To All,
I have used with some success at high power a halfwave vertical
supported by a helium blimp with 5.5lbs of lift. I will point out just a
few items.
1) Halfwave or not the thing still requires a ground system. I used 4
ea. 135ft radials tied to common ground point at center.
2) To match the 2,000 Ohm impedance I used a parallel tuned circuit with
about 6 in Length 3 in O.D. of 14ga 7 TPI Air-Dux or B&W type coil and
less than 300pf at 15KV air variable capacitor. The antenna lead or "hot
end", "cold end to ground" and 50 OHM cable tapped up from cold end up
for best 50 OHM match.
Results...In one contest it worked very well when predominant angle
appeared to be very low. Other times my old top loaded shunt fed tower
outperformed it in all areas. (one year I had both and they were
separated by a half wave or so)
I would suggest not higher than 3/8 wave or better yet 1/4 wave length
operated against good radial ground system. Day in day out you will have
better luck scattering signal off at 30 degrees into the ionosphere.
There will be times where very high angle and very low angle will be
better. I do not however see an advantage for extremely low angle on 160
(below 15 degrees).
I will make one further note of interest. Always use a antenna of this
type at EARTH GROUND or you will discover St. Elmos Fire! Any wind or
precipitation will generate a couple hundred thousand volts of static
build up. This Will contribute to failed pin diodes in your transceiver
which are almost always capacitively coupled to the antenna in modern
rigs.(PIV 100V)
Not to bark about the recent thread. Experience tells me that the
majority of contesters exceed PIV and Current ratings of PIN diodes on a
regular basis. Leaking or shorted diodes do nothing to help in terms of
BCI birdies across band.
73
Bob KG7D
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