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When you put power into an antenna, a current flows out of one conductor
of the feedline to supply that power. An equal and opposite current flows into the other conductor. In the case of a grounded vertical, this means that whatever current flows into the base of the antenna also flows through the ground -- where the feedline shield is connected. Due to the resistance of the ground, this results in I^2 * R power loss. If the antenna's radiation resistance is comparable to or lower than the ground resistance, the fraction of applied power that's lost is significant, so it's common to lower the ground resistance by using radials. Radials become increasingly important as the vertical gets shorter, because a short vertical has a lower radiation resistance. However, the feedpoint radiation resistance of a half wave vertical is very high -- typically higher than the ground resistance. For a given power input, a relatively small current flows into the base of the antenna, so very little current flows in the ground. Consequently, the ground loss is low, and there's no need to decrease its resistance with radials. Roy Lewallen, W7EL Roy, I've always maintained that when I don't know that the ground resistance is zero, I want as little current flowing in it as possible. My inverted "L" is a voltage fed half wave on 160 meters, about 70 feet up and 170 feet out courtesy of a couple of strategically placed Oaks . I have measured that feed impedance as being in excess of 2600 ohms and feed it with a remotely tuned "L" network. I used it for several years just fed against 60 feet of 6 inch well casing, and then, bowing to conventional wisdom as advertised on 1850 KHz, added an elevated counterpoise beneath the whole thing. (About 12 feet in the air) I didn't notice any change in signal reports, but that impedance sure changed a lot. Did I change something besides the ground resistance? Regards W4ZCB |
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