Dave Oldridge wrote in
9:

Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:
....
If you made the Y measurements using the audio output of a narrow
band receiver, it is very hard to make high resolution measurements
(eg to 0.01dB resolution) with say, a multimeter.

It is. You need a good AC voltmeter with decent digital accuracy and
resolution and you have to average a bunch of readings.

I put some notes together on a perspective of the noise measurement
(sampling) process and its statistical uncertainty, they are at
http://www.vk1od.net/fsm/nmu.htm .

It is my experience that a digital voltmeter probably samples for
something around 100ms, and with a 2kHz wide noise bandwidth, you might
expect an uncertainty of near 0.5dB at the 90% confidence level. Just
watch the readings bounce around.

Sure, if you average 100 of those measurements (actually, you should get
the root of the sum of the squares... because it is power you average,
not voltage), you might reduce that uncertainty to around 0.05dB... but
it is not a very practical manual method, if recording accuracy (ie
writing down the wrong number) doesn't get you, environmental drift will.

Owen