Bill, This actually kept me up last night!
For “Normal” FT8 operation, your solution makes 100% perfect sense. The only downside I can see is that you lose that 200 Hz or 300 Hz at the top end of the band. That is a very fair trade-off. But, in DXpedition mode with a Fox and Hounds, it gets murkier. First, consider the Hound end. Let’s assume that the Hound knows that his/her radio rolls off at the low end and really should be offset by 200 Hz, so s/he shifts the VFO by 200 Hz. If the Fox is using the default TX frequency of 300 Hz, that fools the Hound’s system into believing that the Fox is now at 500 Hz, where the Hound’s receiver hears well. That all should work well. But, imagine that the Fox has five streams running. The lowest would appear at 500 Hz for the Hound, with subsequent ones beginning at 560 Hz, 620 Hz, 680 Hz, and 740 Hz. If the Hound does not get through in three tries or the Fox fades so that Hound doesn't receive a transmission from the Hound, won’t the Hound get moved to the “repechage” band to get another couple minutes of attempts to send the signal report? When the Fox is at 300 Hz and up, that repechage band is 300 Hz above the Fox TX frequencies, where the Fox can still hear callers. BUT, if the Fox is at 500 Hz TX or above, won’t the Hound get moved 300 Hz *below* the Fox TX frequencies? As in, perhaps, 0 Hz relative to the bottom of FT8 channel? In this case, the Fox would never be able to hear the Hound there. So, when a Hound offsets her/his frequency by 200 Hz to accommodate the receiver low frequency roll-off, s/he essentially eliminates use of the repechage band. That may be the price a Hound pays for not having a properly working FT8 radio. That and remembering to call above 1200 Hz. Now, think about when a Fox is using a radio where the receive passband rolls off below 400 or so Hz, as is all too often the case. Just look at the most popular radios used on DXpeditions. With a single transmit stream, the Hounds will move to 300 Hz to send their reports. Unless the Hound is at least as many dB stronger than the filter roll-off, the Hound might get lost in the noise when s/he responds. That leads to multiple decode cycles being used, tying up the Fox. The rate drops, not only due to the immediate need for multiple decode cycles that otherwise might not be necessary, but also because the Hound likely will start the same dance again to initiate a contact. That presumes that the Fox does not log the contact while the Hound is in the repechage zone. (My observation is that most Hounds just go off and call again. Why? Dunno. I also notice that very few repechage contacts get made for whatever reason.) I’ve observed the above many, many times. Even just yesterday. It seems to me that there’s a few solutions for this. One is that all Hounds should be armed for bear with large antennas and big power. That way, they’ll overcome the Fox’s receiver filter roll-off. Of course, that precludes weaker signal contacts for tough propagation paths where the Hound only merits a -18 signal report from the Fox to begin with. Plus, it’s kind of against the spirit of FT8 being a weak signal mode usable by stations with more moderate power. A second would be for Foxes to all use radios that receive well to below 300 Hz. Problem solved for the Fox end, which really affects all the Hounds calling, too. That still puts the burden on all the Hounds to have receivers that are flat to below 300 Hz or shift their VFO by 200 Hz and sacrifice the ability to use the repechage band . Maybe that’s just tough on the Hounds who don’t know enough to do that. A third solution is for the Fox to move his or her TX frequency to 700 Hz. With five streams, the topmost one would begin at 940 Hz - still good. Essentially every Hound would have no problem, even if their receiver rolled off below 400 Hz. Any Hound that gets moved to the repechage band would get placed 300 Hz below the Fox TX frequency, which means they’d start at 400 Hz - pretty much within almost all Fox receivers, even with low frequency roll-off. (Isn’t that how it works?) The problem with all of these is that it requires cooperation from everybody to make it work. People would need to read the manual and know where their receiver rolls off. That’s easy enough to discover, but how many FT8 users actually do? Then, they’d need to adjust, repair, or replace their radio to have no meaningful low frequency receiver roll-off. My own solution, if I was king of the WSJT universe, would be to make the default FT8 Fox TX frequency 700 Hz. Almost every Fox just uses the default frequency. Why should they know any different? They’re DXpeditioners, after all, focused on going to some rare place with all the associated challenges and working as many stations as they can. They presume that 300 Hz is optimum - why else would it be chosen? There is nothing magical about 300 Hz. It was a seemingly well thought out arbitrary selection, but hardly a frequency that has significance based on the work of Kepler, Shannon, or the rest. In fact, you could make an argument in favor of 700 Hz just on the basis of spectrum use, since the somewhat rarely used repechage band is now below the Fox rather than between it and the throng of calling Hounds. That may be a trivial consideration, of course. If you believe that only weak Hounds are the ones who get placed in the repechage band, there’s a greater likelihood that they will get interfered with due to the second audio frequency harmonic of loud responding Hounds when the repechage band is above the Fox TX frequencies rather than below it. I don’t know the statistics on this one, of course, so it may be trivial again. But, it’s not a negative. The best part of making 700 Hz the default Fox TX frequency is that it doesn’t require any change in much of anything, other than the actual change of a 3 to a 7 in the code and a couple mentions in the documentation. 100% backward compatible, as best I can tell. That’s my argument. I won’t press on this further. Thanks for reading! 73, Clarke K1JX > On Oct 16, 2021, at 11:12 AM, Bill Somerville via wsjt-devel > <wsjt-devel@lists.sourceforge.net> wrote: > > HI Clarke, > since the K3 has the ability to us a 4,000 Hz Rx filter, I would instead > recommend tuning the VFO down 200 Hz or 300 Hz as a far better solution. You > can even adjust the working frequencies to those offsets. I suggest this for > a couple reasons: > > Switching between data modes and SSB will be much simpler as SSB is used for > SSB and DATA A is used for data modes, > Using the ESSB option is likely to allow any unwanted audio harmonics to be > transmitted, the WSJT-X Split Operating relies on a Tx filter cut-off at just > below 3,000 Hz which will not be the case if you select ESSB transmit. > You say that you might forget to do the setup for a slightly shifted VFO > frequency, yet you suggest several other settings changes that are needed to > operate the way you suggest. Simply change the working frequency values in > "Settings->Frequencies" and you are done with my recommendations, there's no > down side IMHO. > > 73 > Bill > G4WJS. > > _______________________________________________ > wsjt-devel mailing list > wsjt-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/wsjt-devel
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