1:1 Trifilar True (Current) Balun

What it does

Connects an unbalanced 50 Ω coax line to a balanced 50 Ω load, such as a dipole, and forces equal and opposite currents in the two output leads.

How it works

Three windings on one core. Two carry the signal to each side of the load; the third ties the output's virtual center to ground reference, so common-mode current sees a high impedance. Performance depends strongly on coupling: with k = 0.999 the simulated loss is 0.02 dB at 14 MHz, but with k = 0.99 it rises to 1.4 dB (simulated, both).

Building it

• Core and turns aren't in EMRFD. QRPedia's pick is 10 trifilar turns on an FT-37-43, for receive or low power.
• Twist the three wires together before winding to keep coupling tight. Pins 1, 3 and 5 are the dot ends.

Tuning and testing

Terminate the output in 50 Ω and measure return loss at the input with a return-loss bridge across the bands.