Topband: Voltage at top of vertical and scaling?

[Don Kirk]

Hi Jim,

Let me take a crack at this based on some online reading I just did, and
the examples I found were based on treating the antenna as a transmission
line (W8JI as well as others treat it this way).  I would estimate the
voltage at the tip of 1/4 wave vertical running 1500 watts to be 3437 Vrms
= 4861 Vpeak.  This is just an estimate using some general assumptions.

Full Details: The above assumes the feedpoint impedance is 35 ohms, and
that the characteristic impedance of a single wire vertical acting as a
transmission line is considered to be between 400 to 650 ohms (I used the
average value of 525 ohms).  Using 525 ohms as the characteristic impedance
of the single wire vertical transmission line, this would translate into an
impedance at the tip of the 1/4 wave wire to be 7875 ohms which appears to
fall into the range of expected tip of antenna impedance values stated
online (easy to transform the feedpoint impedance to the tip of the antenna
since we are dealing with a 1/4 wavelength but you have to take a leap of
faith that the characteristic impedance of the single wire antenna acting
as a transmission line is 525 ohms).  If you then treat the single wire
vertical as a lossless transmission line, the voltage at the tip of the
antenna calculates to be 3437 Vrms (4861 Volts Peak) when running 1500
watts.

The online examples I saw were lacking a lot of detail, and some included
some obvious errors.  Therefore I had to take the best of each example to
come up with my above approximation (assuming the online examples were on
the right track).

I hope others will step in and correct me if they feel I have not
interpreted things correctly.  I'm probably sticking my neck way out on
this one :)

73,
Don (wd8dsb)

On Tue, Apr 16, 2019 at 8:38 AM Jim Miller <jim at jtmiller.com> wrote:

> Is there any data on what the voltage might be at the top of a full size
> 80m vertical at 1500w input?
>
> Is there some zener like ionization at the top that limits the voltage?
>
> If so is there a way to estimate the voltage at lower points along the
> vertical? I assume the scaling wouldn't be linear since starting at the
> bottom on 36 ohms (sorta) and going to "infinity" at the top would preclude
> that.
>
> Thanks
>
> jim ab3cv
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