Topband: Antenna isolation measurements

[Michel Spelier]

Prior to the CQWW160, we did some (topband) antenna isolation measurements
between:
-a quarterwave inv L (top at 15m down to 11m) and
-a new unterminated beverage (0.8m high) of different lengths (via a 9:1
transformer) and 2 different locations.
The Bev used PVC-covered 0.6mm thick twisted pair telephone cable.

The power source consisted of the K3 (with its integrated wattmeter) feeding the
inv L tru a remote-fed CG 3000 ATU. The rx was a spectrum analyser connected to
 the Bev.
The very broad "main lobe" of the inv L was pointing to the Bev in all setups.

Setup1:
Bev length=120m & inter-ant distance=35m (distance between inv L feed and Bev
transformer)
Isolation: 33dB.
Conclusion after A/B-switching on the K3 Rx: no improved Rx noticeable (high
noise) and ext Rx input at risk. (too high input level)

Setup2:
New location
Bev length=75m & inter-ant distance= 120m
Isolation: 50dB.
Conclusion after A/B-switching on the K3 Rx: still no improved Rx noticeable but
low enough input level not to use a frontend saver.

Setup3:
Same location as above but with improved Bev grounding compared to Setup2.
(added 2nd grounding bar and longer, more ground radials over the four  5m ones)
Bev length increased to 144m & inter-ant distance again 120m.
Isolation: 43dB.
Conclusion: the S/N of the Bev is much better than the Tx/Rx inv L for the
expected heading, even signals out of the general bearing have better S/N,
others are not audible or much weaker. (expected behaviour from its radiation
pattern)

Setup3 was successfully used last weekend during the CQWW160CW. It was the first
time, a dedicated Rx antenna was put to use.
The isolation measurement on Setup3 was done 2 days after the contest without
apparent impact to the K3 ext Rx. (we expected <=50dB and not 7dB less)

We hope these measurements provide sufficient detail to be of interest for
others.


73,
Michel, ON7EH