Re: LF: VLF QRM puzzle - why not the same in a split screen on SL?

["James Moritz" <james.moritz@btopenworld.com>]

Dear Roger, LF Group,

These vertical stripes are indeed due to harmonics of the mains frequency, 
specifically the 179th harmonic at nominally 8950Hz. If you watch these 
harmonics with, say, "QRSS3" spectrogram settings, you will see wandering 
"carriers" at multiples of 50Hz, that drift around in a range of a few 10s 
of Hz near 9kHz, and may change frequency at a rate of several Hz per 
minute. However, with a millihertz-resolution spectrogram, this drift rate 
is fast enough that some energy appears in most of the displayed FFT bins on 
the screen during one FFT window period, then drifts completely out of the 
displayed range. Hence the result is a vertical streak of noise. 8970Hz/179 
is apparently close to the limit of how far the mains frequency is allowed 
to drift; I usually see these bands of noise a few times a day around 
8970Hz, but it rarely reaches 8976Hz
Cheers, Jim Moritz
73 de M0BMU