On 12 Aug 2003 14:38:34 -0700, (Fred) wrote:

Richard Clark wrote in message . ..
On 11 Aug 2003 13:07:12 -0700, (Fred) wrote:

A few years back I read a paper which described an electronically
tuned HF Helical antenna. It was a shunt fed Helical monopole which
employed electronically switched PIN diodes to add or subtract helix
turns. As far as I remember a 6ft helix antenna was resonant between 4
to 30 MHZ by switching the PIN diodes. Diameter = 19", 20 turns, Q50,
average power = 400W.
Now the question to the antenna experts: was such an antenna ever
developed and manufactured. If yes, who did.
Thanks es 73
Fred
wb6iiq / dh1nwr

Hi Fred,

Seems unlikely. PIN diodes (or any diode for that matter) would have
a very difficult time switching 300W voltage potentials far in excess
of the diode's own intrinsic boundary potential. PIN diode switching
is most often employed in receive circuits where the signal level is
considerably smaller than the switching potential. Then, on the other
hand, they may have huge power sources capable of blocking the RF
potential swing anticipated at the node the PIN diode serves (talk
about cost!).

73's
Richard Clark, KB7QHC

Thank you Richard,
the PIN diodes they choose are UNITRODE (by now TI) UM4010 with a
resistance of 0.17 OHM and a maximun back bias voltage of 1000 V
(2000V breakdown voltage). The current handling capacity is 10A at an
ambient Temp of 70 degree C. You are right this thing had to be
expensive and you need about 5 or 6 of them. I couldn't find anything
on this device on the TI website, probably obsolete.
Regards
Fred, wb6iiq

Hi Fred,

Try
http://www.microsemi.com/literature/rfbrochure.pdf

73's
Richard Clark, KB7QHC