Antenna Potential

Author: Edmund A. Laport

The root-mean-square potential V_a at the base of an antenna of measurable impedance Z_a = R_a + jX_a, for a power input of W watts, is simply

If the antenna has an electrical height G less than 75 degrees, the approximate potential V_m on the upper end is

(This may also be used as a rough guide to the estimation of upper-end potential for antennas of heights from 110 to 225 degrees. It should not be used at all in the region of a potential minimum in the distribution occurring within 15 degrees each side of the point 90 degrees from the upper end of the antenna.)

For potentials at intermediate points, with the same limitations set forth above, adequate practical accuracy is obtained by considering that the potential varies cosinusoidally with distance from the upper end.

The antenna potential implied here is the potential at a point on the antenna itself with respect to ground. This is a physically unmeasurable quantity but has some significance in relation to guy insulation. It has further significance in certain extreme applications of very large power, or a very small antenna conductor, or both, where the electric intensity at the surface of the conductor is required in the computation of potential gradients and the probability of corona formation.

FIG. 2.33. Geometry of antenna and guys on which potentials of Table 2.7 were measured. (After Brown.)

Figure 2.33 shows the dimensions and layout of a tubular steel radiator, with the locations of guy cables and insulators.9 The potential across each insulator was measured with a voltmeter, at known power input, operating at frequencies that gave this antenna electrical lengths of 51.5, 71.0, and 192 degrees. The voltages for 1,000 watts antenna input are shown in Table 2.7.