Arvid Evans <arvevans@...>
There are several alternative ways to build various parts of the BITX
design. For my BITX40 I chose a PTO type oscillator for the VFO.
This uses a layout similar to that done by Jom - WA6OTP
<www.wa6otp.com\pto.htm> for the mechanical construction, but is a BJT
based Clapp oscillator circuit with emitter-follower VFO buffer. A 32
tpi (turns per inch) brass screw is threaded into a coil of #28 wound
on a soda straw and covered in hot-melt glue. The present tuning
range is about +350 KHz for my version. This covers all of 40 Meters
(7.0 - 7.3 MHz) in 28 turns of the knob. I am still tweeking this to
try for 10 KHz per turn, but that is not quite working yet.
A picture has been posted (see PHOTOS in the BITX20 forum pages under
K7HKL) to show the mechanical design. A schematic has not yet been
finalized because the oscillator is prototype and still being modified
for tuning range and temperature stability.