pin. A typical RS-serial port transient shield has cut-off frequency of 100 MHz to 2 MHz depending on line impedance.

Incoming POTS TELCO line needs multi-stage transient filtering; three 350 V GDTs, 2....5 ohm 5 W series resistors (some prefer to use PTC's but I doubt they react fast enough), 160 V TVSs to ground and across the lines and good low-inductance ground-path to drain-off the surge.

 This ADSL line shield works along the same principles and follows muti-stage structure of 24 V DC DIN-block, except it is built for symmetrical (balanced) line and passes the 10 kHz....2 MHz ADSL-bandwidth with just few dB of attenuation at band's upper edge. It uses resistors instead of inductors in series and has a rectifier to isolate the middle stage's Zener-diode's capacitance (no MOVs or TVSs can be used for ADSL) from the lines and uses low-capacitance Surgectors or SIDACtors at the equipment side. The GDTs can be of any current-rating, but the bigger, the better. Lower voltage GDTs would also work as long as there is no POTS DC or ringing voltages on the line. The Zener chains are made using diodes of  fairly low power rating, so if they short, try ones with more power dissipation capability - the same goes for any other part used here. I recommend you apply some faster diodes (with low capacitance) for the rectifier bridge than 1N4007. The zeners are not ultra-fast either, so maybe TAZ's could be applied here, but watch the capacitance across the lines and to ground. This filter should be located close to house main ground-bus and have short GND wire. For additional ruggedness, the spark gaps can be separated by 5 W 1 ohm additional series-resistors from the DGTs.

The TECLO line's experience likewise transients from magnetic coupling from lightning as AC lines discussed below, with the unfortunate difference by the nature of the star-network: every copper pair runs all the way from the PBX to the customer. TELCO cables have their braids grounded at every 1.1 km (here) and unshielded air-lines should have GDT shields at both ends, but the cables are usually way longer (up to 35 km) than AC utility lines and the transient voltage problems are thus worse.

Sometimes it is hard to pick which you want; use effective transient shields and melt the thin copper pair wires (which usually the phone company has to repair for free since leased-line) or if the GDT shields are blown up or don't exist, listen to the loud static shots during lightning from the unplugged TELCO socket (which may save the copper from melting, but may bring other troubles). Those old long and expensive-to-maintain TELCO lines in the countryside are something the phone companies nowadays wish to get rid off and just make money with Cellulars and ADSL inside the 7 km  DSLAM coverage - no cut overhead lines or rotten poles to fix. When no money is spent on inspecting and replacing the blown lightning transient shields along the overhead air-line, the customer gets to handle all the transients blowing-up his own TELCO plug-shields, phones, modems and in-house wiring and as a result, eventually gives up filing line-fault reports and terminates the land-line phone contract - making the phone company very happy.

AC power line shields

The utility AC systems are built in totally different way in USA and Japan with each customer practically having their own transformers and short low- voltage lines, so the mechanism causing lightning induced voltage surges to overhead low voltage line's does not apply there. Neither is the model applicable to Norway or other poor soil conductivity places where PEN conductor is not grounded. Still the transient shields work by same principles everywhere, but how they are connected, it's a different story. Please note the examples apply for areas with annual lightning densities of approximately 1 ground strike/km2/year.

EN SFS 6000 standard, the low-voltage directive, says: in areas with less than 25 thunderstorm days /year (which we do not have in Finland) and connected with over-head air-line (AMKA...), AC overvoltage transient shieds are not required. But, this standard ignores sensitive electronic equipments.

With the introduction of Aerial Bundled Conductors (ABC, ALUS, AMKA..) the decrease of line's self impedance and the laid-up structure of the cable reduced surge voltages in the cities (now mostly with buried cables) and suburbs, with the short line sections and frequent grounding points at every consumer, to levels where routine multi-stage transient shielding was longer necessary, except in places where lots of expensive and sensitive electronics are used. In the densely populated suburbs using the neutral-grounded system, the PEN conductor gets connected to customer's grounding at about every 50...200 m effectively limiting the induction loop length (or area). Also the line sections are short since even a thick ABC cable can only supply a limited number of households, some of which may be having high loads in the form of electric heating and saunas. As a result, lightning induced voltages usually remain within range that equipment (level shields) can handle, unless the ground strike occurs closer than some 100 m from the overhead LV line, which is statistically not that frequent.

Situation outside suburbs in the countryside was bad in the past; the typical 3 km-long  high-impedance overhead open-wire line was a real nuisance to a customer in particular at the end of the line. Lightning induced incredible surge voltages at the line's-end and damaged even the most basic electric appliances and the entrance fuse & metering panel which sometimes spit out small balls of lightning. As those open-wire lines have mostly been swapped to low-impedance aerial bundled cables or buried cables over the past 35 years, transient problem has been alleviated, but is far from being fully solved, mostly because nobody wants to pay for it. Here those local utility companies refuse to fund or install for free any transient protectors on their LV lines and the insurance companies show no interest on this grey-area either in any way as far as I have heard of. Only the part of labor in shield installation is tax deductable in Finland. Fair or not - if someone wants to live in the countryside, it's mostly his problem and he pays to resolve the poor "quality" of power he buys.

The 20 kV 3-phase overhead air-lines form the back-bone of medium HV-level county-wide power distiribution in Finland. The general understanding is the 20 kV to 400 V transformer's low-voltage neutral should be properly grounded for many reasons. But, actually emphasis has been put to consumer side grounding and during construction-phase, the transformer ground may be temporarly omitted providing adequate number of consumer ground electrodes are connected to PEN conductor.

The older pole mounted transformers that are not using buried low voltage cabling, have no ground wires running down the poles, even every guy-wire have insulators. The HV spark-gaps are bonded to low-voltage's neutral via transformer casing. The low-voltage aerial bundled cable's neutral messenger conductor is used for grounding the transformer secondary by a single wire ran down from the next pole 25 m down the LV line and plough'd to rocky sandy soil towards a road no farther away than 8 m. How good grounding can that be?

The reason for this arrangement is the HV-switch's primitive mechanical constuction - when broken, causes a risk for the operator (high step-voltage etc...) below the transformer and in these cases the transformer ground-lead must be taken at least 20 m away from the equipment as air-line. The second reason is the high impdance grounding arrangement reduces voltage transients on the transformer and give longer operating life.

The rules say the transformer's secondary's neutral can be grounded no farther than 200 m away with grounding impedance of 100 ohms or less - when soil permits it. One buried wire may be just about 100 ohms, but is minimal and it appears as transient surges from the 20 kV spark gaps during lightning are conducted along the LV neutral and get  induced on phase-conductors by mutual impedance and  fed down the ABC cable towards the customers - thanks.

Low grounding impedances and resistances achieved by good ground electrodes, do improve the reliability and safety of LV distribution system. Unfortunately, the utility company does not want to use money on early renovation of the line-switch before the transformer and the poles have to be replaced anyway.

The ABC cable's neutral wire is grounded (but not in every country) at supplying transformer and at every consumer, may be grounded every at every 0.5...1 km, so at least 1 km-by-7 m ground-loops are formed. These loops couple magnetic field energy from nearby lightning ground strikes. As the loop (line) length increases from 100 m to 1 km, the voltages raise to 10-fold. Tolerable 1.5 kV surge levels encountered in the suburbs, turn to 15 kV bangs in the country. Where the overhead bundled cable (branch) to the customer exceeds 500...1000 m in length and the household has sensitive electronic equipment that should be protected, power line transient shields may well prove to be useful. Of course the need has to be scaled to local lightning densities to reckon the damage frequency. It is easy to predict transient shields are really needed annually in the countryside, while in suburbs they would be just waiting for that rare real-close strike which may not occur for the next few decades.

The installation of MOV blocks by the utility company on the last overhead line's pole before the customer, costs about 350€, but prices may vary. Mid-level transient shield module for main fuse & metering panel cost 260€  + the installation costs - but those old panels have no room so it will cost some more.

Incoming AC power lines should be protected preferably in 3 stages:

As said before, installation of arresters and filters to utility AC entry panels and other fixed household AC wiring must be done by a professional electrician. These protective devices should be connected (to a good ground) with as short ground connection wires, as possible to eliminate unwanted current-limiting parasitic series-inductances, but inductance is needed in between each class of AC shields in the form of at least 5 m (5 uH), preferably 10 m (10 uH) of line in between, or an inductor block (7.5 uH at 10 kHz) of adequate current handling capability per fuses used on the AC supply side - stepped AC transient filter system does not work if installed with short cabling between each module. Applying merely equipment-level shields to a system that has no preceeding B- or C- stage shields, but would need them, will not be a successful strategy: the overwhelming surge currents exceed what the small MOVs can handle and are driven out of their normal operating area where voltage limiting is imperfect sometimes destroying the MOVs at first strike transient, after which they quit working.

Surge test signal forms

The common-mode transient damping of surge protectors is measured at 1 MHz and normal-mode (differential) surge is measured at 100 kHz. The latter surge is low-pass filtered by inter-cable capacitance and so the power-spectra contains lower frequencies, while common-mode surge travels fast on the whole cable, which capacitance in case of air-line (distance to GND is big), is lower and the low-pass roll-off effect occurs to a lesser degree. The farther the surge travels (along the air-line), the less the power spectra contains higher frequencies and the lower the voltage and surge power.

AC stabilizer-transformers offer tens of dB of transient damping, but are not cheap, small, quiet, run cool.... A UPS, if proper type (ON-LINE, not stand-by type), usually also gives galvanic isolation. They are quite useful in damping common and differential mode surges up to 2 or 4 kV-level entering from the AC line towards your radios and PSUs.


If the tower & house does not have a low-impedance ground-bus, a strike on the tower will cause a GPR (Ground Potential Rise), where substantial amount of strike current gets driven out of the house via AC and TELCO lines as whole household wiring's voltage potential (including the ground) jumps to tens or hundreds of kilovolts. This usually causes massive equipment damage. The same works the other way around: if the lightning strikes on your incoming AC power line and the house has "poor" grounding and the radio tower is not bonded well to house ground-bus bar, the surge may look for paths to ground via the tower's grounding and pass though your radios on the way.

On HV power lines with overhead lightning conductor, GPR causes the classic insulator flash-over during a lightning-strike on the overhead lightning conductor, as the whole tower structure's potential gets elevated to 0.5....5 megavolt-level due to "inefficient" grounding (the overhead conductor's can't prevent voltage near strike point to surge up (line Zs=400 ohms)). The surge voltage arcs over the insulators to the phase conductors and some of it travels away along the line (also with 400 ohm Z) and the arcing,  preserved by AC HV, shorts and/or grounds the phase conductors - activating  ground-fault and over-current sense to trigger fast circuit-breakers to drop the HVAC off momentarily from the line, so the insulator (spark gap) arc-over can extinguish. The air temperature needs to drop below about 1500 °C for air to stop being conductive (ionized). Though HV lines are irrelevant to ham radio operator, it's interesting to hear the lower HV air-lines, up to about 70 kV, suffer mostly from magnetically coupled lightning surge-voltages causing insulator or spark-gap arcing, which triggers line protection circuit-breakers momentarily. An open-overhead-line 7 m above ground develops an 80 kV surge-voltage via inductive coupling from a 30 kA ground-strike 100 m from the line (1/10-distance increases voltage by 10-times). Above 70 kV AC operating level the HV line insulators and other equipment can withstand everything else atmospheric, except a direct strike to the line or the GPR. This gives a good idea on the magnitudes of inductive surge-voltages on long air-lines. Shielded buried cables also pick up energy from lightning strikes: a ground strike 200 m away from the cable produces several kV surges and when braid is grounded, up to 100 A current spikes on the cable's shield. Any shielded cable longer than 15 m should have it's shield grounded at both ends.

DC-shorted aerial - why?

HF (or any) antennas that are not DC-shorted at feed-point, pick up static DC charges (coax is a good capacitor) are considered to be against rules and should be modified to be DC shorted or have GDT across the feed-point (cut dipoles direct-fed with coaxial, direct-fed GPs.,etc.). If you can choose between building a DC-shorted and non-DC-shorted HF aerial, select DC-shorted and ground the coaxial's braid. I mentioned before lightning surges (direct, induced, influenced) are (V)LF (~0.01 -  1 MHz)  broad-band RF energy, so what's the DC-short good for? Well, from the viewpoint of upper HF band frequencies or VHF/UHF, 0.01 to 1 MHz is "almost" DC...., but even DC-shorted antennas will not protect equipment from differential-mode surge currents of direct strike.

I wonder why radio manufacturers don't  use a crowbar-relay shorting the receiver's antenna connector when the radio is unpowered?


ex. GDTParts left of a telco surge protector after fist nearby hit of a 13 km aerial telco line by Telia-Sonera who had informed the customer the line's own "transient protection has been checked and is OK" - REALLY? Every transient protector used has blown to pieces on a first <1 km ground strike or indoor cable pairs have melted. Perfect way to get rid of expensive-to-maintain long landline customers - they move to cellular broadband or Wimax as soon as they are available.

A blown surge protection component means it has done it's job, most likely saving your equipment from more extensive damage, but if it happens too often, use more protective stages and devices with higher power or current dissipation or redesign the system. MOV's in power circuits needs to be fused, if a shorted MOV can cause damage or stop equipment from operating when the MOV eventually permanently short-circuits. MOVs tend to get shorted after passing many-enough current surges, but even a single one -  if too big - may short it. Some new MOV types include internal fuse. Their condition would call for monitoring.

 I have never seen a ham radio station that would have been shielded from lightning related damages professionally to the full extent and I am afraid I never will. When a house is not built originally to be a radio station, you make compromises. About 70 % of the installations only have tower-base grounded, some kind of modest grounding for the radio and that's all.

Fortunately I have experienced little equipment damage at home from lightning since Nordic countries have a short summer season with moderate thunderstorm activity. Some years ago a two-stage telco line surge-suppressor shorted, telephone and a card-modem were damaged from a (induced?) lightning transient that came in from the telco line, but I have dealt with lightning related troubles at work - mostly related to use of  long telco lines, or coaxes and on rare occasion, a devastating direct strike to a street light pole spreading the surge along the phase conductor to electronic equipment and on sensitive scale sensors located near the pole, vaporizing PCB foils 500 m away at the other end of RS-485 line, etc.

The general attitude on add-on transient shields I hear is: "where they are needed, they blow-up at first strike and are no-good and cost money...". On troublesome installations the plug-socket transient-shields they sell at shopping malls and the internal transient protection inside TELCO equipment (modems), will not be sufficient. They are not useless, but are meant to be used only as last stage equipment-level shields and to survive would need rugged coarse shields ahead of them (see photo above) to dissipate the energy step-by-step. My own DSL has been up 24/7 since August 2003 using a 2.5 km mostly-buried cable - lets see how long...

in Finnish, suomeksi: http://www.sral.fi/files/ukkossuojaus.pdf
and on pages 13...16:  http://www.digita.fi/binary.asp?path=1930;2500&field=FileAttachment
in Swedish: http://www.hvi.uu.se/Lightning/skydd_av_radiostn.html
basics about lightning protection in English: http://www.lightningsafety.com/nlsi_lhm/ExplosiveFacilities.html
Extensive quide on lightning protection by Dehn
Power Quality Application quide: Earthing systems, Fundamentals of Calculation and Design
Link to most recent news on lightning-related incidents

Antenna entrance GND patch-panel for the ham shack of  6 m, 2 m and 70 cm 200 W-class power ham station with 2 m & 70 cm pre-amps and separate TX/RX feedlines. (note the insulators and fire-proof back board)

This panel provides DC to HF blocking for TX lines, 20...60 dB of filtering for low-voltage 0.1... 2 MHz differential-mode surges and 0.15 to 0.7 dB TX insertion losses, 1 to 4 dB RX insertion losses with 3-stage transient suppression, VLF- to HF- band blocking for RX lines with DC power by-pass for pre-amps. Common-mode surge damping depends primarily on low-impedance grounding of the panel (note: 2 parallel GND wires used!). Please take care there is a fuse or other current-limiter at the equipment supplying the DC power to the RX coaxials in case the TranSil or MOV short-circuits. Note: TX connectors will not pass DC so switched pre-amps supplied with DC over the TX/RX coax (such as SSB-electronic SP2000 with sequencer), will not work through this panel as it is now built.


  Copyright 1997... 2014  Ilkka Yrjölä.