Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:

Dave Oldridge wrote in
9:

Near as I could measure it, the NF of the receiver after my mod was
1.2db. I had to resort to boiling and freezing water and a tiny dummy
load to measure it at all.

I haven't tried hot/cold tests using ice and boiling water, I didn't
think it was practical.

Only just and you need a good 4-digit or better AC voltmeter to do it at
all. I wasn't after accuracy, just a ball-park estimate and I know I got
it fairly close because the receiver did show a marked increase in noise
when any decent antenna was connected.

You finally measured a receiver noise temperature of 50K with hot and
cold loads of 270 and 370.

That means a Y factor of 1.059dB.

If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF
would be 1.66dB.

Yep...the most I'd be willing to commit to with that measurement would be
that it was below around 2.5 and PROBABLY fairly close to my measurement.
I measured the voltages alternately 25 times and took a mean to try to
smooth out the errors.

With this configuration the sensitivity of NF to changes in Y are
extreme, 0.4dB change in NF per 0.1dB change in Y around that point.

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 have done these tests with a liquid nitrogen cooled load and room
temperature load, and that gives more practical Y ratios, 3.7dB for a
1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y.
This
still demands high resolution measurement of noise power.

Yes, anything less than 4 digits is just about useless.

--
Dave Oldridge+
ICQ 1800667