[Mobile-Portable] Unwanted Antenna Coupling between close antennas

[Ray, W4BYG]

Sonny,
Thank you for the thoughtful comments.  I appreciate your response. I'll 
give the subject more consideration.

Regarding the placement of my antennas on the new truck:  I have been 
HF-VHF mobiling for almost 55 years with 5 or 6 various installations.   
I have also been involved in some very interesting antenna pattern tests 
on mobiles.  My conclusions: most any antenna works best when mounted in 
the center of the vehicle roof, thus that's where my Comet VHF/UHF is 
located.

The CB antenna performance is much less of a concern because it's use is 
almost always very close to any transmissions I would receive when on 
the highway.  (I use it only for congested traffic information, usually 
gained from truckers).  So the CB antenna performance is not as much 
concern and is mounted between the Comet and the rear of the cab (only 
about a foot away).  (BTW:  The purist might argue, but I can see no 
affect of the CB antenna near field location on the Comet radiation 
pattern.)

Regarding HF antenna placement:  I have found that my Hf mobiles 
antennas work best also near the center of the vehicle and work quite 
poorly when mounted low and off to the side and rear of a vehicle 
(especially if mounted at the bumper level, like so many people do 
today), because of excessive ground losses.

My last install was on a 2006 4Runner SUV.  There a Hustler 20 or 40 
meter 8' antenna was mounted on the car top carrier.  It was about 14' 
high at the top.  Although that was a concern driving around with local 
trees, it worked super when on the highway.  I could often break into DX 
pile ups in less that 2 or 3 trys.

Now the current concern was how can I install a similar HF antenna on a 
really nice new pickup truck (my "Tennessee Cadillac"), that also has a 
$1000 toneau cover over the bed?  I'm now thinking I will try a nice 3rd 
party car top carrier attached to the toneau cover and mount the antenna 
on 2 crossover rails a couple of feet behind the cab, giving due thought 
as to needed bonding/grounding, etc. (Haven't tried it yet, just 
thinking it thru).

Anyway that's "my story and I'm sticking to it", until I learn something 
better.
Regards,
Ray, W4BYG
Cleveland, TN

But: I'm no longer young enough to know everything!




On 7/28/2016 8:02 PM, N1KHB--- via Mobile-Portable wrote:
> Hi Ray,
>     I think you've been rather thorough asking the  right questions of your
> own approach as well as doing what I consider to be the  right basic tests
> at least in the practical shoot-from-the-hip sense which is my  usual
> standard and style too. :-) One point however is that it  appears you've tested at
> more or less arbitrary frequencies. I might  recommend that you do the same
> tests with harmonically related frequencies. One  example I can cite is a
> situation that exists even to this day. Our local 2  M repeater has an output
> of 145.29 MHz. There's a low power transmitter  operating in the AM
> broadcast band used by our DOT to issue driver alert  messages that the state
> considers useful to drivers by listening to their car AM  receiver. I don't recall
> the frequency as I have it set on a push button memory  position. That AM
> transmitter (I don't know that it's even licensed) easily gets  into my 2
> meter radio when set at 145.29 and with a quarter mile or so, and it's
> intelligible no less! Who'd have thought such a problem would exist considering
> the two different operating modes and with such a high harmonic content? The
> main point is that a test on an arbitrary frequency might well yield
> different  results than on harmonically related frequencies. Maybe the  undesired
> signal gets into the front end, maybe it's getting into the IF. I  don't know
> and don't especially care since the result is still the same. Give it  a
> try and let us or at least me know of the outcome.
>     I'm also asking in a rhetorical manner what the  reason is that the
> antennas need to be so close to each other. And finally, what  questions are you
> trying to answer of the HF antenna installation method?
>    HTH.
>   
> Best,
> Sonny N1KHB
>   
>   
>   
>   
> In a message dated 7/28/2016 6:50:50 P.M. Eastern Daylight Time,
> w4byg at att.net writes:
>
> 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!