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Re: LF: Selection of spot frequencies for DX

To: [email protected]
Subject: Re: LF: Selection of spot frequencies for DX
From: "Vernall" <[email protected]>
Date: Tue, 25 Mar 2003 08:56:37 +1200
References: <[email protected]> <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi Jim, Alberto, Rik and others,
 
I agree that the frequency stability of the mains network is a point to consider further.  However, I think there are two interference mechanisms at play in the LF band:
 
1.  Genuine harmonics of the mains frequency, from the likes of peak rectifiers in domestic equipment.  These are a mixture of 50 and 100 Hz components, but are "phase locked" within themselves, so the net result is an apparent set of 50 Hz "lines"
2.  Sidebands of "asynchronous" switch mode power supplies, where the relatively unstable switching frequency is "hum modulated" by the rectified mains (mostly 100 Hz sidebands for a full wave rectifier, which most are).  Worst case is for the "carrier" of the switch mode supply to be in-band with the LF frequency of interest.
 
The sidebands of a switched mode power supplies seem to wander over any part of LF, and look like "snakes" on an Argo screen.  The pic from Alberto (see below) gives a good hint that there is a mains harmonic as well as a sideband from an asynch. switch mode supply.
 
When I first found the line very near 137.7000 kHz on 120 second dot Argo, during the evening, I phoned up my friend Andrew ZL2BBJ to see if he was experimenting with his LF DDS, and he was.  He had it set to 137.7000 kHz as a marker for forthcoming UA9OC tests.  He had no antenna connected, but he did have scope probes and wires on the bench, so I thought my receiving setup was performing very nicely as his QTH is about 10 km from mine.  However, the weak line I got carried on after he assured me that he had turned off power in his shack!  I doubt very much if there is a radio transmitter on 137.7000 with virtual world wide coverage, and that is when I considered that high order mains harmonics may not be negligible when we are digging well down on conventional noise floors.
 
My present opinion is that the New Zealand mains network does have fairly tight frequency stability, but I'll look at doing further tests.  However, if the mains frequency has worse stability in other countries then "avoiding" mains harmonics becomes less of an attraction.  136 kHz is the 2720th harmonic of 50 Hz.  For harmonics to not sweep plus or minus 25 Hz means the stability would need to be better than about 0.009%
 
73, Bob ZL2CA
----- Original Message -----
Sent: Tuesday, March 25, 2003 2:43 AM
Subject: Re: LF: Selection of spot frequencies for DX

James Moritz wrote:
Not sure about ZL, but here in the UK the 50Hz nominal mains frequency drifts around quite a bit - perhaps a few tenths of 1%. Seen on a fairly coarse resolution spectrogram, the harmonics give a series of wavy lines spaced by 50 or 100Hz, which drift enough to give overlapping coverage during a period of a few minutes. But with very narrow resolution spectrograms, this just adds to the "wide band" noise level, rather than producing well-defined spectral lines, because the frequency does not remain within one FFT box for long enough to produce a line.
Just for curiosity, I visualized the 53rd harmonic of the 50 Hz here in my QTH,
and this is the result.  An unidentified signal is also present.

73  Alberto  I2PHD


JPEG image

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