Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

Fifth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Installation / testing


The balun packaged in a non-conductive housing was designed to have minimal stray capacitance to ground to minimise common mode current with asymmetric loads.


Above, the balun is attached to the exterior side of the antenna feed entrance panel using a male to male N adapter, done up very tight. The feed line connections are liberally coated with marine grease to prevent ingress of water and oxygen, a measure to reduce corrosion.

Common mode current


Common mode current was measured at 7MHz at 100W, and is very low, about half that achieved with the ATR-30 (internal balun) on the other side of the antenna feed entrance panel using the medium sized feed through insulators seen on the panel.

In this configuration, the fly lead from the ATU to the antenna feed entrance panel is 0.5m, and RG142 cable is used for voltage withstand.


Above is a thermograph of the balun after half an hour of 100W on and off a minute about. The antenna feed entrance panel is the bluish area, and the balun just right of the writing is barely warmer than the panel.


  • Duffy, O. Mar 2013. Bowral G5RV for local contacts. (offline).
  • ———. 2012. Balanced ATUs and common mode current.