[Elecraft] KX-1 BC interference on 20m

[Don Wilhelm]

Stephen and Ed,

I was thinking along those same lines.  On 20 meters, the KX1 LO signal 
is below the RF signal, so a strong BC station in the 4.1 to 4.3 MHz 
range could push itself through and be heard on the upper end of the 20 
meter band.  If you have no stations on that part of the spectrum, you 
can likely reject the assumption of an image response.

The KX1 design faced many challenges in order to stuff all that 
performance in a small space, so the design is a bit of a compromise. 
The front end suffers from responses in the presence of strong RF energy 
just like any other Gilbert cell mixer connected in the receiver front 
end (with no RF amp stage for isolation).  My understanding is that the 
Gilbert cell mixer can generate 'funny stuff' in the presence of strong 
RF energy fields like might be found in Europe - in reading EMRFD, I 
find that Wes Hayward does not even recommend the gilbert cell mixer for 
use in receivers that will be used on the Eastern side of the US for 
this very reason.
In addition, the DDS does have some spurs that can be mysterious - if 
the offending signal tunes more quickly than normal signals, you are 
likely hearing the result of one of the DDS spurs.
The KX1 is a good design, but it just is not perfect in all ways.  You 
may be hearing the result of one of these imperfections - OTOH, you 
*could* have a problem there.  Comparison with another KX1 under the 
same circumstances is one way of telling whether a problem exists or it 
is just one of those design compromise situations.

73,
Don W3FPR

Ed - K9EW wrote:
> Hi Stephen,
> 
> It might help if you could identify the SW stations you're hearing,
> and find out what frequency they're on.  What you could be hearing is
> a very strong signal on the receiver's image frequency, and the only
> thing you could do would be to add more selectivity to the front end
> to attenuate the signal on the image frequency.  Ask Elecraft, they
> would know.
> 73,
> ed - k9ew
> www.k9ew.us\
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