Topband: Antenna isolation measurements

[on7eh]

Setup 3 below was retained (Tx antenna and 120m long beverage feed location are still 120m apart ) but 
the 160m inv L Tx antenna was converted into a quarterwave 80m vertical over the same radial wire field.

The isolation on 80m measured 44dB. (on a freezing day)

73,
Michel, ON7EH


From: Michel Spelier 
Sent: Tuesday, January 27, 2015 10:29 PM
To: topband at contesting.com 
Cc: on7eh at skynet.be 
Subject: Antenna isolation measurements

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 trasnformer). 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 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 much better Bev grounding than above. (added 2nd grounding bar and longer, more ground radials) 
Bev length increased to 120m & inter-ant distance again 120m. 
Isolation: 43dB. 
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) 
We hope these measurements provide sufficient detail to be of help for others.  

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 also 50dB) 
73, 
Michel, ON7EH