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In this famous quote by Alexander Pope, “doctors” are not only physicians, but highly learned men in general.
We have two recognized experts on short antennas, Brown and Trainotti, saying the opposite thing. Brown says that a ground screen near the base of a short antenna is not necessary, and Trainotti says that a ground screen is necessary. I think that Trainotti is the one who is right. I will not argue which man is the more highly regarded. I will only argue the technical facts. Here is another famous quote by Alexander Pope:
“Some judge of authors’ names, not works, and then
Nor praise nor blame the writings, but the men.”
A common ground structure used for AM broadcasting is 120 radials about 1/4 lambda long. It is frequently thought that the ground resistance of such a structure is about 2 ohms. There is more exact information about ground resistance in Figure 16 of Trainotti’s “Short Low-and Medium-Frequency Antenna Performance,” published in the October 2005 issue of IEEE Antennas and Propagation Magazine. Figure 16 shows that for a fairly high earth conductivity of 10 mS/m, and at 1 MHz, the common ground structure has an equivalent loss resistance of .7 ohms. The high earth conductivity reduces the ground loss resistance to less than half of the typical 2 ohms because the total ground impedance is the earh impedance and the radial array impedance in parallel. The ground loss resistance would be higher for lower earth conductivity.
Brown experimented with 113 radials .4 lambda long. He did not determine the loss resistance of this structure, but simply characterized the ground loss as “insignificant.” Trainotti’s Figure 16 has some information about the kind of ground structure studied by Brown. Interpolating from his 120 radial curve to 113 radials, and using a radial length of .4 lambda, the ground loss resistance is about .5 ohms. The ground conductivity used by Brown is five times what was used by Trainotti, so Brown’s “insignificant” ground loss resistance is higher than .5 ohms (perhaps an ohm or more).
I calculated that the ground loss resistance cannot be more that .182 ohms if the Brown’s claimed efficiency is to be obtained. It is not possible that Brown’s ground loss resistance is nearly that low. Rich determined with his NEC program that the maximum loss resistance permissible in order to get Brown’s results is .149 ohms. This is even worse for Brown’s assertions than my calculations.
Trainotti did not say that 120 radials 1/2 lambda in length is virtually a perfect ground. I think that this impression is caused by his statement that all of the earth within a half wavelength radius of the antenna forms part of the return circuit of the displacement current of the antenna.