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Those sources also don't tell me anything about
"velocity factor" as far as I can tell. I don't expect those who are totally invested in and entangled by "velocity factor" to understand this. But they continue to fulfill my expectations. (Richard C. will probably even predict with some accuracy their next card to be played...) Cheers, Tom ======================================= Yes, the velocity factor doesn't change with Length/Diameter. But it is sometimes convenient to discuss the effect as such. Actually everything happens at and near the ends of the wire. The short length of wire to be pruned to bring about a state of resonance is the same regardless of the number of half-waves in the anenna. It is sometimes referred to as the "End Effect". Think in terms of the directions of the electric lines of force at the wire ends. They are not all radial lines of force. Some of them extend outwards in the direction of the wire. In the same way as magnetic lines of force appear when a bar magnet is sprinkled with iron filings. This, at the ends, and only at the ends, has the effect of increasing capacitance to the rest of the Universe. The wire behaves as if its longer than it actually is. Hence pruning is necessary. When several half-waves are connected in series it is not necessary to prune each of the half-waves. The electric lines of force are all in radial directions at their junctions. The "end-effect" occurs with any length of antenna. There are only two ends. Obviously, as the diameter/length ratio increases so does the effect. The flat ends of the antenna support a greater number of lines of force in line with the antenna. The effect slightly reduces efficiency. When the antenna is pruned to bring it into resonance it is accompanied by a reduction in radiation resistance. This is most noticeable at UHF and above where very fat cylindrical antennas are used. Sometimes elipsoids are used for high power transmitting antennas. I trust my description/explanation has not further confused the issue. ---- Reg. |
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