[TowerTalk] Second radio tri-band antenna options?

[Dick Green]

> Seems like the obvious options are a tri-band (10,15,20 meter) vertical
fan
> dipole (not too much fan) or a tri-band 1/4 wave vertical with 3 or 4
> elevated
> resonant radials.

I use a multiband vertical with my second radio and it works reasonably well
for gathering multipliers. However, I lust for a second tribander, either
rotating or fixed S or NE.

Do yourself a favor and build your antenna switching system to be able to
use any antenna on either radio. You'll be glad you did. In addition to that
capability, I have a special switch that swaps the tribander and vertical
between the two radios.

> Would anyone like to hazard a guess about the behaviour of my second
radio's
> receiver, when I am running full power into the stack of beams?  Is 50' of
> separation and vertical polarization going to be enough, or do I need some
> kind of filter to protect the front end?

It would certainly be a guess. Separation is probably the biggest factor,
but there are a lot of things that affect how much interference you will
see, including types of antennas, gain, F/B, F/S, typical directions of
operation, the type of radios in service, and the specific band combinations
you expect to use. Basically, you have to build it to find out how bad or
good it's going to be. 50' is not very much separation. Most of my antennas
are 70'-100' apart, and I still get some interaction, although considerably
less than I feared. My minimum separation is about 35', between the vertical
and the 80M vee, and those antennas just can't be used together.
Fortunately, I never use that combination (if one radio is on the vee, the
other would use the tribander, not the vertical.) I get virtually no
interference at low power. The problems are strictly at high power. There
are only a handful of unacceptable band combinations. I built a pair of
automatically switched bandpass stubs that fixed that. In general, I can get
within 1 or 2 KHz of the harmonic -- not bad. Others have used transmit
and/or receive bandpass filters with good success.

My advice is to build the best antenna setup you can afford/fit, try to
separate the antennas as much as possible and worry about the interference
later (and think hard about making that second multiband antenna
directional.)

73, Dick, WC1M



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