Spade tuning was used in at least one early regenerative radio receiver. That is a sheet of brass or copper was rotated across the surface of a pancake coil to vary the inductance. I tried the same idea myself using a sheet of aluminium between two parallel pancake coils connected in series. That arrangement should allow a rather large reduction in inductance. I found that I could get a large tuning range doing that. Such an arrangement of pancake coils can also be used as a coin detector.

I have a book on regenerative radio receivers here:http://www.webstore.com/91359,owner_id,other_items

and some other things here:

http://circuitprofile.scienceontheweb.net/

http://code.google.com/p/lemontree/downloads/list

Probably better to go with the later VXO receiver design, but using the PTO rather than the VXO. That receiver has a much better front-end. Adding the diplexer mentioned in the article would improve its performance too.

If I build a PTO again I'll likely use a more conventional oscillator configuration. Also, for 40 metres and above I'd break up the inductor into a fixed toroid plus the PTO coil with some experimentation to get a good bandspread.

thanks for this PTO insight.I followed and got it built.It is a very nice and simple design.Might even build the featured receiver .

thanks,

73