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VK2ZRG > TECH 08.04.06 12:04l 39 Lines 1791 Bytes #999 (0) @ WW
BID : 2352_VK2ZRG
Read: DL1LCA GUEST
Subj: Re Noise figure query
Path: DB0FHN<DB0FOR<DB0MRW<DK0WUE<7M3TJZ<ON0AR<IW8PGT<SP7MGD<VK7AX<VK2TGB<
VK2IO<VK2WI
Sent: 060408/0928Z @:VK2WI.#SYD.NSW.AUS.OC #:11111 [SYDNEY] FBB7 $:2352_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:2352 Date:08-04-06 Time:9:26Z
Hello Gordon VK3XX and readers,
Thanks for that Gordon. What you say is all fairly right. For a terrestrial
communication circuit the best way to improve the signal to noise ratio is to
reduce RX feed line loss or increase TX power/antenna gain. When the effective
antenna temperature is 290 Kelvins or so, for every dB that you reduce the RX
system noise figure, you get a dB improvement in RX signal to noise ratio.(SNR)
For EME or satellite work however, the effective antenna temperature is
usually less than 290 K, so that you will get bigger increase in SNR than the
improvement in noise figure.
My noise figure query was because I had a dilemma with one part of my
programme. I had a line on the screen that gave the receiver noise power
in the specified bandwidth. I realised that any loss in front of the first
amplifier stage won't increase the noise power but will just reduce the
signal level (when feed line and antenna temperatures are 290 K).
However if the noise figure of the first amplifier was raised to give the
same system noise figure without any preceding loss, then the RX noise
power will be increased accordingly, but the signal level will be unchanged.
Last night I decided that knowing the receiver noise power was of little
value, and that the minimum discernible signal (MSD) was a more useful number
to know. The MSD is the same whether the system noise figure is determined
primarily by feed line loss or amplifier noise.
So I've changed my programme a little to give the MSD and the dilemma is
resolved.
73s from Ralph VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
Taglines by Colin Coker G4FCN
C Program run, C Program Crash...ReWrite in Pascal!
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