[amsat-bb] Re: Pre amp question

Bruce Robertson broberts at mta.ca
Wed Dec 13 02:56:23 PST 2006

Quoting Steve Raas <sraas at optonline.net>:

> As I prepare for getting on the birds.. I decided to test out this
> AG-25


(Once again, it has to be said that I'm new at this stuff, too, and I'd be
happy for the folks on this list who have taught me over the last two years
to correct any errors.)

What you describe in your letter would, I believe, correspond to the
situation in which the noise figure of your preamp is not much better than
the noise figure of the front end of your receiver and there is not very
much loss between the preamp and the radio (i.e., not very long a run of
cable). Is it possible that you hooked it up with a short run of cable in
the shack, shorter than you usually use for terrestrial work? Perhaps your
receiver nf has improved. If the preamp and the front end have the same
noise figure and there is significant loss in the cable, however, you will
win with this setup. 

>From my experience, if your cable is low loss, a 145 preamp isn't as
necessary as a 435 one because thermal noise is higher and the cable won't
ding you as much. My preamp went funny and it wasn't a complete tragedy. 
Who knows, maybe *my* 145 preamp is the pits, too :-) Those ICOM units are
a bit dodgy, though: they don't seem to specify their noise figures very well.

I think all would agree that 435 MHz is the band that *really* needs a
preamp. You want one that has a 1 dB noise figure or less for the usual 15
+ dB of gain. Advanced Receiver Research and SSB USA are known to be fine.
My in-shack ARR preamp is *amazing*. (I have a short run of heliax between
antennas and preamp.) Be sure to get one that switches on xmit.

As far as setting up a station, I'd try building something minimal to start
 so that you can observe the improvement each added element brings. Use a
female-female N connector in place of a preamp on 145 at first and observe
the helpful always-on beacon on VO-52. How high above the horizon does it
need to be before you can hear it over the noise? before it starts to move
the S-meter? (Don't try this with AO-7; it's too weak.) Even on 435, you'll
hear *something* without a preamp if you have an antenna with some
directionality. (And, as I've mentioned in the previous letter, even a 1/4w
vertical will bring in LO-19 and others.)

73, Bruce VE9QRP

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