Dear J.B., LF Group,

 

I did some experiments a while back with different ground configurations; like several other people, I found that once a reasonable ground connection was in place, further additions to the ground system only produce small improvements. Here the soil is clay, so fairly easy to drive rods into, and probably quite low resistivity. I found that a single 1m ground rod resulted in the antenna system having of the order of 20 ohms more resistance than multiple ground rods spaced a few metres apart around the feed point. Adding ground rods one by one had roughly the effect of paralleling 20ohm resistors, and once I reached 6 or so, there was very little improvement to be had by increasing the number – at this point the overall loss resistance was still something over 30 ohms (for a 9m high, 40m long inverted L).

 

When I connected ground rods that were a greater distance from the feed point (10m or more), very little current flowed through these – although they might have a low resistance, the overall impedance of these ground connections is increased due to the distributed inductance of the connecting wire, so they are not very effective. I expect this would apply to your well shaft. You can overcome this, and even up the current distribution, by isolating the antenna feed from the local ground, and equalizing the impedance of the ground connections, either by bringing them to a common “bus” point with roughly equal lengths of wire to all the grounds, or by adding balancing inductance in series with the shorter ground connections (these types of arrangement seems to have been very popular for LF/VLF antennas in the 20s and 30s). I tried this in several different configurations, but in spite of successfully evening up the current distribution over a wide grounding area, it only reduced the overall antenna losses by a further 10% or thereabouts. I also tried a counterpoise supported about 2m off the ground, and covering all available area (about 12m x 50m) – this also only reduced the loss resistance by a few ohms.

 

I came to the conclusion that, once this stage was reached, most of the loss must be due to dielectric losses caused by the electric field of the antenna penetrating poorly conducting materials around the antenna (the ground underneath, buildings, trees, etc.). This was supported by the loss resistance decreasing with increasing frequency (it went from around 300ohm at 10kHz to 16 ohm at 500kHz as I recall). Increasing frequency leads to reduced antenna voltage for a given antenna current, so lower dielectric loss. A similar antenna put up during an expedition in an open field site, clear of trees and obstructions, had a somewhat lower loss resistance (maybe 20 ohms), so I think the majority of the loss occurs in the actual soil under the antenna. As Alan points out, this type of loss can be reduced by increasing the antenna capacitance (more wire). It can also be reduced by increasing the height (further from the ground = reduced field at ground level) and keeping the wire away from trees and buildings. It would seem that, at least in my circumstances, there is little to be gained by improving the ground system. Commercial LF antennas are much higher and longer than amateur ones, so will have much lower losses from this source – for these antennas, a few ohms of loss in the ground system will make a big difference.

 

Cheers, Jim Moritz

73 de M0BMU