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Re: question about the DEM 13 ULNA 2.4GHz preamp



Hi Mark,

I'm going to insert my answers into your message:

>From: "Mark L. Hammond" <hammond@camel.campbell.edu>
>The details on this preamp are here:
>http://www.downeastmicrowave.com/PDF/13ulna.PDF
>
>I have one of these (pre-built) units  in the following configuration:
>
>helix feed->13ULNA premp->Transystem 3373 downconverter on a BBQ dish.
>
>My question is this--does an increase in S-meter background noise 
>level/floor (compared to without the preamp installed) on my 2M rig 
>guarantee that the preamp is working correctly?

ED: Generally you should see an increase in receiver noise {no signal
input} when you power up a preamp before either a convertor or receiver.
This is because the preamp is generating some internal noise [NF>0 dB],
plus amplifying the antenna noise [sky+ground pickup].  If you replace the
antenna with a 50 ohm load, you should see even more noise since the load
acts like a 290K thermal source [like adding 3 dB to your NF].  Likewise if
you point the antenna at the ground or a thick stand of trees, you should
see a significant rise in noise [and s-meter].

>Or is it possible that one of the components has actually failed (like the 
>PHEMT), but yet it still causes an increase in the S-meter?

ED:  Not likely.  If the preamp is burned-out normally there is no change
when you apply power.  On occasion I have had a blown gate on a GasFet that
raises noise when turned on, but amplifies no signals.  When I troubleshoot
receivers, I insert a signal before and after an amplifier stage to see if
the signal is stronger when inserted before.  If stronger after, then
likely the stage has failed [the stage has no gain].

>The reason I ask is this--when I first got the unit, I installed the preamp 
>BACKWARDS in between the helix feed and the downconverter, and powered it 
>up.  (I'm feeding DC into the preamp through the external feed-through cap, 
>NOT via the coax)  And AO-40 signals have been quite poor, and I'm not 
>certain that my preamp is actually working....

ED: Since you are not feeding DC down the coax there is no harm to
connecting it backword.  You will not see any increase in noise if you
turned it on.  Even if you feed voltage thru the coax, most preamps have a
series coupling capacitor on the antenna connector that blocks DC voltage.

>Does having the preamp in line WITHOUT powering it allow signals 
>through?  I would expect yes, with perhaps a tiny reduction in signal from 
>loss through the unpowered preamp.  Yes, I need to and can test a few of 
>these things out, but it's a bit challenging to do it based on my current 
>configuration/power scheme.

ED:  When the preamp is in the circuit but powered off, it acts as a huge
attenuator; little or no signals will be passed.  Certainly not AO-40
signals.  This is why you hear nothing when you burn out a preamp.

>I know I'm showing my ignorance on how the preamp works, but I hope to 
>learn something from constructive replies.
**snip

ED: AO-40 will be very weak due to the large squint angle right now.  My
"bare" Drake hears nothing!  Your new preamp is probably doing fine.  Have
you compared listening with no preamp.  I'll bet it is better with the
preamp in-line.

>Thanks for any comments.
>
>
>Mark L. Hammond  [N8MH]
>
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