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


Well I read all the replies you have gotten, and are fine suggestions for
electronic designers.  But these are two units that have been designed with
kit builders in mind.  You didn't say if you built them or bought them
assembled.  I think if constructed reasonably well they will work well
either directly connected or separated.  The LNA has around 15 dB gain so
that should overcome the NF and any mismatch issues as long as you use
suitable interconnecting cable (keep losses to under 6 dB...this leaves
about 10 dB net gain to overcome the convertor NF of maybe 1 or 2 dB).  {if
the Conv-NF = 2 dB than it will only add 0.2 dB to the NF of the LNA if
there is 10 dB net signal gain ahead of it}

First, I have never heard this "connection" concern/problem? about these
units.  If you are using the standard DEMI enclosures and connectors then
there should not be any signal leakage that might cause signal feedback
(the other reason I could think that one would be concerned about).  Since
by your call you are located in the USA, why not call Steve Kostro at
Downeast Microwave and ask if he sees any reason for concern?  He designed
them and sells them!

I would guess most folks would mount the convertor with a few feet
separation from the LNA for mechanical mounting convenience.  You could
even install the convertor inside the house (but you want to be sure that
the connecting coax loss is <6dB if you do this).  One advantage of doing
this, would be that the convertor is then in a more stable temperature
environment (which may minimize temp related freq drifting).

Ed - AL7EB 

At 11:32 PM 3/27/03 -0800, Eric Rawson wrote:
>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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