I can always depend on you going over my figures. ;-)
One thing you might get a challenge on is sky temp at 70cm. My original use of Tsky=150K is probably too conservative for some locations but may be closer to reality in urban areas. I believe the celestial sky noise is considered to be 70K at 70cm by eme'rs. I will concede that 100K is probably too high for very well constructed antennas.
So with quiet locations and better antennas the improvement in sensitivity can exceed the improvement in NF. This can be realised by dish antenna as a rule (I generally use 29K for antenna temp on eme dishes above 1-GHz). For a dish, the edge illumination needs to be down near -20dB to achieve that kind of antenna temp. For satelllite reception -15 dB edge taper is sufficient to get antenna temp of 50K. Achieving this kind of clean pattern with a yagi antenna is much harder.
Yes, thanks for providing the S+N/N as this is indeed what one reads on the s-meter. The amount of dB/S-unit varies greatly by radio (theoretically it should be 6-dB/S-unit). I find on my FT-847 that the calibration of the s-meter is anywhere from 6-db to 3-dB per unit and is non-linear. One needs to calibrate their individual radio to trully know what the s-meter is indicating. S0 to S1 on my radio is about 5-dB.
At 04:58 AM 9/29/2008, you wrote:
----- Original Message ----- From: "Edward Cole" [email protected] To: "Tim Tapio" [email protected]; [email protected] Sent: Sunday, September 28, 2008 9:48 PM Subject: [amsat-bb] Re: Telemetry
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
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