[TowerTalk] P-P voltage on an antenna
Jim Lux
jimlux at earthlink.net
Tue Feb 3 10:28:23 EST 2004
At 10:57 AM 2/3/2004 -0500, Eric Scace K3NA wrote:
> W1MK and I wonder if anyone can point us to an authoritative reference
> for an answer to these two questions:
>
>1. An antenna model predicts a current of I amps at a particular segment
>on an antenna. What is the maximum (peak) RF voltage V
>that will be found at that location on the antenna?
>
> The practical application is to understand what voltage must be
> tolerated by any support (e.g., insulator and guy wire) attached
>to the antenna at that point.
>
>2. Similarly, what is the maximum (peak) RF voltage V to be found at the
>tip of an antenna element?
>
> I'm sure there is a way to calculate this ... but I can't lay my hands
> on it quickly.
>
>-- Eric K3NA
This is actually tougher to calculate than you think. You might get a
start by looking up or calculating what the feed point impedance of an "end
fed" half wave is, for the dimensions of your antenna (typically a few
thousand ohms). Then, if you know the power, you can calculate the voltage
from E = sqrt(P*magnitude(Z)). This would probably give you an "upper bound".
In a real antenna, near other real objects, you need to consider that there
are image charges in those neighboring objects, the fields of which tend to
cancel the fields from the antenna, reducing the local E field.
One way to approach it is to generate the near field data (E field) then
sum the values moving away from the antenna towards "free space". The
voltage is the sum of all the "distance * volts/meter" numbers. The
problem is that the field drops off quite quickly as you move away from the
element, so the numerical integration approach may give you an under
estimate. The other sticky part of this can of worms is that the fields
you're summing are also not all in phase (so it's not like calculating DC
fields).
For a rough approximation, where the distance involved is very small
compared to a wavelength, you can probably safely assume that the phase is
constant, and just sum.
You can also calculate the voltage from the charge that is moving
around. If you know the charge distribution, you can calculate the voltage
(again, by numerical integration) (which is essentially how the near fields
are calculated in NEC, anyway).
I'm sure there's a detailed analytical treatment of a trivial case (i.e. a
short dipole) in the usual electromagnetics texts (e.g. Balanis, Kraus)
although I don't recall it in Kraus, at least in an explicit form.
The upshot, though, is that there isn't a nice equation for this.
Jim, W6RMK
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