Over the weekend I put a temporary counterpoise under my antenna, to see how much effect it would have and make some rough measurements. The counterpoise consisted of 11 parallel insulated wires about 45m long, spaced about 1.2m, to make a rectangle 45m x 12m. These were supported at a height of about 2m above the ground, and virtually filled the garden. The antenna was my usual inverted L, currently at a mean height of about 9.5m and 40m long. Due to the position of the antenna in the garden, the layout is asymmetrical, with the counterpoise extending 3m to one side of the antenna, and 9m to the other side.

With no counterpoise, the antenna loss resistance at 136kHz was 37ohms. With the counterpoise as above, Rloss dropped to 32ohms, a reduction of about 14%. With antenna current of 5A, 1A (ie 20%) of RF current was returned through the counterpoise. Removing alternate counterpoise wires to increase the average spacing to 2.4m led to Rloss of 35ohms, and 12% of the antenna current flowing in the counterpoise. Reducing the counterpoise to 45m x 6m with 1.2m spacing of wires, located centrally under the antenna, led to Rloss of 34ohms and 12% of the antenna current in the counterpoise.

So a small reduction of loss was achieved by the counterpoise - it would seem likely that, if the area of the counterpoise was increased and the spacing of the wires reduced, a large reduction in loss could be achieved. The counterpoise acts like a screen between the field of the antenna and the lossy ground - however, since only a small fraction of the antenna current flowed in the counterpoise, it is clear that my counterpoise was only intercepting a small fraction of the total field of the antenna, so a much greater area would be required to produce a substantial efficiency improvement. If this greater area was available, a similar increase in efficiency could probably be more easily obtained by increasing the size of the antenna top loading, or a modest increase in height. In my case, a much more practical way of obtaining the same improvement in radiated power would be to increase the TX power by 14% - it really is very awkward having your whole garden covered in wires at head height!

Cheers, Jim Moritz
73 de M0BMU

As Jim no doubt knows the "standard" counterpoise, or radial system, for MF broadcast stations in the USA is 120 radials of a quarter wave length. This is what is probably required to collect all the current coming from the vertical.