[CQ-Contest] Re: 2-Radio Interference

AD4TR at aol.com AD4TR at aol.com
Sat Dec 23 07:49:22 EST 2000

 I built the coax stubs using the info from K1TTT website. I cut it to his 
specs and they work great. 
  Martin ad4tr

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>From Michael Tope" <W4EF at dellroy.com  Sat Dec 23 06:16:16 2000
From: Michael Tope" <W4EF at dellroy.com (Michael Tope)
Date: Fri, 22 Dec 2000 22:16:16 -0800
Subject: [CQ-Contest] Re: 2-Radio Interference
References: < at pop-server.kc.rr.com>
Message-ID: <00fc01c06ca7$dc07a280$6401a8c0 at neptune>

Hi Marty,

Discerning fundamental overload in the receiver from harmonic/spurious
products generated in the transmitter is quite easy. Put a step
attenuator in front of your receiver and change the input attenuation
from 0 to say 6 dB. If the interference heard in the receiver drops 6
dB then its generated in the transmitter (or at least at a point ahead
of the receiver). If its generated in the receiver, then it will drop
by a larger amount than 6 dB. Second order products generally drop 2
dB for every 1 dB decrease in applied signal, and third order products
will accordingly drop around 3 dB for every 1 dB drop in signal
strength. Note that using the attenuator built into your radio may not
work for this test unless its ahead of all active circuits (including
any PIN diodes used to switch receiver input filtering).

If you put bandpass filters or stubs on both rigs, you should be see a
benefit irregardless of where the QRM is being generated. If its
generated in the transmitter, then the filter on the TX side will
attenuate it. If its overload of the receiver thats causing the
problem, then the filter on the receive side will help to attenuate
it. Single stubs made from good coax will provide about 25 dB of peak
attenuation. Higher values can be achieved with multiple stubs
cascaded between 1/4 wave lines, but these arrangements can get a
little complex as you in many cases need a separate set of stubs for
each band.  If you are running low power (<200 Watts), I would go with
the Dunestar type bandpass filters. For QRO, you can run a combination
of stubs on the transmit sideand bandpass filters on the receive
side. This is what we do at W6UE. A combination of ICE bandpass
filters and LMR400 coax stubs allows us to run two stations each at
1500 W with monoband yagis only 10 feet apart (10/15/20 meters). When
all the connectors are tight on the stubs and we remember to turn on
the ICE filters, we have no discernable interstation interference.

Good Luck,

Mike, W4EF..............

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