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

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Thanks for this explanation Jim.

People on this list are so helpful.

73s
Roger G3XBM

On 7 April 2011 19:33, James Moritz <james.moritz@btopenworld.com> wrote:

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

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