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How far should one separate a LNA and its associated downconverter, and Why?

I own a 2.4 GHz LNA (DEM 13ULNA) mounted at the focus of a 2' x 3' BBQ dish.
The dish is modified for circular polarization (aluminum screen covering,
5-turn helical feed).  The LNA connects via a short run (~2') of coax to my
downconverter (DEM 2400-144).  When I bought these last year, I was told
that the coax run between the LNA and the downconverter should be "more than
a few feet".  When I asked why, I was given a mumble about
"interaction"...it was rather vague.

Hence, my question.  Is it really necessary to separate an LNA from its
associated downconverter with some length of coax? If so, why?  What,
exactly, might happen if you, for example, mounted them within inches of
each other?  And how far apart must they be separated (i.e., how many feet
of coax) to avoid this problem, whatever it is?

Eric Rawson, KN6KC

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