The transmitter had an unbalanced output. The antenna I was using was an 80 meter doublet fed with 300 ohm transmission line. I needed some way to couple the unbalanced output of the transmitter to the balanced 300 ohm transmission line. To do this I built an antenna coupler that used a link coupled coil with a tapped secondary. The transmission line was connected to various combinations of taps until the transmitter loaded properly.