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From: David Jefferies
Date: 11 Jan 2002
Time: 17:13:49
Remote Name: 212.1.144.114
Taworri,
correction, the equivalent coax to the wire antenna would have inside diameter 0.1764 lambda, not radius. Thus the fields fall to zero at about 0.088 lambda from the conductor surface, and so your disk top loader capacitance figure breaks down at about this radius. EG, for a 20 metre antenna of length 2 metres and top loading disk, you could stand a disk radius no more than about 1.7 metres although even this might be pushing it.
The effect of the top loader is to draw current from the top of the vertical section; thus the radiating part, the vertical wire, has higher average current. The most this can do to the radiation resistance is to increase it by a factor 4. However, looking at the Smith chart, if you start from a large shunt capacitance load, you land up with lower series capacitative reactance at the generator end. Thus more of the drive voltage appears across the radiation resistance.
regards
David.