[Antennas] Questions!

[Adam Farson]

Chris,

In fact, I have an antenna which incorporates a range-switching relay
(although it is not currently erected.) This antenna is a Cushcraft AV-80
vertical, which has a tap-switching relay to select the CW or SSB portion of
the band. I used to work the European 80m DX window on the AV-80 from my
former QTH in Florida.

The AV-80 is a base-loaded vertical, with a top capacity hat. The radiator
is 11m tall, and elevated mounting (base 3m above ground) is recommended.
The loading-network box incorporates a relay which switches a tap on the
loading coil to select the CW or SSB sub-band. The relay contacts switch a
relatively high-current, low-voltage point. The relay can be powered either
from a separate control line, or by putting +12V DC on the centre conductor
of the feedline.

The point here is that a relay is fine for switching a
low-voltage/high-current point in an RF network. Relays are not practical
for switching "stretchers" onto the ends of a dipole, as the RF voltage at
the end of a half-wave dipole (or a quarter-wave monopole) is likely to
exceed the breakdown rating of any practical relay. Hence the use of
parallel traps; these present a very high impedance at their resonant
frequency, thus effectively isolating the "stretcher" from the basic
radiator without the complications of an electro-mechanical switching
device.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/