[NLRS] Preamp questions - response

[Jim Froemke]

Hi Jason. I've been there and done that. You're starting out correctly by
asking questions before you make a decision (I didn't).

In my case I purchased a good used 2 meter pre-amp/relay combination and
mounted it on the mast using RF sensing. At first it seemed to help out with
my receive situation due to the long (240') length of coax between the
antenna and ham shack. Upon further inspection it made my transmit situation
somewhat worse and masked the real problems I had.

The reasons that one considers adding a pre-amp are to set the receive
system noise figure, compensate for lossy transmission lines and compensate
for poor sensitivity in the front end of your transverter. The unanticipated
problems resulting from adding a pre-amp are additional loss on transmit
(due to the relays and additional jumpers/connectors), reliability problems
(due to sequencing), pre-amp overload (due to strong signals), overload of
the transverter/transceiver front end, the need for additional control/power
cables and antenna pattern distortion (due to more metal near the antenna
boom).

I decided to upgrade my transmission line and move the power amplifier
(which also contains a receive pre-amp) from the shack to the base of the
tower thereby reducing the transmission line length between the power
amplifier and antenna. After doing this I could not see any benefit of the 2
meter pre-amp at the antenna versus the pre-amp in the amplifier and I
eliminated the transmit power loss (due to the separate preamp
relays/jumpers).   During the three years I used the 2 meter pre-amp I
burned it out twice (sequencing problems on CW). After these experiences and
the cost of two repairs, I sold the pre-amp and have been happy ever after.

My advice with regard to 2 and 6 meters is to spend your hard earned money
upgrading transmission line and if necessary antennas as these will improve
both your transmit and receive performance. If your current problem is a
poor transceiver front end, upgrade that or add a hot transverter ahead of
it (you'll probably due this over time anyway). You'll pick up more noise
with your antenna pointed at the horizon then you can make up from a low
noise pre-amp. (If you're into EME and/or satellite communications where
your antenna is elevated above the horizon you'll probably benefit from a
pre-amp.)

With regard to 1.25 and 0.7 meters, the situation is not as clear. I
continue to see a noticeable benefit with mast mounted receive pre-amps and
may have to further shorten/improve the transmission lines in order to
eliminate them. Here again my power amplifiers (mounted at the base of the
tower) have receive pre-amps built into them but, are not currently being
used. I'm using the SSB mast mounted pre-amps
(http://www.ssbusa.com/gaasfet.html) on these bands with good results but my
long term goal is to eliminate them also (KISS).

Good luck with your decision.

73, Jim, K0MHC/0
EN26ha
----- Original Message ----- 
From: "Jason Godfrey" <godfrey@aptsolutions.com>
To: <nlrs@mailman.qth.net>
Sent: Monday, September 15, 2003 1:09 PM
Subject: [NLRS] Preamp questions


> Hello.
>
> I was able to play with an external preamp on 2m on Sunday and was
impressed
> on how much easier it made hearing others. Makes me want to add a preamp
into
> my normal bag of tricks.
>
> I was wondering what the list members are using for a preamp setup. I am
> particularly interested in how you handle switching. Most of the models
I've
> seen are receive only. (The one we tried Sunday was carrier sense, but was
> limited to 25w drive.) Looking at DEM's site the preamp isn't too
expensive,
> but the RF relays are, particularly since two are required.
>
> Thanks in advance for any advice
> - Jason, N0RPM
>
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