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LF: RE: Pre-amps.

To: [email protected]
Subject: LF: RE: Pre-amps.
From: "Talbot Andrew" <[email protected]>
Date: Wed, 25 Apr 2001 08:39:58 +0100
Reply-to: [email protected]
Sender: <[email protected]>
All noise is reduced in proportion to the bandwidth, both external and equipment (thermal) noise.   The power of thermal noise is given by kTB, where k is Boltzmans constant, 1.38E-23, and B in degrees K.   Gives in -174dBm per Hz.
For all but very tiny, lossy, antennas, atmospheric noise usually dominates so a fantastically low noise figure is not essential.  A tuned loop gives an output vastly greater than thermal noise, but when untuned and if it is particularly lossy as well, then the levels may be comparable.  The MAR6 has a noise figure better than 4dB at frequencies up to 1GHz so gives a good account of itself at LF, but this level of performance is unnecesasry at LF if using a tuned loop.
Andy  G4JNT
 -----Original Message-----
From: Laurie Mayhead [mailto:[email protected]]
Sent: 2001-04-24 18:42
To: [email protected]
Subject: LF: Pre-amps.

Hi All,
I have been thinking about weak signals on 136 using QRS and DFCW. These techniques enable signals to be read which are buried 30db down in the noise(see Extreme narrow bandwidth techniques ON7YD ) Does this mean that signals 30db down on atmospheric noise are seen,if so what about the noise from the RX pre-amp ? In my case this is about 20/30 db less than the Ant. noise,so would I be looking at the pre-amp noise or is this also reduced by the narrow bandwidth ?
I should like to improve the noise performance of my pre-amp.
filter combination. and would appreciate any information on high performance circuits or amp. units. Is the MAR6 likely to be worth while. 
By the way if my thoughts are correct this may be why high Q loops are reported to be superior to lower Q types, higher signal output overriding amp. noise. Any ideas ?      73s  Laurie.  

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