Re: Microwave Noise

Chris Bartram G4DGU

Hello again, Andy,

I can certainly see that off-air HF noise could include artefacts of the non-linear nature of the propagation medium - I'm old enough to remember the 'Luxembourg Effect' on MW! IMD2 does nowadays seem to be taken more-or-less seriously,  particularly since the advent of direct sampling receivers.  But much VHF and up equipment still seems to ignore it.

The use of FFT techniques could easily lead to measurement inaccuracies/uncertainties in a situation where the 'noise' spectrum was non-stochastic. I may be a bit of a dinosaur, but I still use a zero-bias diode detector with proven square-law response, a thermal wattmeter, or alternatively a wideband true RMS voltmeter for making noise power measurements. True RMS voltmeters like the Racal-Dana 9300 or 9303 are available surplus at quite low prices, and with a simple downconverter can be used as a back-end for noise ratio measurements. However that's getting a bit off-topic.

Chris G4DGU

The question really came about after reading an article in the latest QEX about noise monitoring and WSPR, where they use two methods of measuring noise, both of which I've adopted in the past.   The RMS of the time domain signal, measured during the quiet period of no WSPR transmissions.  And the FFT techniques where the bins are put into order of power and noise deemed to be the level in the bin at the 30% point ( I use the lower quartile plus 5dB - thoughts on this differ but the delta is minimal)

In that article they mention that often the two measurement methods give significantly different results, and at other times are very close.   This is put-down to the non-Gaussian nature of noise at HF.    Hence my thoughts at higher frequencies.

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