[TowerTalk] Peak Voltage at the tips of ants ?

[Lux, Jim]

On 10/4/22 8:07 AM, jim.thom jim.thom at telus.net wrote:
> Ok, simple question.  With 1.5 kw  (measured at feedpoint)  applied to the
> 50 ohm input  of the (full sized) dipole / yagi... what is the PEAK V at
> the tips ?   Assume 1:1 swr at the feedpoint.
>
> Same question, but ant is not full size, and may have various forms of
> loading schemes used.  (Say 50-80%) of full size.
>
>
> Ok, what about  a single, full size  1/4 wave vertical ?   Or a loaded
> vertical ?
>
>
> I can't find a straight answer anywhere.


That's because it's not an easy question to answer.

The typical model starts with constraining the current at the ends to be 
zero, and then you can solve for the impedance, and that gives you the 
current and voltage at the feedpoint.

One approach is to run a code like NEC, and look at the electric near 
field, but that doesn't give you a "voltage relative to 'ground' " it 
gives you a field.  You can integrate that out to a large distance and 
get an approximation.

You can also approach it from a "transmission line" sort of approach, in 
that the 1/4 wavelength arms are sort of a transmission line, but it 
turns out that antennas aren't transmission lines, because they radiate, 
so the energy at the ends is not the energy at the feedpoint.

A similar problem arises in tesla coils, and there, you can get 
reasonably good estimates by numerical integration, which have been 
confirmed by measurements.  The typical setup is to have a e field 
probe, and then drive the tesla coil, *off resonance*, with a known 
voltage, then measure the e field. That gives you a reasonably accurate 
measurement of volts on the probe relative to volts at the top of the coil.

You could do the same for an antenna.  The key is driving the antenna 
well away from resonance (at DC even) so you get the right probe/antenna 
scale factor, which is highly dependent on all the parasitic 
capacitances.  Then you can drive it at resonance and find out the 
number in use.