Hi, we some times see drifting lines spaced 60 or 120 Hz apart that are
harmonics of the 60 Hz line frequency. The drift at 60 Hz (or 50 Hz in your
case) is very small but when multiplied 2 or 3 thousand times will show up.
I would not expect power supply problems with the transmitter to show up
this way. The sidebands should have the same stability as the line (though
100Hz from the carrier would be times 2 , 200Hz times 4 etc) unless they
were running on a backup generator that was having some problem holding the
load :-).
Sandy
WB5MMB
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From: Alan Melia[SMTP:[email protected]]
Reply To: [email protected]
Sent: Friday, February 04, 2000 4:41 PM
To: rsgb_lf_group
Subject: LF: 73.4kHz Rugby ...more thoughts
Hi All, yes Mike I do take on board that things have got considerably worse
during the last 12 months. I decided to listen tonight (Friday) to see if I
could detect any traces from you. In fact after logging one weak continuous
trace about long enough for a CQ or an over, I spotted some strange quirks
in the display. I spent the next hour monitoring the sidebands of Rugby.
The 100Hz sidebands are not I think anything at all to do with the power
supply. The transmission is shift keyed with a shift of 100Hz and the
sidebands are a function of the shift not hum from the PSU. So I am afraid
we are going to have to put up with them. I suspect though I do not have
any
recording to check, that the transmission has changed from FSK to PSK aand
this may account for some of the extra spread (?? conjecture) but.....
Has anyone actually looked carefully at the 100Hz lines?? On my dispay they
were zooming about all over the place. I have one trace with an 18Hz
'drift'
over a 4 minute period. Now it is my appreciation of angle modulation that
the spacing of the sidebands are determined by the modulation (increase the
mod frequency and they move apart) and their position is determined solely
by the carrier frequency (if the carrier drifts 10Hz all the sidebands will
move in the same direction by 10Hz)
{Please, someone, kick my rear if I have misunderstood my Bessel functions)
Unless I am mistaken what I see is that there is in fact a serious fault on
the transmission!!
It also occurs to me that is there is an instability manifesting itself as
a
drift over 10s of seconds there could also be 'noise' modulation.
As a result (with my tongue stuck firmly in my cheek) I intend to inform my
local RA office (which is a sub-office of Birminghan and so covers the
site)
in writing, of the faulty Rugby transmitting equipment, causing
interference
to amateur users. I know we have no protection against primary users, but
we
should not have to contend with badly maintained transmitters. I also
intend
to copy the letter to the RSGB and to the RA at Canary wharf. I will
suggest
that as this interference from an govenment transmission has persisted for
a
period which could be upwards of 12 months, perhaps they would consider
delaying the closure of the band to amateurs, to allow our experiments to
be
continued. (I am on good terms with the local RA 'boys' )
That should be good for a bit of a laugh. Before I send this missive on
Monday I would like confirmation that some of you can also see this
sideband
line 'drift', just to confirm that it is not a product of my own equipment.
It is interesting to note that my unsynchronised (VFO) signal generator
(TF2002B) stays within a Hz or so for hours on end.
I suspect if they clean it up then we will be able to work between the
tram-lines again. It is interesting that they were obviously looking for
something on Tuesday. You may remember that CFH went off for a day soon
after it reappeared. This was due to a power supply instability on the
exciter according to Jon and was creating all sorts of havoc. So the
professionals DO have problems.
Another 18 months on 73 yet?? well we can hope.
Cheers de Alan G3NYK
[email protected]
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