On 10/4/22 8:30 AM, Lux, Jim wrote:
On 10/4/22 8:07 AM, jim.thom jim.thom@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.
There's a post on stackexchange that attempts to derive the voltage
using a lossy transmission line model from Laport, but I'm not convinced
that you can use the "feed point Q" to calculate it as he does, or use a
lossy transmission line as a model. Here's why: The fractional energy
lost in a lossy transmission line is equal all along the line (in terms
of the fraction per unit length). In an antenna, though, the energy lost
is neither a constant fraction, nor constant as you move along the
antenna. (there's a comment from Lewis Kelsey that comments that the Z
of an antenna isn't constant)
Another answer attempts to look at it as if you were feeding a full
wavelength doublet. That's not a bad approach to calculating the feed
point impedance of a half wavelength vertical radiator, but doesn't
account for the power radiated by the antenna, so it will be an over
estimate of the voltage. That is, the power at the end of a halfwave
dipole is a lot less than the power at the feedpoint.
There is another answer on stack exchange which hints at an approach.
Use a NT card in NEC to connect a 100 meg resistor (actually a 1E-8 mho
admittance) between the end of the antenna and "ground" (or the
feedpoint). Then you can get a direct measurement by looking at the
voltage and current in the resistor, which is given in the NEC output.
https://ham.stackexchange.com/questions/1410/what-is-the-peak-voltage-at-the-tips-of-a-dipole-antenna
It is interesting that there's no obvious published analytical solution
from the late 1800s, when everyone was doing these analytical
solutions. Maybe the search engines don't find them. I would think that
Pocklington, Rayleigh, Hallen, Schelkunoff or one of those guys would
have derived it.
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