[amsat-bb] Re: Telemetry

Edward Cole kl7uw at acsalaska.net
Sun Sep 28 12:48:55 PDT 2008


You realize that the "low-noise figure" preamp is most affective when 
placed at the antenna and that this is because the feedline will 
degrade the noise figure.

How the noise figure is affected, is that the amount of loss in the 
feedline is directly added to the noise figure of the preamp.  So if 
your preamp is 0.5 dB NF (a common value for a good Gasfet preamp) 
and your feedline loss is 1.5 dB (what a medium long run of RG-8 or 
RG-213 will have at 70cm) then your overall receiving noise figure is 
about 2.0 dB (actually there is a small amount of degradation 
contributed by your radio's noise figure (which may another 0.1 or 
0.2 dB if your preamp gain is 20-dB or more).

So when you place your preamp at the antenna, your receivng system NF 
will be close to that of the preamp.  You do not need super low loss 
feedling if the preamp is at the antenna.  Use what you have 
now.  The feedline loss will add directly to the NF of your radio in 
the shack, but the total will be reduced by the gain of the preamp.

I plugged these numbers into my NF program and the result is:
preamp in shack:  system NF = 2.08 dB
system sensitivity: -139.3 dBm  (assuming 2-KHz SSB bandwidth, 150K 
of sky temperature, and 100K of antenna temperature)
preamp at antenna:  system NF = 0.58 dB
system sensitivity: - 140.9 dBm

So the NF is dramatically reduced and one can copy signals about 1.6 
dB weaker (about 1/2 S-unit).  If signals are S7-9, this will not be 
significant but if signals are S0 to S1 it may make the difference 
between copying the signal or not!

By all means move the preamp to the antenna.  No change in feeline is 
need for reception improvement (assuming here that you are not 
planning to use this antenna for transmitting).

73 Ed - KL7UW

At 04:22 AM 9/28/2008, Tim Tapio wrote:
>Hello all!
>Two questions on telemetry..
>1.       I was considering dedicating a 70 cm "eggbeater" with a preamp as a
>good, omni-directional antenna. So far though, it doesn't seem to pick up
>below 12 degree elevations. Currently the preamp is in the shack (I know, at
>the antenna) and my coax is not optimum. If I use heliax, put the preamp at
>the antenna, is this feasible or should I plan on a steerable beam?
>2.       I'm having a dickens of a time figuring out what the telemetry
>frequencies are. I've looked at DK3WN's site which has the decoding software
>but I seem to be having issues finding frequencies, am I looking in the
>wrong places?
>Thanks for your time!
>73 de Tim, K4SHF FM04
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