On Fri, 9 Sep 2011 21:10:08 +0200, "Antonio Vernucci"
wrote:

For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big
HF yagi on its top). The home-made tuner, placed at the tower base, has a
motorized variable vacuum capacitor in series and a (properly selected) fixed
capacitor in parallel. I can so remotely tune the variable capacitor across the
3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the
band.

Hi Antonio,

Describe the "shunt." (I presume it is a long wire from the tuner,
standing off 10cm or so, up maybe 5 - 10 meters where it attaches to
the tower? Can I also presume that the tower goes to ground at the
base?)

The antenna works fine and I receive good reports.

And so it would seem this would be the end of the post. But no.....

The only thing which worries me a bit is the very high voltage that develops at
the antenna end

End? There are at least two, and perhaps you mean the end of the
shunt at the tuner? If by "end" you mean up at the very top, then
high voltage would be expected (but worry about it would not be
expected).

By this point, there are a lot of gaps in the description (at least
for me).

(about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to
about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be
a source of significant losses.

You worry about loss, when you receive good reports?

High voltage reveals loss in heat. Corona would be an example, but
you don't describe corona. You also don't describe heat.

To solve my doubt, I started by precisely measuring the capacitance of the tuner
capacitors, and I could then easily calculate the antenna impedance at 3.500
MHz, which resulted to be (18.2 + j 656) ohm.

I think you have a sign problem. You are describing a short antenna
with inductive reactance (given the height and wavelength, very
improbable).

However, this might be the feedpoint impedance at the lower end of the
shunt - maybe, maybe not.

ABOUT CURRENT
With 2 kW applied, one can easily determine that the RF current flowing through
the antenna is about 10.5A, quite higher than the 6.3A figure one would get
should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect
some more loss in the conductors due to the fairly high current.

Only in the shunt wire. Only if you are using wirewrap wire. Measure
the RESISTANCE. Multiply by CURRENT squared. You get WATTs. How
many? That is your loss. Now, if you can measure the resistance of
your tower, stand very far back because it is about to fall over from
being rusted out.

ABOUT VOLTAGE
The 10.5 A current flowing through the big 656 ohm antenna reactance causes the
antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with
a (well insulated) screwdriver, you would see a nice Tesla-like arc.

With this statement, it is obvious that you are not touching the
screwdriver to the "end" of the antenna (some 15-20 meters above your
head).

Current and voltage are not uniformly equal along the length of the
antenna. Your beam at the top acting as a top hat might yield some
uniformity, but that doesn't come from simple math analysis.

Instead, what you describe is a feedpoint impedance calculation.

Any idea on whether the high RF voltage present on the antenna could cause some
significant loss?

You are working very hard to find a problem that isn't there.

73's
Richard Clark, KB7QHC