El 09-09-11 21:10, Antonio Vernucci escribió:
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


Hello Tony,

It is not the voltage that will introduce loss (as the field lines
will go into air and frequency is low), but it is (corona) discharge.
Whether this occurs, depends on presence of sharp edges and
especially sharp (double curved) surfaces with small radius, think of
bolts, threaded rods, etc that point away from the tower into the air.

A thin end of an antenne element can also cause breakdown as it points
far into the air (away from other solid structures) and the voltage
maximum will be at the elements of your HF antenna.

In case of your 8 kVrms (that is 12 kVp), an antenna element with
10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at
the end. If the end of the element is just straight (just cut
aluminium), you will exceed 3kVp/mm and corona will occur.

If (corona) discharge occurs, you will notice a sudden change in VSWR
with increasing power.


--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM