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[Antennaware] Antenna amplifier noise

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Subject: [Antennaware] Antenna amplifier noise
From: "Andy Ikin" <andrew.ikin@btopenworld.com>
Date: Sat, 25 Apr 2009 13:25:43 +0100
List-post: <antennaware@contesting.com">mailto:antennaware@contesting.com>
Hello Folks,

Is the following, a valid method to either measure amplifier excess noise or 
make a meaningful comparison?

Assume one measures the S/N of a AM Rx to be say 10dB S/N at 30 percent 
modulation for an input signal -101dBm.

Then one re-measures the Rx S/N with a 20dB noiseless pre-amp. Would the Rx 
sensitivity, 10dB S/N now be a theoretical -121dBm????

If so, then, if the 20dB amplifier had a NF of 3dB, the measured Rx sensitivity 
for 10dB S/N would be 3dB less i.e. -118dBm!!!!!

For example, the well respected DXeng RPA-1 HF Pre-amp. has gain of 15.8dB at 
500kHz. This amplifier increases the RX sensitivity 10dB S/N to -112dBm. If the 
amplifier was noiseless then the RX sensitivity for 10dB S/N would be 
-116.8dBm! So the difference between the noiseless amplifier and the measured 
sensitivity is 4.8dB. Can I assume that 4.8dB is the NF??? I would have expect 
a NF of over 4dB as this amplifier uses resistor feedback.

I also have a transformer feedback amplifier, gain 14.6dB at 500kHz. This 
amplifier increases the RX sensitivity 10dB S/N to -113dBm. If the amplifier 
was noiseless then the RX sensitivity for 10dB S/N would -115.6dBm. So the 
difference between the noiseless amplifier and the measured sensitivity is 
2.6dB. Can I assume that 2.6dB is the NF???

The Rx is an NRD525. A HP 8568B Spectrum Analyser and Marconi 2019A signal 
generator is used for amplifier gain measurement and Rx sensitivity. 

I assume that there is probably a flaw in the above rational, but the figures 
don't seem too "way out"!!!

73

Andrew
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