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Re: Downconverters and coax imp.



Jeff Yanko wrote:

> Most D/C would normally use 75 ohm coax as their transmission needs. If I put a preamp at the antenna and the D/C a few feet from it, then back to the rig, I would need to run some low loss coax like LDR400(?), etc. and that and most of the coax in this catagory is 50 ohms.  Does it really have much of an affect on the overall system?  From what I've been reading, the difference between 50 and 75 ohm coax is negligiable except for over extreme long runs, over 150 feet.

With a good preamp at the antenna, you have set the noise figure at a 
nice low value, and provided plenty of gain into the front end of the 
downconverter.  The downconverter adds even more gain.  You have gain to 
spare in the coax run from the downconverter to the receiver.  In fact, 
many folks find that even after a long run of lossy coax, they still 
have too much gain at the IF receiver, and they end up inserting some 
ADDITIONAL attenuation in the line, so that the "zero signal" level is 
close to SØ on their S-meter instead of S7 to S9.

So, even though a mismatch exists between the 75 ohm downconverter 
output and the 50 ohm radio input, there will be a mismatch whether you 
use 50 ohm or 75 ohm cable, and in any case, it will not impair 
performance.  Your cable need not be low-loss, but well-shielded cable 
would be preferred.  I was considering RG6-QS (quad-shielded), but 
decided that LMR-200 was easier to find, easier to work with, very high 
quality shielding, etc., etc., and still didn't insert a whole lot of 
loss at 123 MHz!  You also have to consider the RF connectors that will 
go on either end, probably an F connector on the downconverten end and 
maybe a PL-259 or N connector on the radio's end -- make sure you can 
get connectors of each type that will work with whatever cable you choose.

73 de W0JT

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