Voltage at the end of an antenna: there's a thread on the EZNEC forum here:
https://groups.io/g/eznec/topic/78616046#29
where several suggestions were made. Model it with a current source with
I=0, and watch out for where you connect to your antenna. Dan, AC6LA,
discussed making sure you have it on an appropriate segment.
regards,
Dave N3AC
On Tue, Oct 4, 2022 at 11:45 AM Lux, Jim <jim@luxfamily.com> wrote:
> 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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