[Meteor-Scatter] Using WSJT/FSK441, advice please

Wed, 30 Jul 2003 10:09:00 -0400

Hello Steven.  One thing that we learn to do is to pay close attention to
the spectrum display (right hand window), especially when getting pings with
no decode.  Everyone does not have good frequency calibration.  If the
signal you are receiving is more than about 250 Hz off frequency, the
decoder in the WSJT program cannot process it and may report inaccurate DF
values as well.

For instance, suppose the signal is off frequency by +400 hz.  This offsets
the tones so that the lower 3 tones are nearly aligned with the upper three
tone slots.  WSJT is likely to report a good DF in this case as it does not
see the highest tone at all and the others are all close to the expected
frequency.  A similar situation happens with the received signal is low in
frequency.

For weak pings, it is almost impossible to tell that the frequency is off by
the sound heard in your speaker or headphones.  In fact it takes some time
and experience before you will begin to recognize the sound of 'too high' or
'too low' even with strong pings.  Thus the spectrum display is the only
tool you have to decipher this condition.

Looking at the spectrum display, you should observe a significant 'spike' of
signal aligned close to each of the vertical yellow lines at the top of the
spectrum window.  But if the DF was off by +100, for example, then you would
observe these spikes to be about 1/4 of the way between each line and the
next line to the right side of the display.

But when the signal is off by about +400 hz, then the top tone is usually
completely suppressed by the receive filters and will not show in the
spectrum display at all.  But you will then observe that the lowest 'spike'
is missing.  That is your clue that the station may be way high in
frequency.  It is best to observe it several times before making
adjustments, because sometimes the message being sent will result in not
much usage of that lowest (or highest) tone.  But if it happens
consistently, and you are not getting any good decodes, then it will be a
good idea to adjust your receiver offset to move the spikes down or up so
that they align with the lower or higher set of the yellow lines.

If the station is off by more than 4-600 hertz, it becomes much harder to
analyze and you may have to go hunting by trying +500, +1000, -500, -1000
etc until something starts to make sense.

Of course the problems you (and Dom) are reporting could be not due to
frequency problems at all, but you need to be aware of this issue in case
you are just having a bad run of luck with several off frequency signals.

Hope it helps.
73, Russ K2TXB

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Stephen Ayling
> Sent: Wednesday, July 30, 2003 6:39 AM
> To: [email protected]
> Subject: [Meteor-Scatter] Using WSJT/FSK441, advice please
>
>
> Hello Everyone,
> About 2 weeks ago I began to use WSJT in FSK441 mode for meteor
> scatter on 2
> metres, fortunately it seems, also coinciding with a period of good
> reflections, and I completed qsos easily with several stations at good
> distances. As a result of this initial flush of success I thought all was
> working satisfactorily, despite my setup being rather on the minimalistic
> side for m-s working.
> Subsequently conditions seemed slowly to deteriorate, infrequent
> pings which
> were short and weak, and I was able to decode only nonsense from
> them, even
> though my settings in WSJT should have resulted in at least some fairly
> reliable decodes. This situation continued every day, and I began
> to realise
> that all was not OK. Even though I changed to another computer with
> different OS, processor and RAM, nothing improved; pings, though  above
> noise, decoded gibberish.
> Then I analysed the captured .wav files from the first successful qsos,
> pings were generally 10dB plus above noise, and duration several
> hundreds of
> milliseconds; so why was I not decoding properly pings
> approximately in the
> range, say  2-8dB and 20-100mS long, even though, as I have already said,
> program settings, W, S, DF, Tol, etc suggested I should be able to ?
> Somewhere in my setup there must be a lack of sensitivity. Pings either
> strong or long decoding OK; but pings either weak or short not doing so.
> My 2 metre transceiver is the Yaesu FT290R. On this rig the agc is fixed.
> There is no agc switch and no possibility to select different agc
> time-constants, or indeed switch it OFF. Is this the problem ?
> I would welcome any comments and advice on the correct use of agc,
> particularly from anyone who uses the FT290R with WSJT/FSK441.
>
> My vy 73 de Steve, SO5AS, ( KO02jd ).
>
>
>
>
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