3698 was chosen by CWB for those reasons:

  • to be as close to the top of the band as possible
  • to be clear from QRM especially SSB users
  • to be almost always empty, not in use
  • improved propagation compared to lower band

The beacon on 3699 is very helpful in this respect. While it would be too close to go on 3699.5 due to people maybe listening to the practice in wider filters, 1kHz is absolutely ample room for CW especially given that most RX these days not only have narrow filters, but also can swap side bands. So, monitoring 3698 you can choose to at the same time keep an eye on conditions by listening upper and wide, or those listening to 3699 if there is an emergency announcement on 3698 they may hear it if listening wide on lower (most CW is lower on old rigs, and/or below 10MHz), but also can easily switch in a filter and be clear of any QRM whatsoever.

All activity on every HF band in CW when not crowded 1kHz is ample seperation, when crowded as little as 200Hz is ample seperation. This frequency is less likely to be in use at any random moment than others on the 80m band. That all being said as to the reasons behind choosing 3698, it does not need to be used as a QSO frequency, it can be used for calling then QSY down.

It is a go-to frequency in EMERGENCIES and in Emergency Communication Exercises. Not a general calling frequency.

In Proper Perspective

A 1kHz seperation of CW signals is the same as a 15kHz seperation on SSB, it is overkill. If anyone doesn't know why:

A CW signal is less than 200Hz wide. An SSB signal is at the most 3kHz wide. That is a bandwidth difference of 15 times. 5 CW circuits can fit comfortably within 1kHz. 5 SSB circuits can fit comfortably only within 15kHz. SSB stations therefore usually position themselves 3 to 5kHz apart, while it is common to find CW signals just 200Hz apart. Even though an SSB signal 1kHz apart can be distinguished, but QRM would be considerable. Likewise if a CW signal is 200Hz apart and is very strong and there is no narrow filter and AGC is on, copy of the weak signal may be difficult. Thefore most CW stations tend to place themselves at least 300Hz apart where possible. In CW the brain can seperate signals that are even just 50Hz or less apart. CW filters of 300Hz and less are very common. On HF bands it is common for CW signals to be 500Hz apart, and in contests much less, often even 100Hz. A CW signal that is 1kHz away will not be heard by any amateur RX of worth, no matter how strong it is, if the correct CW side band is chosen even without filtering, or otherwise if a normal 500Hz filter is used. In addition notches allow CW signals to be easily notch out that are off frequency.

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