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:
Two questions on telemetry..
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?
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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