[TowerTalk] station to station interference

Wed Sep 28 07:57:12 EDT 2016

Don't rule anything out.  a couple of the really odd harmonic and
sub-harmonic stuff I have found over the years are:

1. leakage between 419 bandpass filters.  The control lines and power lines
are not filtered so if they both are run from the same power supply the rf
from one tx is directly coupled to the other.  The same for the control
lines, if they both are driven from the same level converter rf can couple
from one to the other through it's power.
2. harmonics from bad joints on a tribander.  In this case the tribander was
an old telrex model that happened to be in the main lobe of 40m when it was
aimed at Europe, it would radiate harmonics on 20, 15, and 10.
3. a subharmonic caused by an unbonded hardline.  This took months to find.
A piece of large jacketed hardline feeding a 6m antenna was not bonded at
the top of the tower and it was on the same tower as my 20m stack.  Some
part of the feedline must have been 1/2 wave long on 40m.  when transmitting
on 20m above some minimum power level that depended on which antenna or
combination was selected it would resonate on 40m and make a fairly stable
subharmonic.  It was also sensitive to rain so the threshold power when it
would go into oscillation would vary with the weather.  This would also show
up on 15m as a slightly less stable 1.5*freq harmonic.  Fix was to bond the
top and bottom of the hardline to the tower.
4. another subharmonic caused by power to daiwa power/swr meters.  Another
long hunt for a subharmonic of 20m kept pointing back to the shack.  I spent
hours looking for another unbonded hardline or similar resonator as it had
the same basic characteristics as the previous one... except it wasn't
sensitive to rain.  I was disconnecting stuff on the 10m station because I
had a strong hit on it there on a very small sniffing loop.  When I
unplugged the 12v to the daiwa cn-801 meter the signal dropped off but it
didn't go away completely.  I put things back together and found that when I
plugged in the daiwa meter the signal came back up.  I unplugged all 6 of
the meters and with each one the signal got weaker until it went away.
Those daiwa meters are interesting as they use an unpolarized 12v input.
They route the power to the internal circuitry though a floating full wave
rectifier.  That rectifier combined with the length of the 12v supply cable
made a nice subharmonic generator.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net:7373

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Steve
NR4M
Sent: Wednesday, September 28, 2016 00:18
To: Towertalk at contesting.com
Subject: [TowerTalk] station to station interference

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.

--
73 de Steve, NR4M

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