Hello All

The method of using an HF frequency, dividing down then using high power
fets in class D or E then a LPF is not totally satisfactory and only permits
cw mode.
Keying this type of tx is also a problem as described in recent discussions
on the reflector plus the problems suppressing the odd harmonics 3/5/7 th
etc because of the square waves inherent in the design. Some of these
problems are being investigated and rectified but it still leaves a non
linear output and suitable only for cw or rtty.
This approach is used by most and  was a quick fix to get most of us going
on 73 and 136 khz  but maybe the time has come for a more traditional
approach like a linear  transvertor used in conjunction with an HF
transceiver and a high power linear amplifier with appropriate bias
switching for class C for more efficiency on cw.
This would enable us to use any mode generated by the hf transceiver plus
any other mode generated by a computer ie psk 31 and other psk variations.
It also solves all the keying problems.
The disatvantage is of course that with a linear approach you only get 50%
efficiency compared with 90% with the switched mode system.
I am currently thinking of progressing along these lines and building
something suitable when time permits.
The recent discussion on this media about SUPER amplifiers is very
interesting and am awaiting the big thump from one of these on 73/136 soon.
de G3KEV