[TowerTalk] Peak Voltage at the tips of ants ?

Tue Oct 4 13:11:29 EDT 2022

Re: voltage at the end of an antenna

I just re-ran a dipole at 7 MHz with 1 amp source, to see how it responded.
Here are my recent notes:

1. Run everything with a Mininec ground. I used uninsulated 12 AWG copper
for elements.
2. L-networks don't radiate, and I couldn't connect them to the end of the
antenna. So I used lots of segments on the antenna, and put the network on
an adjacent segment.
3. I used R=.0001 in the Series line, and Series configuration. On the
Parallel line, I used Parallel config.
4. Line 2 was a 1' grounded vertical with 3 segments.
5. Source 2 was on Wire 2 at the 50% location, with I=0 and a Current
Source.
6. I used EZNEC to look at results. I had about 70 volts at Source 1 (the
center feed), and 346 volts on Source 2.
7. You could stick sources like this all along the antenna if you watched
your segmentation. I haven't tried it, and can't immediately see how to do
that without duplicating on small verticals and no-load L-networks.

regards,

Dave N3AC

If you're using AutoEZ - Dan, AC6LA, can make sing, and can probably
extract that source voltage for Source 2. If he told me, I forgot how so
looked at EZNEC itself.

On Tue, Oct 4, 2022 at 12:01 PM Haring family <dcharing at gmail.com> wrote:

> 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 at luxfamily.com> wrote:
>
>> On 10/4/22 8:30 AM, Lux, Jim wrote:
>> > 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.
>>
>>
>> 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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