Bob.
The counterpoise is intended to provide an effective grounding (earth) for the set during operation. This helps with the efficiency of the signal being sent out over the aerial. One could accomplish this by pounding a steel grounding stake into the ground and connecting the set to that, however, in a combat zone that action would have been as popular as having to go out in no-man's land during World War One and pounding barbed wire stakes into the ground. The counterpoise was used as a quiet alternative to all the noise making a hammer produced.
The Working Instructions for the No. 19 Mk III explain the counterpoise as follows:
" The use of an elaborate earth e.g. a radial earth, with the spokes not less than half the wave-length being used, will improve radiation markedly. Even a simple earth-pin near the vehicle will effect some improvement in radiation, and will avoid the noticeable drop in aerial current which would occur through people near the truck touching the parts of its chassis to which the set is earthed. An earth will also, in many cases, improve the signal-to-noise ratio on receive."
Hope that helps.
David
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