[amsat-bb] Re: Telemetry
i8cvs
domenico.i8cvs at tin.it
Mon Sep 29 05:58:19 PDT 2008
----- Original Message -----
From: "Edward Cole" <kl7uw at acsalaska.net>
To: "Tim Tapio" <tim at timtapio.com>; <amsat-bb at amsat.org>
Sent: Sunday, September 28, 2008 9:48 PM
Subject: [amsat-bb] Re: Telemetry
> Tim,
>
> 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!
Hi Ed, KL7UW
I agree with your calculation and I got the same result working out your
numbars using my hand held calculator:
Preamplifier in the shack:
Noise Figure NF= 2.08 dB
(2.08 / 10 )
Noise Factor F= 10 = 1.61
Equivalent Noise Temperature = (1.61-1) x 290 = 178 kelvin
Preamplifier at the antenna:
Noise Figure NF = 0.58 dB
( 0.58 / 10 )
Noise Factor F = 10 = 1.14
Equivalent Noise Temperature = ( 1.14 - 1) x 290 = 41.4 kelvin
The improvement in S/N can be computed only considering the
equivalent temperatures because into the KTB equation K and B
are constants and so:
178 + 150 + 100
Improvement in S/N = 10 log [ ----------------------] = 1.67 dB
10 41.4 + 150 + 100
Actually you see that assuming a sky temperature of 150 kelvin and an
antenna temperature of 100 kelvin the improvement of S/N = 1.67 dB
only and it is very close to the difference between both Noise Figures
and infact 2.08 - 0.58 = 1.5 dB
But using good antennas as for EME the sky temperature in 70 cm can
be reduced to 30 kelvin and the antenna temperature to only 50 kelvin so
that the improvement in S/N going from an overall Noise Figure of
2.08 dB to 0.58 dB is greater than the difference between the Noise Figures
and changing the sky temperature and the antenna temperature in the above
formula for the EME example the S/N improvement rises to 3.27 dB
The above improvements are calculated in S/N ratio but the S-meter
readings are in (S+N)/N ratio and S/N = [(S+N)/N ]-1 in factor so
that as an example if the S-meter reading is 3 dB or 2 in factor then
S/N = 1 or 0 dB because the signal equals the noise in level.
On the reverse side if the computed S/N ratio is 1.6 dB or 1.44 in
factor then (S+N /N ) = (S/N) + 1 = 2.44 or 3.88 dB wich is about
1/2 of S-unit as you pointed out but if using an EME antenna the
computed improvement in S/N= 3.27 dB or 2.12 in factor then the
S-meter reading will swing to 2.12 +1 = 3.12 in factor or 4.9 dB i.e.
about one S point.
>
> 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
>
I agree and I added the above calculations to show how easily one can
realize that the right place for a low noise preamplifier is as close as
possible to the antenna connector.
Best 73" de
i8CVS Domenico
> 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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