Baluns and ATUs

Hams often use an ATU with integrated balun as a general purpose device to adapt an antenna fed by two wire open feed line to deliver an unbalanced 50+j0 load required or preferred by common modern transceivers.

If feed line radiation on transmit or feed line pickup on receive is to be minimised, the objective is to make the currents in each conductor of the feed line equal in magnitude and phase, but opposite in direction.

This objective is best achieved with a balun with very high common mode impedance, a so called ‘current’ balun.

If an effective balun has almost equal but opposite currents flowing at its balanced terminals, any current paths between the output terminals of the bare balun and ground will compromise balance of the feed line currents in the general case. To some extent, such current paths are unavoidable as it is just not possible to build a practical balun that does not have some stray capacitance to ground, but any construction that contributes more stray capacitance to ground than necessary compromises the balun’s ability to force nearly equal but opposite line currents.

Placing the balun in a metal enclosure is arguably a compromise, and the best implementation might well use a non-conductive enclosure, and locate the balun some short distance (eg 300mm) from the ATU using a coaxial pigtail for connection.

One questions whether the recent ‘true balanced ATU’ products that incorporate an internal balun on the transceiver side of the impedance transforming network are optimised for feed line balance due to the considerable stray capacitance from network components to the conductive enclosure between the balun and the balanced line terminals. Though many reasons are proposed for this type of configuration, they don’t stand rigorous scrutiny.

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