Tesla (No, not the car)

[NZ0I]

I've been recently diverted to another somewhat related project: a loaded vertical dipole antenna design for 80m ARDF fox transmitters. I've been learning lots of cool stuff. When not working on that I'm continuing progress on the dual-band transmitter. Meanwhile Jerry's been involved in testing and characterizing the experimental 2-band receiver model.

While working on the loaded vertical dipole I encountered the need for an accurate online inductance calculator. Eventually I found one: http://hamwaves.com/antennas/inductance.html

Reading through some of the things that differentiate this inductance calculator from the rest, I noticed a reference to a Tesla coil (with a cool picture) most of the way down the calculator's web page. That picture reminded me of an experience that Jerry related to me a few months ago, in which a QRP fox transmitter he was working on (the transmitter was powered on and transmitting at the time) suddenly "exploded" giving him a painful RF burn to his hand. The apparent explosion was not destructive (other than to Jerry) and the transmitter continued to function properly with no repairs required - aside from some bandaged fingers.

The exact cause of the apparent discharge wasn't clear, but presumably resulted from a high-voltage arching near the transmitter's output filter. Perhaps a self-resonant condition as described on the inductance calculator site is to blame?

Some lessons to be learned: 1) an inductor's inductance, and hence the Q-factor, near resonance are enormous! and 2) Be careful when working around RF signals, even low power ones!