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Walter Maxwell wrote:
Roy, it seems to me everyone has missed an important point concerning a circumferential component of the current. We know that the current flowing on the radial wires is radial in direction. What seems to be missed is the current that returns to earth between the wire radials. That current is going to flow in the direction of the lowest resistance. As such it's not going to flow radially alongside the currents flowing on the wire, because the radial resistance of earth between the radial wires is much greater than the resistance of the wires. Consequently, currents reaching earth between the wires will find a lower resistance by traveling toward the nearest radial wire instead of continuing in a perfectly radial direction. This new direction of current flow will not necessarily perfectly circumferential, but will certainly be somewhere between radial and circumferential. Walt, I hadn't missed that phenomenon, but didn't mention it because it doesn't produce a circumferential current. If you look at the current flowing from the earth to each radial wire, you'll see that the sum of these currents will be purely radial, assuming that the system is symmetrical, i.e., radials are equally spaced and equal length, the ground is homogeneous, and the radiator is vertical. Consider a bit of current returning between two radials, which is a little closer to the radial on the right. It'll detour to the right, giving it a rightward component as well as an inward radial component. But for every such bit of current, there's another one the same distance from the radial to the left which will have leftward and inward radial components. The radial components are in the same direction (inward) so will add but the circumferential ones (leftward and rightward) cancel, leaving a net radial current flow. You can say that the returning currents bend to the right or left as they propagate toward the antenna base, but not that there's a systematic circumferential current flow -- no current crosses from radial to radial in a clockwise or counterclockwise circular pattern like Richard implied. I recall reading a paper which showed that connecting radials with circumferential wires actually degrades a ground system's effectiveness, but I wasn't able to lay my hand on it when I looked. Roy Lewallen, W7EL |
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