If I'm reading correctly, the "varactor" is a pair of plain old
rectifier diodes! I never would have thought of using power
diodes for this application. I guess I thought there would
have been way too much capacitance to be able to reasonably
resonate them at 100 MHz. Good stuff!

About 45 years ago, Popular Electronics had a construction project for an
FM-band transmitter. It used a reverse-biased silicon rectifier to modulate
the carrier.

I assume two back-to-back diodes increases the linearity of the modulation.