[Lowfer] Re:CWSpacings and QRSS Considerations

[Alan Melia]

Hi Stan, I am not sure I have read all the comments on this so please excuse
if I get the wrong slant.

After just 6 ot 7 years of looking for very weak stations, and beacons I
would conform that a steady carrier is almost mandatory. There may be higher
speed CW ID in it at intervals but you must appreciate this reduces the
average ERP so could look like a 6dB or more fade. At first sight that
doesnt seem important but when you realise that 3 to 4 dB in ARGO will take
you from "solid" to not detected it could be important. Very accurate
frequency setting is useful, but for identification of a generally "below
the noise" signal there is nothing to beat a small systematic frequency
shift. Most products and spurii just show as stable lines, the shift is a
good indication of a "man-made" signal .....it does not need to be keyed
with the ID even, just slow enough to be detected with the slowest
waterfall.

If propagtion study is the obective, you need to think carefully about what
you monitor. Little useful information can be gained from recording the time
when a signal happens to pop above the noise level. You really need to have
a more continuous record of the signal even if it is below the level that
will carry useful information. You need ideally to know how far below
detection it is....difficult.

I like to separate out fading from propagation, multipath fading can occur
at any state of propagation. Shall we say that on MF and LF propagation is
defined by the excess path attenuation (over "spreading loss"), and fading
is imposed on top of that my variations in phase of arrival of signals over
two different paths. The depth of the fade is purely a measure of how
similar are the levels (losses) on the two paths.....a 20dB fade means the
signals are within 90% the same strength. Thus the only useful value is the
peak level. If the two paths have the same losses this will at best be 6dB
above the strength on a single path.

Whilst subjectively MF does seem yo have very short fade periods, a test I
did recently over a 700km path showed extictions for 30 to 45 seconds, which
might be thought to be a disaster, but there where hour-long periods with no
significant fades later in the night. It may be that in the evening when
most hand keyed operators are listening, the ionosphere is still moving
significantly, so deep fades are more noticable and "serious" to information
exchange. Later in the night this may settle leading to better chances for
very slow "modulation"

It would seem that to beat the fading, a form of "ARQ" would be the best
system for information exchange, simple repetition would not work too well.

This discussion of modes on MF is interesting as our power level (at present
in the UK on 500kHz ) do not allow for any fading margin, as would typical
commercial power levels at these frequencies, hence a lot of "drop-out" is
experienced.

It would seem ( so far) that my observations on signal absorption at night
in the D-layer ( as a result of Geomagnetic activity) are applicable to
500kHz as well

Cheers de Alan G3NYK