Topband: how high are very high voltages on antennas?

[Tom W8JI]

> It is often mentioned that "very high RF voltages" are present at ends of 
> dipoles, voltage nodes on loops, etc. but I haven't been able to find any 
> guidance in the form of numbers.  How high is "very high"?

An antenna is also a transmission line, and so the voltage depends on losses 
along that line, the line surge impedance, and the current along the line.

This is why thin wires in antennas (antennaE are on insects, antennaS are on 
radios) with low radiation resistance have very high voltages, especially 
when loss is low. Since radiation resistance of a full wave loop is fairly 
high, voltages will not be extreme. A delta is a little worse than an 
expanded area loop with a little less gain, but not much. All are about 
equal to a regular dipole.

http://www.w8ji.com/delta_loop.htm


> Assuming QRO power into a full size apex up delta loop, corner fed, 
> vertically polarized, what will be the voltage at the apex?  I haven't 
> found a way as yet to have EZNEC give me a value.

Connect a second current source to a small wire running away vertically from 
the corner for 1/8th wave, and set the source to zero current. The voltage 
given in the Source Data button tells you the voltage.

> If I wanted to put a relay at a voltage point to switch a stub, how would 
> I float the 12v DC coil from the RF present?

Why would you switch in a stub? Switching in a "hanging" wire at the apex or 
at the center of the bottom horizontal wire (at a voltage maximum) will 
lower the resonant frequency. Or are you switching in a long stub, to make 
it an entirely different mode of operation?

73 Tom