22 October 2007

Antenna tuners are reversible

I'm back on posting after a few weeks without any shack or lab activity.

I had few minutes to spare, so I wanted to do a measurement on my homebrew LC switched antenna tuner. I wanted to see what kind of impedance range it can match. How? Read on...

The LC circuit is a bidirectional quadripole, so it can be used in any direction: close one end on 50 ohm and measure R-jX on the other.

I used the MFJ-259B to do measurements, so I have no idea if "X" is inductive or capacitive. I closed the antenna side on a simple 50 ohm resistor (2x100 ohm in parallel) and flipped switches all around to see what the instrument showed.

On all HF I could read low R with highX (probably the equivalent of a short whip) as well as moderate R&X (a long non resonant, non half-wave, wire). I was concerned the capacitance swing was somehow wrong, but my feelings were wrong and the tuner actually does the job.