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

[Bill Coleman <aa4lr@arrl.net>]

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. 
> 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?

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 
> 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.

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 
> 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.

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 
channel interference.

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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