GaFET Preamp for XV144 transverter

[Edward R Cole]

Ed:

Here are a few relevant numbers:
XV144:  NF = 0.8 dB
SP144VDG:  NF=0.55 dB, gain=24 dB, 25w max rating
Coax Loss for 25-feet:
RG-58= 1.25 dB
RG-213= 0.55 dB
LMR400= 0.38

With SP144VDG at antenna:
system NF=0.61 dB using 25 feet of RG-58
system NF=0.60 dB using 25 feet of LMR-400
so it doesn't matter what coax you run

Assuming sky temp of 250K at 2m (probably the best one will see
somewhere out in the desert)
receiver sensitivity for SSB is in the range of -138 dBm, which is excellent.

For urban noise levels sky temp of 500K, receiver sensitivity is -136 dBm
---------------------------------------------
with SP144VDG at XV144 and 25 foot of coax to antenna:
system NF= 0.93 dB using 25 foot of LMR-400
system NF= 1.10 dB using 25 foot of RG-213
system NF= 1.80 dB using 25 foot of RG-58
coax makes a big difference in NF.

sky temp=250K:  sensitivity = -138.4 dBm with LMR-400 and
sensitivity= -137.6 dBm with RG-58
sky temp=500K:  sensitivity = -136.4dBm with LMR-400 and sensitivity=
-135.9 dBm with RG-58

I looked up in QRZ.com and Trinidad, CA looks pretty rural so maybe
you can use the lower sky temperature figure for estimating performance.

So what does this tells us:
Preamp at antenna is best (we knew that) and one can run cheap coax
(without degrading NF and sensitivity).

If you keep preamp in the shack then you want to use good low loss
coax for best performance.

Considering that you may not have a lot of stations nearby and making
long-distance contacts involving weak signals, I would put the preamp
at the antenna.  The SP144VDG is self-switching with RF sensing so
only 12v need be provided.  This is a very simple setup so why not
install it at the antenna?

I will say that I normally use external coax relays with plain
preamps, but I have installed on a 2m yagi a RF sensed ARR preamp for
simplicity for use with a FM 2m radio.  It works very nicely with
stations 70+ miles away running FM simplex.  It is "almost" as
sensitive as my my four-yagi eme array+ preamps ( but not quite).  I
run 80 foot of LMR-400 from the preamp to the radio.

One sidenote: 25w is not very much power to operate long distance on
2m.  I typically run 120-150w.

73, Ed - KL7UW

-------------------------
From: "Edmund Wright" <[hidden email]>
To: <[hidden email]>
Subject: [Elecraft] GaFET Preamp for XV144 transverter
Message-ID: <6F6BA2EC60E14E61940E7F5854935CAD@Edmund>
Content-Type: text/plain;       charset="iso-8859-1"

I have recently finished aligned and tested an Elecraft XV144
Transverter.  I have an Advanced Receiver Research SP144VDG GasFet 2
meter preamplifier.  Its specs are as follows:

     Noise Figure  < 0,55 dB
     Gain 24 dB
     Compression +12 dBm
     Transceiver Power  <25 watts

Should I use my preamplifier between my Isopole 144 MHz vertical
antenna and my XV144 transverter antenna connector?  I will have
about 25 feet of coaxial cable between the preamplifier mounted on
the antenna and the XV144 transverter.

I will appreciate any comments or suggestions

Ed Wright
AA6LZ


73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     [hidden email]

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