Thanks, all seems promising to me and I look forward to seeing it!
So far as I'm aware a mag loop, particularly on a lower HF band, could well
need retuning for a 3kHz change as might a heavily electrically loaded antenna.
While modern rig frequency changes are indeed likely to be quick there will
be a finite time involved for PLL's and the like to settle before the rig
will transmit. If an auto ATU is in use and that frequency change means a
new minimum SWR match solution comes in automatically then there's also a
finite time needed for that. In other words sequencing will likely become
critically important?
Maybe a special RC version could optionally upload sufficient data for
offline collision analysis?
Alan G0TLK, sent from my mobile device
On 2 September 2021 19:19:12 Phil Karn via wsjt-devel
<wsjt-devel@lists.sourceforge.net> wrote:
On 9/2/21 2:26 AM, alan2--- via wsjt-devel wrote:
Hi, I've been following this thread with interest and have a few
questions/comments please:
If the proposed protocol frequency changes are set to use the wider
bandwidth of a SDR receiver linked to a T/R switch with a standard voice
rig as Tx as I saw in one post, those frequency changes will presumably
need to use CAT control. Will that be reliably fast or stable enough with
such relatively frequent changes, for all rigs?
You would not need this to implement my original idea, which is to hop each
transmission *within* a single SSB bandwidth. The "hopping" would be done
in the transmit waveform generation software, and it is only within the
range that the receiver already demodulates everything anyway.
The only concern would be if the per-transmission hopping is performed over
a new, wider FT8 allocation that requires a SSB radio to be retuned beyond
a single SSB bandwidth. It would work best with a separate receive SDR able
to demodulate the entire thing at once, but even then the transmitter would
only have to move one or two SSB channels, and then only on each
transmission. I don't see this as a problem for any modern radio. Whether
it's a problem for very high-Q antennas I'll leave to the antenna experts.
How many antennas, and for what bands, have to be retuned for a change of 3
kHz?
Is there a way of getting some indication of how many collisions are
currently occurring in any user session, firstly to try and obtain some
real world data on how big and frequent the issue might be, and secondly if
possible what the decode conditions were? I base that on the vagaries of
HF propagation that I suspect might be the principal controlling factor and
of course are entirely unpredictable.
That's a *very* good question. We can infer some of this by looking at
receive SNRs when many transmissions were needed. If you need 5
transmissions to get through, and then you do with a high SNR, the cause
could be either a series of collisions or a very rapid propagation
improvement. Note that you don't need to look only at the transmissions
addressed to you; you can look at the SNRs of the same stations talking to
someone else (or calling CQ). And you can look at those sent after your own
QSO.
I point out that there may actually be a more serious "collision" problem
at a higher protocol level. If you call CQ, and then five stations answer
you, you can pick only one to answer. The other stations may keep calling
you, especially if they can't hear the station you're actually working, and
they may collide and/or cause QRM to other users. Because they get no
response from you, they may also think they're colliding when you're just
ignoring them. They might even increase power (a bad idea).
Per-transmission hopping will definitely reduce the pile-on effect that
causes so much QRM when a sought-after station calls CQ. Remember, he'll
hear you no matter what frequency you use within the SSB bandwidth. Think
of my idea as something like automated, random split frequency operation.
(There's a lot of wisdom and experience in conventional ham analog
operation. We should use as much of it as possible).
The pileup problem could also be addressed at a higher level by
automatically answering each calling station in turn, or even better by
answering them all *at the same time*, i.e., with multiple transmit
waveforms on separate frequencies. This latter approach would require a
*great* deal of care to avoid intermodulation distortion as the transmitter
would no longer see a single tone with a 1:1 peak-to-average power ratio. I
would only do this in a SDR transmitter carefully designed for the task.
There's already too much distortion in the common setup using analog audio
between computer and transmitter.
Are collisions an issue at VHF and above where propagation is different?
If this gets implemented it would be good to have it switchable in and out,
so users who are interested can compare what's happening.
I assume congestion is much lower at VHF, but repeated collisions on the
same frequency can still be a problem if you answer on the caller's
frequency, especially during a contest. So there's still a strong case to
be made for randomizing the frequency of each transmission. And of course
it should be a selectable option.
Phil
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