On 9/9/2011 12:10 PM, 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.

Generally, your losses are going to be IR losses. High voltage implies
low current, and low loss.

However, when you start talking kilovolts, you might have significant
loss from corona discharge.


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.


That's the feed point Z of the basic radiator? or of the whole
assembly, including the shunt feed wire/tube?


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.

Yes, that seems reasonable, but what conductor is that 10 amps flowing
in? What is its AC resistance at 3 MHz?


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 (?).

Corona is definitely a source of loss. Humidity doesn't change corona
much, but does change leakage across insulators (i.e. the crackling one
hears under HV power lines on damp mornings).

However, is it a *significant* source of loss? How much stuff is at the
HV? What's the actual surface field? (Running a screwdriver near it
perturbs the field)



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?

HV loss is hard to estimate. You could make your shunt wire a shunt
tube or rod and basically eliminate corona (A rule of thumb is that
10-15 kV/cm radius will have virtually no corona.. so for your 6kV, a
1cm diameter tube is in the right ballpark) Skin depth at 3.5 MHz in
Aluminum is .043mm, and the usual rule of thumb is to make the tubing
wall thickness 3-5 skin depths. 0.12-0.20 mm seems about right. Copper
could be thinner wall (skin depth is less)

I'd worry more about loss across dirty insulators.
And even more about IR losses.




Thanks and 73

Tony I0JX
Rome, Italy