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-------- Original Message --------
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<th valign="baseline" align="right" nowrap="nowrap">Subject: </th>
<td>Re: LF: Selection of spot frequencies for DX</td>
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<th valign="baseline" align="right" nowrap="nowrap">Date: </th>
<td>Tue, 25 Mar 2003 08:56:37 +1200</td>
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<th valign="baseline" align="right" nowrap="nowrap">From: </th>
<td>"Vernall" <a class="moz-txt-link-rfc2396E" href="mailto:vernall@xtra.co.nz"><vernall@xtra.co.nz></a></td>
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<th valign="baseline" align="right" nowrap="nowrap">Reply-To: </th>
<td><a class="moz-txt-link-abbreviated" href="mailto:rsgb_lf_group@blacksheep.org">rsgb_lf_group@blacksheep.org</a></td>
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<th valign="baseline" align="right" nowrap="nowrap">To: </th>
<td><a class="moz-txt-link-rfc2396E" href="mailto:rsgb_lf_group@blacksheep.org"><rsgb_lf_group@blacksheep.org></a></td>
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<th valign="baseline" align="right" nowrap="nowrap">References: </th>
<td><a class="moz-txt-link-rfc2396E" href="mailto:5.1.0.14.0.20030320183946.0260e158@gemini.herts.ac.uk"><5.1.0.14.0.20030320183946.0260e158@gemini.herts.ac.uk></a> <a class="moz-txt-link-rfc2396E" href="mailto:5.1.0.14.0.20030324122153.00abb360@gemini.herts.ac.uk"><5.1.0.14.0.20030324122153.00abb360@gemini.herts.ac.uk></a>
<a class="moz-txt-link-rfc2396E" href="mailto:3E7F197A.4060502@usa.net"><3E7F197A.4060502@usa.net></a></td>
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<div><font face="Arial" size="2">Hi Jim, Alberto, Rik and others,</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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:</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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"</font></div>
<div><font face="Arial" size="2">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.</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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. </font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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.</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">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%</font></div>
<div><font face="Arial" size="2"></font> </div>
<div><font face="Arial" size="2">73, Bob ZL2CA</font></div>
<blockquote
style="border-left: 2px solid rgb(0,0,0); padding-right: 0px; padding-left: 5px; margin-left: 5px; margin-right: 0px;">
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-stretch: normal; font-size-adjust: none;">-----
Original Message ----- </div>
<div
style="background: rgb(228,228,228) none repeat scroll 0%; font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-stretch: normal; font-size-adjust: none;"><b>From:</b>
<a title="dibene@usa.net" href="mailto:dibene@usa.net">Alberto di Bene</a>
</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-stretch: normal; font-size-adjust: none;"><b>To:</b>
<a title="rsgb_lf_group@blacksheep.org"
href="mailto:rsgb_lf_group@blacksheep.org">rsgb_lf_group@blacksheep.org</a>
</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-stretch: normal; font-size-adjust: none;"><b>Sent:</b>
Tuesday, March 25, 2003 2:43 AM</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-stretch: normal; font-size-adjust: none;"><b>Subject:</b>
Re: LF: Selection of spot frequencies for DX</div>
<div><font face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><font
face="Arial" size="2"></font><font face="Arial" size="2"></font><br>
</div>
James Moritz wrote:<br>
<blockquote
cite="mid5.1.0.14.0.20030324122153.00abb360@gemini.herts.ac.uk"
type="cite">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.</blockquote>
Just for curiosity, I visualized the 53rd harmonic of the 50 Hz here in
my QTH,<br>
and this is the result. An unidentified signal is also present.<br>
<br>
73 Alberto I2PHD<br>
<br>
<img height="161" alt="" src="cid:part1.03060002.05050108@verizon.net"
width="473">
<br>
</blockquote>
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