I did not describe the RF ground because I consider it not relevant to my
question.

Anyway, the ground system is formed by 64 quarter-wave radials.

73

Tony I0JX
Rome, Italy

"dave" ha scritto nel messaggio
. ..
You did not describe your RF ground.



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. The antenna works fine and I receive good reports.

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

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.

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.

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. I am
wondering whether such a high voltage could constitute, by itself, a source
of
extra loss. Please note that the wire coming down from the tower is connected
directly to the vacuum capacitor terminal, with no stand-off insulator. So, I
do
not see a place where power can get dissipated due to the high RFvoltage,
other
than perhaps in the humid air (?).

I could probably avoid this situation by changing the tap height on the
tower,
but I would run the risk of not being any longer able to tune across the
whole
3500 - 3800 kHz band adjusting one of the two capacitors, and not both.

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

Thanks and 73

Tony I0JX
Rome, Italy