A second, even more obvious, combination of available crystals fell out my fingers into the keyboard. Vy vy vy obvious, I even have both crystals in stock, for QRP and QRSS!
- 7059 => 21060 - 14000
Yes, there are cheap 14MHz crystals available (check here: http://www.reichelt.de/) and the 15m QRP-crystal should be in any of our parts collection anyway.
Here's, what I see, could be done:
A Pierce oscillator for 14MHz. This pulls the frequency up, so that we get into the 20m QRSS-range. For FSK, the 14MHz oscillator should be modulated. In practice, this is a ordinary 20m MEPT, no strings attached. Usually, one would expect the frequency to be 14000.800kHz
An oscillator with the 15m QRP crystal should be constructed such, that the frequency can be pulled downwards in a stable and controlled way to a frequency 21059.xxkHz. Let's make that 210593.
Now, all we need is a mixer, to subtract the 14MHz signal from the 21.059xMHz signal. Tiny downside here: the FSK will be inverted.
Such a design would provide a dual band MEPT. The thing, equipped with two finals and two filter stages, would even allow for simultaneous transmission on both bands.
A further idea is, to switch between the two bands according to the ambient light. This could be done by either the beacon keyer, a separate micro-controller or some analog contraption using OP-amps or transistors.
My personal favorite is the beacon keyer, since it would be rather simple to revert mark and space according to the band.
Now, where's my soldering iron???