[CQ-Contest] Re: Topband: Sept/Oct NCJ article on DX Prowess of Receivers

Tom Rauch w8ji at contesting.com
Tue Aug 31 16:27:05 EDT 2004

> 1.  5 kHz spacing (BDR and IMD) is far too wide for most
> contest interference in my opinion.  I prefer ARRL's 1 kHz
> measurements as being much more realistic, especially
> for contesters.  When do you last recall a contest with
> signals spaced only 5 kHz?  In my opinion, signals
> spaced at ~500 Hz is much more common, especially
> in the major contests.

Bill and all,

(You might repost this to cq-contest if it bounces Bill.)

Every receiver problem I have ever experienced (other then
when I am transmitting while listening on 160 meters) has
only been with signals closer than 2-3kHz on CW or 10kHz on
SSB. I've never had a problem in any receiver, no matter how
poor, with clean signals 5 or 10KHz away. Everything that is
a problem *with the receiver* is always something within the
roofing filter passband of my receivers.

A 5kHz IM3 test checks receivers with interference 5kHz off
frequency and 10kHz off frequency. That's almost silly
worrying about to me, because my own transmitter barely
causes a problem on 80 meters at 5kHz spacing. On 40 meters,
I can full duplex with 3-4kHz spacing using my IC-751A and
FT1000D. It is unlikely other signals will be as strong at
my house as my own transmitter.

Consider my experience with a new FT1000D I bought in 1999.
I operated about an hour or less in the "low signal density-
low signal level" Stew Perry CW contest and had to give up.
Adjusting front end gain to just comfortably hear background
noise, I received bleeping and blooping from mixing of
signals within the FT1000 roofing filter. Correcting the
FT1000 design errors to move IM3 DR into the 80 dB range
cured the problem. I never have a problem with a clean
transmitter now, no matter who is close and how crowded the
band is.

I know popular opinion in Europe is moving towards
wide-spaced testing being important. I just can't imagine
how that can be real. If receivers are bothered by a mix of
wide spaced BC stations, it would stand to reason mixing
products of those stations would fill the band. Instead I
see it claimed to produce unidentifiable noise.

I know a few transceivers operating on 10, 12, or 15 meters
near our Interstates are bothered by CB radios (probably
hundreds of watts minimum to several kilowatts maximum) when
some vehicles pass by, but there are questions as to the
spectral purity of high power CB radio and amplifiers.

I'd like to know what receivers  are bothered by skywave
signals 10kHz or further away, and what the exact operating
situation is. I'd like to try to duplicate that situation
here. I already know about IC706's and IC756's, I am talking
about better quality receivers in the 80dB IM3 and blocking

73 Tom

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