Hi JC, I did not publish any absolute Noise Figure values, my measurements that I published are all just differences in Noise Figure.
It actually is much easier to measure the difference in Noise Figure between preamps that have such high gain (approximately 40dB in this case) since the noise floor generated by a 50 ohm resistor at room temperature is easily measurable (at least indirectly) after 40 dB of gain (It's -106.89 dBm for a 40 dB preamp using a bandwidth of 500 hertz if the preamp adds no additional noise). Even if you can't accurately measure -106.89 dBm level signals, you just need to be able to be repeatable in your measurements between preamps (accuracy and repeatability are not the same thing), and your system must be able to detect differences in output signal from the preamp in units of dB (easy to do with an SDR receiver at these levels using attenuators to verify changes in dB on the SDR display). In my case all of my measurements were made with my Kenwood TS-180s operating on 160 meters, and then making measurements via the wideband IF output port of the Kenwood TS-180s that's connected to an SDR receiver (the HDSDR software also allowed me to do long term averaging, and it even allows you to display power spectrum density). My second method of measurement used the S meter on my TS-180s, but just as an indicator (actually digitized the voltage supplied to the S meter). I recorded the signal strength on the S meter that the preamp generated when the preamp input was terminated with the 50 ohm resistor, then I went back and adjusted my signal generator connected directly to my TS-180s via attenuators until it provided the same S meter reading. Using the signal generator and precision attenuators I was able to calculate the voltage (and therefore power) into the TS-180s that the S meter recorded when driven by the preamp. I used this technique to compare Noise Figure between preamps, and I made numerous measurements over a 6 day period and then averaged the data to reduce my variation using this method. I did in fact test the use of #31 mix cores (10 to 14 turns) on various lines (power supply lines, 3 foot short feedline between preamp and receiver input, etc. and saw no difference in signal levels. I even eliminated the Kenwood power supply, and fed the TS-180s with a lab grade linear power supply to eliminate slight changes in supply voltage over time. P.S. the Kenwood TS-180s is one of the better radios made in regards to noise floor (measured -139 dBm by Sherwood). It would certainly be much easier to make these measurements using a spectrum analyzer, and I probably will acquire one of the lower cost Rigol spectrum analyzers in the near future. In the meantime I would love to have someone independently make measurements comparing these amplifiers that either supports or disproves my measured values. 73, Don (wd8dsb) On Tue, Nov 8, 2016 at 7:24 PM, JC <n...@comcast.net> wrote: > Hi Don > > Measuring noise figure below 1 db is a very complicated work. > > First you need to remove all common mode noise, your noise floor should be > better than -135 dBm at least with a 40 db preamp connected to the radio > input and a 50 ohms load. > > You need chokes everywhere and most important shield on 12V, RF in and RF > out. Check the AM band you hear carrier coming in , noise is also coming > in. > > Second the NF is very different on 1.8 MHz , measurement near 10 Mhz, does > not tell you the right NF on low bands. > > In order to have Noise Figure Uncertainty near .2db it is necessary a good > quality lab equipment, calibrated and two hours per measurement for each NF > reading. This assuming the temperature does no change during the test. > > > Your Noise Figure Uncertainty based on your video could be 5db or more. > > > 73's > JC > > N4IS > > _________________ Topband Reflector Archives - http://www.contesting.com/_topband