OK Kenny. I understand now. Not all of the TX will be under control of
the transceiver at the operating position that is using the in-band
antenna, so you need RF protection while also receiving in-band. I think
your phased vertical idea could work. But, a pair of flag antennas would
also work well. And they're ground independent too.
You would want endfire arrays here. As was mentioned, about 100-foot
separation is a good starting point, and that's inline of course. In the
case of either the 2 vertical array or the 2 flag array you could use
coax sections cut to the appropriate length considering the velocity
factor of the coax to get the pattern that you want. One other thing
about the flag option is that the antennas can be either unterminated,
and the pair will give you a really deep null at low elevations, or you
can add VACTROL termination and adjust it on the fly. In your case with
the TX going on I think I'd install the VACTROLs, tweak the terminating
resistance to the best values, and then measure it and replace the
VACTROLs with a several watt non-inductive combination of fixed
resistors that give you that same value. No TX worries then. Terminated,
the flags will give you a deeper back null than the verticals will.
The flags don't need to be huge. You didn't specify the bands of
interest but a 10-foot by 25-foot flag with the bottom wire 3-feet off
of the ground works well from mediumwave to over 40 meters, maybe
higher. I have a pair of these in an inline configuration and
unterminted I can sweep a null from horizon to horizon. Terminated I can
deepen the null and swing the max null angle some against the horizon.
I have my flags setup so they're moveable. I used 4 5-gallon plastic
buckets and put a 3 foot PVC pipe with a cap centered, bolted the cap to
the center of the bucket bottom, and then filled the buckets with
concrete. I can move them and they stay put even in high winds. I have a
10-foot piece of PVC in a PVC coupling on top of the bucket cemented
pipe section and on each end that supports the antenna loop. The flags
are fed in the lower corner via twisted-pair transmission line and that
is back to the shack where any transformation or preamping is done. And
then on to the SDR. The termination is at the opposite corner. I modeled
and tested corner feed vs center side feed and termination and I did not
notice any difference in performance.
You could do something similar with a KAZ or Superloop and that would be
longer but need only one support at the center.
Just a few more thoughts for you to consider...
Rick Kunath, K9AO
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|