Dear Daves, LF Group,
Thanks for the overnight reports - the beacon was shut down at 0925utc here.
I see several spots in the database from AA1A - the first time M0BMU has
been copied in the US this season, thanks Dave.
In answer to YMC Dave's questions:
1) I don't know of an actual figure for required frequency stability, but
more than several Hz drift during a 2 minute period would be a problem. But
a drift of 1Hz in 2min is small enough. Some of the HF people have problems
with rigs heating and cooling between transmit and receive cycles and the
resulting cyclic drift. Using a converter and receiver with seperate
oscillators will certainly tend to multiply drift problems.
2) I have not so far had much success with internet based clock
synchronisation utilities, so at the moment I am just setting my PC clock
against an MSF clock at the start of each session - this is adequate, but my
PC does drift a few seconds overnight. Having said that, I was being
received OK by a number of other stations this morning. The transmission is
supposed to start on each even minute, and have a duration of 110.6 seconds.
3) WSPR seems to have good dynamic range, provided the RX and sound card
gains are adjusted so that overload does not occur. Certainly, it is
possible to receive rather weak signals giving just a faint trace in the
noise on the waterfall display at the same time as strong "red" traces
occur - a range of 40dB or more. Even if a strong signal does cause
overload, the randomised transmit periods mean that only a fraction of the
weak signal time slots will be affected. I have not noticed any problem with
decoding strong WSPR signals here, although there are not really any "very"
strong signals.
The "false decodes" occur from time to time, apparently more when
narrow-band QRM is present. If you read W1JT's description of the various JT
modes, he has made a trade-off between excluding decoded weak signals that
contain a lot of noise but could be correct, and falsely decoding noise that
by chance resembles a signal. In practice, they are not really an issue, due
to the fact that the callsign is encoded with other information (for WSPR
the locator and power output) and these would have to be consistent with the
callsign for the signal to be accepted as valid. Even if the falsely decoded
callsign is plausible, it is extremely unlikely that the locator would be
correct, and astronomically unlikely that the same decode would be generated
from noise at multiple receiving stations on multiple occasions. Using these
types of criteria, false decodes are periodically stripped out of the WSPR
database.
A number of people have had problems with strong signals failing to decode,
on HF as well as 500kHz. We did some investigation of this last winter;
there may be more than one cause, but one issue is that some sound
cards/operating systems appear to generate significant timing errors, which
over the relatively long transmission period of WSPR accumulate to mess
things up. This results in some receivers failing to decode some
transmitters. One way to test this is to receive a strong WSPR signal, but
then after about 40s or a minute from the start of a transmission disconnect
the audio to the PC or turn the gain down to zero. There is enough data in
the shortened signal to decode, but less time for timing errors to
accumulate. If this results in successful decoding, then a timing error of
this sort seems likely, which could be at either TX or RX. Weak signals may
decode fine, because parts of the signal are "missing" due to noise and
fading. So another possible test is to reduce gain until the strong signal
is just a weak trace on the waterfall in the receiver noise, and see if
correct decoding occurs then.
Cheers, Jim Moritz
73 de M0BMU
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