Dear Owen:
Others too may remember the use of a noise source comprising a gas tube
crossways in a piece of waveguide (with 50 ohm probes) to estimate noise
figure in the VT days. One would fire the gas tube and it was estimated to
have a very large, "known" noise temp. Boiling water, while a known temp.,
would not have been hot enough.

Ice water was critical in the use of the HILLMS receiver used to measure
absolute flux. The antenna was a very long horn antenna resting on the side
of a deep gully. In those days, a horn antenna was one of the few antennas
with a predictable gain. The receiver switched at a low frequency between
the antenna and a load kept in ice water. The gain of the receiver was
stabilized with a huge amount of negative feedback. Once a day, the source
would pass through the antenna's beam and a strip chart recorder would
indicate the difference between ice water and the source's temp.

Today, with much lower NF, and much more EM pollution, different techniques
might be used.
73 Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Owen Duffy" wrote in message


Dave Oldridge wrote in



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.

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.

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.

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.

Owen