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Date: Tue, 02 Jun 2009 22:45:48 +0100
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: Re: LF: 136k/500k Grounding experiments (long!)
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A very interesting set of experiments Jim.

I also found  that long copper rods driven into the found as deep as you 
can get them worked best for 73 and 136kHz and that radials in a 
suburban environment were not much good at all. 
However, I did some experiments on 73kHz at Amberley museum some years 
ago. This QTH was once a chalk quarry and I found it impossible to get a 
ground stake earth in chalk to work. I finished up using a single 
counterpoise 300m long just lying on the ground. To get it to work I 
used a loading coil just like the one used for the antenna itself, which 
was also about 300m long strung across the a valley created by the 
mining activity. Both coils and the matching transformer were adjusted 
for maximum antenna current.

Interestingly, a Tesla coil group set up a series of serious Tesla coils 
at the museum for a demonstration and they had make a buried earth mat 
arrangement for the large coils. The largest one, which gave spark 
discharge over a metre long was driven by a RF arrangement at 70kHz. I 
checked the signal using the TS850 with a short antenna in my vehicle 
located around 80m distant from the coil - and heard nothing. This 
surprised me - I was expecting a very large signal.

Regards

Peter, G3LDO


> Over the last few days I have been doing some experiments with ground 
> systems with my LF/MF antenna. The purpose was partly to see whether 
> it is possible to reduce the overall loss resistance significantly, 
> but also to test out some convenient types of ground for antennas for 
> portable operation, in particular radial wires. The antenna is 
> currently a top-loaded vertical about 10m high, with about 80m in 
> total of horizontal wires in a narrow, asymmetrical "Y" shape about 
> 50m long, giving a capacitance at 136k of around 500pF, and around 
> 580pF at at 500k due to the increased effect of the distributed 
> inductance of the wires. As a reference, I used the normal ground 
> system, consisting of 6 x 1m ground rods, distributed within a couple 
> of metres of the base of the antenna tuners.
>
> The measuring technique was to initially set the normal series loading 
> coils to resonance at 136kHz and 502khz with the normal ground system, 
> and then measure the antenna resistance between the "cold" end of the 
> loading coil and ground with an RF bridge. The total loss resistance 
> of the antenna (including the loading coils and the normal ground 
> system) was 57ohms at 136k and 24ohms at 502k. Then an alternative 
> ground system was connected instead of the normal ground, the bridge 
> re-tuned to measure the new resistance, and any change in reactance 
> determined from the change in resonant frequency. The reactance 
> measurement is only approximate, but good enough for these purposes. 
> The bridge equipment was all battery operated, and sat on a wooden 
> table, so had minimal effect on the impedance of the ground system. 
> When I set up a similar antenna with similar ground rods in a field a 
> while back, the loss resistance of the antenna was only about 8 ohms 
> in total, so the component of the loss resistance due to the normal 
> ground system is probably less than 8 ohms. The much higher total loss 
> resistance is due mostly to the environmental factors affecting the 
> antenna.
>
> I first tried combinations of insulated radial wires laid on the 
> ground, including 4 x 20m radials, 4 x 40m radials and 8 x 20m 
> radials. It did not seem to matter much how the radials were laid out, 
> whether underneath the top load wires or in completely different 
> directions. Some had bends or doubled back - there is not really room 
> at my QTH for 40m radials! Some were laid on the grass, others along 
> the concrete driveway that runs down one side of the plot. None of 
> this made more than a few ohms difference to the loss resistance, 
> provided the wires were reasonably well spaced apart.  All 
> configurations had significant capacitive reactance compared to the 
> normal ground rods:
>
> 4 x 20m radials, 136k: R = 80ohm , X = -j466ohms
> 4x 20m radials, 502k: R = 28ohm, X = -j113ohms
>
> 4x 40m radials, 136k: R = 66ohms,  X = -j248ohms
> 4 x 40m radials, 502k, R = 25ohms, X = -j61ohms
>
> 8 x 20m radials, 136k: R = 62ohms, X = -j283ohms
> 8 x 20m radials, 502k: R = 23ohms, X = -j76ohms
>
> So it would seem that having the total of 160m of wire as 8 shorter 
> radials is slightly better than 4 longer ones as far a loss goes. I 
> also tried elevating the radials above ground about 0.3m on lengths of 
> cane:
>
> 8 x 20m radials, elevated, 136k: R = 80ohms, X = -j622ohms
> 8 x 20m radials, elevated, 502k: R = 23ohms, X = -j158ohms
>
> The loss resistance at 502kHz is slightly reduced, but the extra 
> capacitive reactance is now getting very high, especially at 136k. I 
> tried adding a "metallic" ground connection by adding a 200mm long 
> steel tent peg to the end of each radial, and driving it into the ground:
>
> 8 x 20m radial + ground spike, 136k: R = 80ohms, X = 0
> 8 x 20m radial + ground spike, 502k: R = 69ohms, X = -j11ohms
>
> So this is effective at reducing the reactance, but does not help loss 
> resistance; in fact, it leads to a drastic increase in loss at 502k.
>
> Instead of insulated radials, I tried a 15m x 0.6m strip of wire mesh 
> (a roll of chicken wire that was lying around), held down flat on the 
> grass with bricks:
>
> 15m x 0.6m wire mesh, 136k: R = 82ohms, X = -j102ohms
> 15m x 0.6m wire mesh, 502k: R = 31ohms, X = -j9ohms
>
> So in spite of  having a large area of metal in reasonable contact 
> with the ground, there is still relatively high resistance, and some 
> capacitive reactance. Since the insulated radials in general resulted 
> in lower loss resistance, I tried insulating the wire mesh from the 
> ground using polythene sheet:
>
> 15m x 0.6m wire mesh, insulated, 136k: R = 65ohms, X = -j320ohms
> 15m x 0.6m wire mesh, insulated, 502k: R = 26ohms, X = -j63ohms
>
> Since the wire mesh would be quite a convenient earth system to use 
> for an antenna over a paved area, or on rocky ground, I tried laying 
> it on the concrete driveway:
>
> 15m x 0.6m wire mesh, on concrete, 136k: R = 57 ohms, X = -j320ohms
> 15m x 0.6m wire mesh, on concrete, 502k: R = 23 ohms, X = -j65ohms
>
> The loss resistance is now as low as the normal ground rod system, 
> although the reactance is higher.
>
> Looking at these results, the ground rods win for the fixed station 
> antenna. The ground here is a permanently wet clay soil, which is 
> probably quite good for ground rods. They give practically the lowest 
> loss resistance (a couple of the other ground systems give a 
> marginally lower loss resistance at 502k, but in practice the actual 
> resonant frequency was increased due to the additional reactance, and 
> the loss resistance of the antenna decreases at higher frequency). The 
> reactance is also lower. Most of the alternative ground systems 
> effectively behave as capacitive coupling to ground, with significant 
> capacitive reactance as well as some additional loss resistance. For 
> the fixed station, if the ground system has a high reactance, most of 
> the ground current will flow through the lower impedance path via TX 
> chassis/mains earth, instead of the ground system. This is possibly 
> why adding ground radials to an existing system often has no effect. 
> One way round this would be to have an auxilliary tuning inductor 
> connected between the ATU ground, and the ground radial sytem. You 
> could tune the inductor for maximum RF current in the radials, or 
> minimum current via the TX ground.
>
> For portable antennas, it is often difficult to drive long ground rods 
> into the earth. It seems that short ground spikes, like the tent pegs, 
> or metallic conductors laid on the surface of the ground, do not make 
> a very good ground connection. The insulated radial wire systems 
> generally give the lower loss resistances for the antenna system, and 
> are quite easy to set up. The effect of the unwanted ground system 
> reactance for a portable station with no other earth connection is 
> that the ground terminal of the ATU is not at ground potential, and so 
> probably also the cases of all the station equipment. For 500kHz 
> stations with fairly low power (say <1A antenna current or so), where 
> reactance is quite low, this probably does not matter much so long as 
> the equipment (and operator) are reasonably well insulated from 
> ground. For a QRO 136kHz station, it is likely to be a different story 
> - with 1kW I can get about 4A antenna current - if I was using the 8 x 
> 20m radials, the 283ohms reactance would mean the equipment ground 
> would have more than 1kV of RF relative to ground! The capacitance of 
> the insulated radials works out very roughly to 28pF per metre of 
> wire, and one wants to make the total capacitance large enough to keep 
> the voltage down to a reasonable level. More and longer radials would 
> clearly be desirable, which are not really feasible in my garden, but 
> might be quite easy at a /P location.
>
> The insulated wire mesh ground required less area than the radial 
> wires, and had low loss resistance with similar reactance, so could be 
> quite promising, especially where the ground is already insulated; on 
> concrete or tarmac for example.
>
> Hopefully some time this summer, I will be able to borrow a field 
> again, and do some further experiments with more space available.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
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