[VHFcontesting] Multi-op interferance on IF freqs

[David Olean]

Hello Paul,
I always try to use a minimum of power at the IF frequency, whether it be 
28, or 144 MHz. 1 milliwatts causes less trouble than 10 watts.  After that, 
you need to shift the IF on the MWs away from 144 MHz. It is also a great 
idea to get the 144 antenna as far away from the microwaves as is possible. 
When I had the 5/10 GHz rig on the same tower as the 144 antenna it was a 
bummer. Even with hardline IF lines, the 144 QRM was too much. Besides 
physically moving 144 away, the change to 145 or another IF helps a lot. We 
are using 145 and 147 MHz to keep IFs separated between stations. Face it, a 
KW on 144 can get into almost any IF cable on the same frequency. Most coax 
has about 95-100 dB shielding. 1500 watts is + 61.8 dBm, so any leakage 
could be very loud anywhere near a 144.100 MHz microwave IF!!
    Finally, you must invest in suitable filters to keep the QRO bottom four 
from hitting the higher bands. 3 X 432 bothers 1296. 4 X 222 bothers 903 
etc. Putting stubs on the preamps will help as well.  Good grounding of IF 
lines will help. We use 3/8" heliax for 144 MHz IF lines running up to tower 
mtd boxes. High power filters can be a problem. There is a good 144 MHz 
filter using teflon board caps that handles 1500 watts OK. I made a third 
harmonic 432 filter to protect 1296. It was double stub teflon coax 
construction. I got over 100 dB notch at 1296.3 MHz. I use a big surplus 
filter on the 222 amp as well. I found it on eBay!
    Lastly, you will still need input filtering on your receivers on all the 
bands. They protect frontends from huge amounts of power that sneaks down 
your feedline when running a KW a few feet away from your antenna. When you 
get it all working, operating a multi op station gets to be fun.

Dave K1WHS
----- Original Message ----- 
From: "paul rollinson" <ke1li at sbcglobal.net>
To: <vhfcontesting at contesting.com>
Sent: Monday, January 26, 2009 1:29 PM
Subject: [VHFcontesting] Multi-op interferance on IF freqs


> Good morning all,
>
> During the last contest we expanded the bands we worked up to 10G from the 
> bottom 4. This created a huge problem.
>
> We had to shut down the 2, 220, and 432 stations to use the upper bands 
> due to interference from the lower, high power stations on the IF rigs for 
> the transverters. 6M had little effect but we only had 100W running. 
> Murphy reduced our power level.
>
> I speculate that a common patch panel and close proximity of the antennas 
> may be the culprit but I'm looking for other opinions and potential 
> solutions. Especially from the big multi-ops that run legal limit on 2 and 
> 432 which we use for IF's.
>
> June is coming and I'd like to start clearing this problem asap. We always 
> run out of time.
>
> 73 and thanks for the input,
>
> Paul, KE1LI
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