[Mobile-Portable] Unwanted Antenna Coupling between close antennas

Ray, W4BYG w4byg at att.net
Thu Jul 28 18:29:33 EDT 2016


Gentlemen (and Gentle Ladies),
I recently read W8JI's .pdf on "Unwanted Antenna Coupling" and found it 
very interesting.  My immediate area of concern is on my new mobile 
installation.

I have 2 antennas mag mounted on the top of my 2016 RAM 1500 crew cab.  
One is a Comet HP-32 dual band U-V antenna (running about 50 watts) and 
the other is a typical base loaded 3' CB antenna (typically running 
about 4 watts).  They are about 1 foot apart, thus worthy of consideration.

My basic tests for any dangerous antenna coupling from one transmitter 
interfering or damaging an adjacent receiver, has been to turn one 
receiver on, listening to the noise level and keying the other 
transmitter at low power, listening for any desensing or change in noise 
level.  I  then increase the transmit power in steps, as seems 
acceptable.  Hearing little or none, I reverse the process and listen to 
the other receiver while keying the other transmitter.

I then tune in a station on the air and use the same process to listen 
for any cross modulation or distortion, on the audio.  I also listen for 
any "key clicks" when keying.

I have used this procedure on a previous mobile installation where I ran 
the same 2 radios above, plus a 500 watt HF rig usually on 20 or 40 
meters, with the 8' center loaded whip within about 2 feet of the other 
two and found little to no affect on the adjacent receivers.

(I've yet to decide on an acceptable way to mount the HF antenna on the 
new truck).

The most change I observe above has been a very slight noise desense of 
the CB receiver from the 2 meter transceiver and in the previous 
installation a little from the HP HF rig on 20 meters.  But the desense 
seemed to be very slight, maybe 1 or 2 db and thus tolerable.  None of 
the "S" meters showed any RF at any time during the tests.

I use a similar procedure in checking the auto AM-FM radio for RF 
susceptability from my radios.

It would seem the difference in frequency spectrum of the different 
rigs, plus whatever bandpass filtering they normally have built into to 
the front ends are basically sufficient to protect the active stages, 
even with the close vertical antenna spacings in my mobile installations.

Anyone have any comments or otherwise suggestions?
Regards,
Ray, W4BYG

-- 
I'm no longer young enough to know everything!



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