Line Bal Test Meter

Posted on Feb 7, 2014

A simple meter to check the balance of currents running in the two legs of a transmission line. It can be used to check the balance of currents between the inner conductor and the outer coaxial conductor in a coax cable as well as between the legs of an open wire pair. (Originally published in `Amateur Radio`, August 2009 - Some errors in

Line Bal Test Meter
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the circuit diagram first published are corrected here. The corrected circuit also follows in the September 2009 issue of "Amateur Radio") A typical amateur radio antenna installation makes use of a simple dipole or other balanced form of antenna fed via a coaxial transmission line. Because the line is unbalanced, some form of unbalanced to balanced coupling is normally necessary between the coaxial line and the antenna. Without this coupling, a condition is set up where currents running in the inner and outer legs of the coax line are unbalanced and a common mode or longitudinal current component is developed along the length of the line, causing radiation from the line. Apart from distorting the radiation pattern inherent to the antenna proper, it encourages annoying induction into equipment and wiring within the radio shack as well as on receiving encouraging induction of vertically polarised near field noise. A typical balancing interface is the choke balun which must have sufficient common mode rejection impedance to minimise the longitudinal current component. Whilst most radio amateurs possess an SWR meter which can be used in series with the coax line to check how well the antenna is matched to the 50 ohm line, it gives no indication that the currents running in the two legs of the line might be unbalanced. The SWR meter can show a perfect 1:1 SWR indicating that the antenna is loading the line with a resistance of...

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