[aprssig] Antenna separation

Ev Tupis w2ev at yahoo.com
Thu Apr 27 06:12:02 CDT 2017


The issue you are dealing with is exactly why I strongly suggested that USA APRS move to 147.585 MHz during the "great QSY" from 145.79 MHz.  Many 2 meter SSB/CW operators with tall towers and living on hills (the very infrastructure that APRS needed) would suffer from self-QRM and likely abandon APRS if it moved *closer* to their operation.  That is exactly how it played out.
Your issue is also fundamental overload, however the real-estate you are working with is significantly less.  You can't separate your antennas by 20 vertical-feet. ;-)

The safety of the radio is determined by the ability of the RXing radio's first amplification stage to absorb RF.  Without knowing what the radios' ability in that regard, there is no way to quantitatively answer your question.

On the other hand, some empirical strategy can be applied, as the known quantity is the size of the vehicle and the RF outputs involved.  You control both.  Adjust both for best performance.

Start by separating she antennas as far as possible, however also consider testing with the antennas in different vertical planes, too (even if they are closer).  To a vertically polarized antenna, vertical separation provides greater isolation than horizontal distance...however in a mobile environment, having the passenger compartment's "bump" in the way may mitigate that.  

Then, reduce power output to a point where you can still communicate (this is an FCC rule anyway, of course) and see if you can live with the self-qrm that remains.
All of this results in a "most acceptable self-QRM" solution.

Best regards,Ev, W2EV




On Thursday, April 27, 2017, 12:03:49 AM EDT, david vanhorn <kc6ete at gmail.com> wrote:So, I'm about ready to push the button on this. 
I'll be limited by the location spots available, but I have one more question:

I'm intending to have the FTM-350 running APRS, and the IC-7000 running voice on 2m/440
I know I should mount those antennas as far apart as possible, but given the 50W output of each, what is the minimum separation for the safety of the radios? 

It's almost certain to end up with the 7000 V/U on the front mount point, FTM-350 on the middle, and the 7000 HF antenna on the back. 

I know the APRS will still punch holes in my reception on the 7000 VHF, but I'm hoping to minimize that. My main concern is not having the front ends damaged by the RF from the other radio.


-- 
K1FZY (WA4TPW) SK  9/29/37-4/13/15
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