[TowerTalk] station to station interference

[Grant Saviers]

The "not reproducible" on demand and wide bandwidth makes me suspect 
some sort of intermod.  The cell tower guys really work hard on reducing 
passive intermod (PIM), using all hardline, premium DIN connectors, 
careful bonding, and premium mechanical hardware.  A cell tower is the 
ultimate multi-multi.  Another potential source is reverse RF into the 
amp circuits, I think intermod is much more likely on solid state amps 
with a lot of non-linear junctions around.  Then there are the 
apocryphal rusty gutter stories.  I have an intermod that translates an 
aircraft AM band frequency to on top of a local 2m repeater output even 
with a super good DCI bandpass filter in the way.  No idea so far about 
the source, but it clearly isn't inside the VHF radio.  Maybe my active 
GPS antenna?  Another rig?  Your experience with RTTY and its extended 
on times at full power making the problem very noticeable is another 
intermod pointer I think.

Some ideas,

You don't mention if the stubs are designed to keep other bands out as 
well as output harmonics suppressed, that might be something to try, or 
the 4O3A high power bandpass filters.
Trying semi vs glowing amps.
Bonding around everything that is a suspect connection, particularly 
rotators.
Getting rid of or disconnecting all wires to anything that might be 
getting a lot of rf into it and creating intermod.  Choke the heck out 
of every other wire connected to something with active circuitry.
"tune up" all mains grounding and bonding all the way to the pole.
trip off AC breakers when the problem is there, try to isolate it

probably many more ideas from others.

Thanks for the contest Q's. Good luck with finding the gremlins.

Grant KZ1W

On 9/27/2016 17:17 PM, Steve NR4M wrote:
> Hello all,
>
> I'm good at some things and not so much, at others and I have problems 
> thinking 'outside of the box', so looking for ideas.
>
> Have a multi mult here in central VA, which seems to work well.
>
> With the RTTY contest a continual problem came to the forefront again.
>
> I am getting rather annoying interference between stations.  What was 
> specifically pointed out to me this weekend was 20 was clobbering 15, 
> but it's usually 40 clobbering 20, 15 and 10.
> I'm guessing it's about 20 to 30 db of wide 'hash' coming from 20, 
> covering all of 15.
>
> My antennas are all resonant antennas and typically 400 to 500 ft from 
> the shack on multiple towers.  40 is probably 500 feet from 20, which 
> is about 150 feet from 15.  All mono band antennas and the shack is 
> generally on the back side of them.
> I have low power band pass filters after the rig and before the amp 
> and coaxial stub filters after the amp.  And, based upon K9YC's 
> narrative on placing stubs, they're pretty close to optimum placement.
> But, don't think it's harmonically related, as I've transmitted on 40 
> while looking at 20 and 15 meter spectrum on my Flex Maestro and what 
> I see looks very acceptable.  No 'whole band' crap and my second is 
> only about 500 hz wide as my 400 hz cw filter totally covers it on the 
> spectrum display and it's not real strong.  This was with 1000 watts 
> out on 40 meters.  The 3rd on 15 is about the same.  20m is looking 
> like a -120dbflex and 15 about the same. Local noise floor about -135 
> to -140dbflex (dbflex = I don't know their reference...)
>
> It doesn't look like I can reproduce the problem when it suits me.  It 
> only seems to rear it's 'ugly head' during a contest, when multiple 
> transmitters are going.  But, even at that, the other day the only one 
> that was actually transmitting was 20 and it was eating up 15.
>
> Any guidance on what I should be looking for would be of great help.
>