Many thanks Guy and David for your replies.
I certainly have something to think about.
Concerning antenna directivity, I am looking to improve the integral amplifier
in my K9AY Phased Array( 500kHz to 2MHz ). There may be an issue that the array
gain may be too low for Trans Pacific Dx at the bottom end of the AM Band. Also
extra gain is needed to over come any feeder pickup and noise in the Phasing
Controller. Hence, I was trying to find a simple way of comparing amplifiers.
Btw, I manufacture MW Loop and K9AY Phased arrays in very small numbers for MW
Dxers.
73
Andrew
 Original Message 
From: DAVID CUTHBERT
To: Andy Ikin
Cc: antennaware@contesting.com
Sent: Saturday, April 25, 2009 5:06 PM
Subject: Re: [Antennaware] Antenna amplifier noise
The noise figure of an amplifier tells us the input referred noise in a 1 Hz
noise bandwidth. A 0 dB Noise Figure (NF) tells us the amp input noise is 174
dBm.
Convert the noise power of the amp output and the input noise of the RX into
linear units (watts), add the powers, and convert back to decibels. Now refer
it to the amp input.
To start, you need to specify the noise bandwidth. Given your example, and
neglecting the 30% modulation as we don't need this, you say the RX S/N ratio
is 10 dB given a signal of 101 dBm. The RX input noise is 111 dBm. For now
let's say the RX noise bandwidth is 6 kHz.
The amp input referred noise, given the 3 dB NF, has a noise power of 133 dB
in 6 kHz. Here is how this is figured:
174 dBm + 3 dB + 10LOG(6000) = 133 dBm. Remember, this is input referred.
The noise at the 20 dB gain amp output is 133 dBm + 20 dB = 113 dBm = 5.0 fW.
The RX input noise is 111 dB = 7.94 fW.
5.0 fW + 7.94 fW = 12.94 fW = 109 dBm. This is the new noise referred to the
RX input. But we want to know the noise referred to the amp input. It is 109
dBm  20 dB = 129 dBm.
So, the RX noise of 111 dBm is now 129 dBm with the amp. This is a decrease
in noise of 18 dB. What is the new NF?
129 dBm  10LOG(6000)  (174 dBm) = 7 dB.
What is the NF of the receiver alone? 111 dBm  10LOG(6000) = 149 dBm gives
us the input referred noise. NF = 149 dBm  (174 dBm) = 25 dB.
Dave WX7G
On 4/25/09, Andy Ikin <andrew.ikin@btopenworld.com> wrote:
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 remeasures the Rx S/N with a 20dB noiseless preamp. 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 RPA1 HF Preamp. 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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