[Today’s run: 1.x mile with wife and dog]
I was reading an old article on varactor diodes in the QRP Classics book, written by Doug DeMaw of course.
I have a nice QRP rig, the Willamette by NT7S, I use it all the time. It has varactor diode tuning. It works great but the diodes change capacitance in a non-linear fashion. So my frequency dial starts out at 14.000, and the intervals between tick marks becomes smaller and smaller, all the way up to 14.090. I make the dial nicer looking by changing the way I mark it up. At the low end I mark every kHz and put a bigger mark on the 5’s. When that starts to scrunch I skip to putting in marks for every even kHz and the 5. At the highest end I just have 10’s and 5’s marked (between .060 and .090).
In the old days they used air-variable capacitors for tuning (a “bread slicer”), and they managed the tuning rate by making the fins of the capacitor a strange ‘D’ shape. The right shape would provide linear capacitance change over the range of motion of the capacitor elements.
So, I’ve been pondering how to linearize the varactor capacitance curve. Really haven’t found a solution yet. (I probably never will, seeing as how there have been a lot of people smarter than me cover this ground multiple times… as part of their jobs… over the years.)
But I thought it was a interesting problem. I’ll let you know if I think of anything.