#### Re: Measuring Noise Figure

Andy G4JNT

I did it once with boiling water and room temperature on what was supposed to be a low noise amp (144MHz) of the late 1980s era.  I saw  a genuine "fraction of a dB" but back then didn't have the means to measure mean noise power reliably or accurately, especially in a wide bandwidth.

Simplistically:
Assume a 1dB noise figure LNA with sufficient gain to overcome any following stages.
1dB is equivalent to a noise temp T of  75K   (  since NF = 10.LOG(T / 290 + 1)

So the delta noise power seen at the output of the system for input at 100C = 373K and  Room Temp 290K
= :
10.LOG[ (373 + 75) / (290 + 75) ]  = 0.89dB

Which ought to be measurable with modern wide bandwidth SDRs.  It wasn't reliable using a diode on the output of an SSB Rx in 1988 !

Andy  G4JNT

On 19 July 2016 at 11:54, Richard Baker perwick@... [ukmicrowaves] wrote:

Any noise measurement you make with an open transmission line is going to be dependant on its length.

Rather than me try to explain the theory I suggest you try the following experiment.

I would be interested in what readings people get.

Richard
GD8EXI

On 18/07/2016, 18:19, "richard@... [ukmicrowaves]" <ukmicrowaves@...> wrote:

"Thoroughly disagree"  Thanks for your reply Chris. I am not clear with which approach you thoroughly disagree with however.

a) the gain method of measuring noise figure
b) measuring without an input load
c) measuring with an input load; or possibly
d) Derek's approach.

73 Richard G3CWI

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