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

Clive Whelan clive at gw3njw.org
Wed Sep 1 14:31:43 EDT 2004

A couple of comments from a self proclaimed non-techie, although equally a
self professed critical user of receivers in contests over  more years than
I care to articulate here!

W4ZV wrote
3.  Given "proper design" (similar to your "not good"
comment on page 11 about IMD DR2 and IP2), I would
rank IMD DR3 as the most important parameter for
contesters for the following reasons:

a.  Unless you have an extremely close neighbor, or
unless your BDR is terrible, BDR is not likely to be as
significant a problem as IMD DR3.  The reason is very
simply that BDR performance is normally MUCH higher
than IMD DR3.  Thus the latter becomes more critical
since you will reach the IMD DR3 limit sooner.  W8JI
cites an example on 160 below where he sees his
maximum dynamic range needed being around 95 dB,
which most of the receivers in your list exceed at 5 kHz.
As long as BDR is above this number, or unless you have
a very close neighbor or are in a multi-multi operation,
BDR of 95 dB at should be sufficient, although I would
add that this performance should be maintained at 1 kHz
or less instead of 5 kHz for the reasons cited in #1 above.>>

I agree 100% with the above. I confess that I was surprised to see the
weighting given to BDR in the NCJ article recently. I have not used one of
the real clunkers such as the TS2000 or IC756Pro Mk1, but I have NEVER
experienced gain compression in any modern receiver I have used. Probably
the last time I heard such a thing was in a surplus receiver from WW2.
However I have often suffered from the "headless chicken" syndrome, which is
a function of  IMD. I should mention here that I am a 99% CW operator, and I
do not actually know what  IMD sounds like on SSB, possibly not recognisable
per se?

My current rig(s) being TS870s of this century manufacture do suffer from
the "headless chicken" syndrome in adverse circumstances, with 15m in sun
spot maxima conditions being notable. However this is such a good rig
otherwise, and the problem is always addressed by attenuation-with adequate
sensitivity maintained-, that I will tolerate it until a better and
affordable rig comes along, which hasn't happened to date.

and W8JI wrote

<<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.>>

Referring to my on experience, I am not sure if this applies. Frankly I have
usually been so busy keeping the rate up in such circumstances that I
haven't sussed out the critical spacing, but my feeling is that it was
definitely more than 3kHz or so. Of course Tom refers to "something within
the roofing filter", and since the TS870 roofing filter is at best 15kHz
wide, then  IMD at 5kHz spacing or more is entirely feasible.

On this subject I have seen suggestions to change the 8.83Mhz filter for an
INRAD device in the TS870 to address this problem. but I can't see how this
can work. Surely the problem lies in the first mixer and anything which
comes through the 15kHz roofing filter ( of poor quality no doubt) is going
to cause IMD before the 8.83Mhz IF is reached. I would be interested in
opinions and/or practical experience in this matter.



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