Topband: Antenna isolation measurements (3)

[on7eh]

To complete this winters' measurement session,
Elecraft's Gary Surrency quoted +33dBm (=2Watt) to be the max input power 
the K3 handles, prior to a (T)RX-failure.
(TR-switch PIN diodes and/or post mixer IF AMP) A LCD warning is also 
issued: HI RFI for "too high RF input".

I.e. our 100W will deliver on 80m, +6dBm (=50-44) at the ext Rx input. A 
safe enough value on both 80m and topband,
leaving some margin for further setup optimisation if there is an answer to 
the following question:

Is there a minimum target Tx/ext RX-isolation value (in dB) guaranteeing 
minimal noise coupling from the Tx-antenna to the Beverage?
Why? By bringing the Beverage feed somewhat closer to the (fixed) Tx 
antenna, the (unterminated) Beverage length could easily be extended,
improving its directivity.
This isolation is an easily manageable quantity here allowing for better 
Beverage position trade-offs on the farmers'field.

Tnx es 73,

Michel, ON7EH


-----Original Message----- 
From: on7eh
Sent: Wednesday, February 04, 2015 10:37 PM
To: topband at contesting.com
Cc: on7eh at skynet.be
Subject: Re: Topband: Antenna isolation measurements

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
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