[CQ-Contest] Sept/Oct NCJ article on DX Prowess of Receivers
aa4lr at arrl.net
Wed Sep 1 21:31:51 EDT 2004
On Sep 1, 2004, at 2:31 PM, Clive Whelan wrote:
> 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.
> 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
> per se?
I'm not sure that it is IMD, but I have experienced some
adjacent-channel interference that covers 20 or more kHz. Using an
Kenwood TD-430S, which is one of the first generation of
general-coverage up-converting receiver designs, when 10m gets rather
crowded, there's a pattern of phantom signals across the dial. Very
strong signals produce a noticeable change in background noise over 50
kHz or more.
I'm not sure what you mean by the "headless chicken" syndrome.
> 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
> the roofing filter", and since the TS870 roofing filter is at best
> wide, then IMD at 5kHz spacing or more is entirely feasible.
Tom has used much better radios than I. The old TS-430S also has
products outside the roofing filter, although these can be reduced
dramatically by inserting the 20 dB attenuator.
> 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
> 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
> to cause IMD before the 8.83Mhz IF is reached. I would be interested in
> opinions and/or practical experience in this matter.
The primary reason the K2 and K2/100 rigs do so well on these receiver
tests is because they use a very simple single-conversion design.
Although the first (and only) IF filter isn't a particularly
sharp-skirted design, it is sufficient to knock out most all adjacent
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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