Dear LF Group,
I seem to be the only station so far that has had some success with both
WOLF BPSK and 7FSK modes over transatlantic paths, so here is my comparison:
W!TAG was able to get perfect copy on my WOLF signal, so BPSK definitely
does work for DX communication; however, he remains the only transatlantic
station to do so, in spite of weeks of trying. In part this can be put down
to the varagies of propagation, but also many people struggled to get WOLF
to work well. The reason for this seems to be mainly the requirement for
accurate frequency and timing calibration (of the order of 1ppm), not a
trivial problem to overcome since the equipment most people are using was
not really designed with this in mind. Operation of the software and
hardware is not intuitive like QRSS/spectrogram programs.Transmitting using
this mode is relatively difficult too; although very simple QRP
transmitters are possible, envelope shaping is required for higher powers
to avoid causing unacceptable QRM, making high power transmitters
significantly more complex. In the context of the European 136k band, the
bandwidth of some 10s of Hz required by Wolf would be a problem if the mode
was being used by several stations simultaneously. None of these are
fundamental limitations - ingenious frequency calibration schemes have been
devised, some success has been had with modified "variable phase" keying to
control sidebands without amplitude modulation, it would be possible to use
narrower bandwidths for BPSK too. However, BPSK does require significant
time and effort on the technical side to get working.
I would not claim any great technical superiority of 7FSK over other modes,
but from a pragmatic viewpoint it has several good points. It can be
generated by a simple extension of existing DFCW techniques, and received
using the spectrogram software that people are familiar with. The frequency
accuracy required is much more relaxed (of the order of 10ppm on receive),
and timing accuracy isn't really an issue. The bandwidth, although greater
than that of QRSS, is small enough. In its short history, readable 7FSK has
already been received by a number of transatlantic stations, so it
definitely works too. The big advantage is of course in the increased speed
compared to QRSS.
As for the SNR vs. bandwidth considerations, other subscribers to this
reflector have more expertise than me - but I would observe the following:
Both QRSS and 7FSK are made up of dots, so the probability of any one dot
being corrupted by noise is the same. However, 7FSK transmits on 1 of 7
possible frequencies, while QRSS uses only one, so it is 7 times more
likely that a corrupted dot will appear in a 7FSK signal in a given period
of time. However, fewer dots are required by 7FSK to send the same number
of characters by a factor of about 5.5, so the relative probability of a
character being corrupted in 7FSK compared to QRSS would be 7/5.5, ie. 1.3
times as likely; not very different - about 1dB. I realise this is only
approximately true, not considering redundancy and so on, and would apply
only when the ratio of corrupted dots to uncorrupted is small, but you
would need this anyway for a QSO, and the tests over Christmas and before
show that these conditions do exist fairly often.
I think long-distance LF communications can be done with either of these
modes, and probably many others too. Theoretically, I think the more
sophisticated modes like BPSK will win out, but will always have limited
popularity due to the more elaborate technical requirements. A mode like
7FSK may not ultimately be as good, but it is quick and easy to get going
and operate. It is interesting that in the US Lowfer community this winter
has seen an explosion in the number of QRSS beacons and reception reports,
while last winter WOLF BPSK was a la mode. The distances being covered
don't seem to be much different, but more people are getting results. Where
there is an unpredictable element involved, like propagation, the chances
of the success of any particular mode are going to be tied up with how
many people have the capability and the inclination to use it - a
technically mediocre mode in use by 100 stations will probably get more
positive results than an excellent one being used by only two stations.
This is no excuse for complacency of course, but factors like ease of
implementation, compatibility, inertia and "does it match the curtains?"
all comes in to how successful something is - just look at the history of
the personal computer!
Cheers, Jim Moritz
73 de M0BMU
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