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Re: DEM preamp seperation from Downverter

At 09:45 PM 3/28/2003 -0800, Greg LaHaie wrote:
>For the record;
>    When I was disappointed in the performance of my 13ULNA preamp 
>installed just ahead of my 2400-144 DEMI mast mounted downverter (4' 
>cable seperation), a ham with identical equipment advised of greater 
>seperation. He found it greatly improved his performance to have 40' or 
>better of cable seperation.
>    I moved the downverter into the shack (50' of cable) and now my 
>system hears very well indeed! And the downverter is much more stable.
>    I was glad to throw away some of the gain in this fashion to achieve 
>overall greatly improved performance.
>    Be your own judge.

Right Greg!

That sounds like you were overdriving the first stage of the convertor
(which is likely a Gasfet RF amp).  Adding some loss lowered the input
level to  keep the stage linear.  I find the same situation when using a
mast-head 2m-preamp with my FT-847.  I found my EME "weak signal"
performance is slightly better with the internal preamp off.  I keep it off
when using my mode-S system as well.

I think DEMI designed their convertor to be used stand-alone without a
preamp.  But as we all know, a dedicated/optimised low noise preamp is not
a "luxury" on AO-40.  The combo probably has too much gain.  My "deaf"
Drake down convertors don't seem to be sensitive to this.  I have a 40 dB
gain LNA ahead of them!

Rick wrote:
>I also experienced 13ULNA preamp instability and solved it by putting a 1" 
square piece of black anti-static foam in the DEM enclosure. It was about a 
1/4" thick. My 13ULNA is connected to my downconverter through a double 
male N barrel connector.<

Sounds like the preamp was self-oscillating.  The small peice of absorbing
material lowered the feed back below the level for oscillation.  I have
heard this trick used by many microwave ham designers when toher methods
would not work.

73, Ed - AL7EB

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