Baluns: what they are and are they needed?

The word balun is a short form of BALanced to UNbalanced transformer. The full name gives a clue to what it does. To understand it fully we need to understand what in means in the context of antenna systems.

Antennas and similarly feeders can be either balanced or unbalanced. The IEEE Standard Dictionary of Electrical and Electronics Terms suggests; "Balanced" signifies electrically alike and symmetrical with respect to ground or arranged to provide conjugate conductors between certain sets of terminals. (4) Pertaining to a relationship between two or more objects that are alike or symmetrical in some respect."

Taking this definition we can see that a horizontal dipole is a balanced antenna, equally a doublet or a square loop could be described as a balanced antenna. The reality is that all antennas are, to some extent, unbalanced. Maybe your horizontal dipole slopes a bit, or perhaps one end is near a tree while the other is near your house, in either case the antenna will not be perfectly balanced. It might be close to balanced however.

We will most likely connect our transceiver to the antenna using a feed line of some sort. Feedline can also be balanced or unbalanced. Co-axial cable is unbalanced (not symmetrical) whereas open wire line is balanced.

We should be interested in what happens if we feed a balanced antenna with an unbalanced line. A common example of this feeding a horizontal dipole or inverted vee, with co-axial cable. In this case, inside the co-ax, all is well. The problem lies on the outside of the cable. The mismatch between the balanced antenna and the unbalanced cable allows an RF current to flow on the outside of the co-ax. Basically the shield of the co-ax becomes part of your antenna system. [Ref: W8JI]

But does this really matter?

It depends... there are several potential problems that can occur when your feedline is radiating due to feeding a balanced antenna with an unbalanced feeder:

The answer to the potential problems of common mode currents is a balun. A balun reduces common mode currents that can exist on your feedline. Some balun designs are a lot better than others at doing this. [Ref: G3TXQ]. The "balun of first resort" used by radio amateurs is often a few turns of co-ax at the feedpoint of the balanced antenna. This is a "solution" that I have never seen used on a professional HF radio station. The reason is, that it is often only marginally effective and its performance is hard to quantify: sometimes it works, and sometimes it doesn't. Such a balun is usually self-resonant in the HF band so it works well across a limited range of frequencies and hardly at all elsewhere.

A better design of 1:1 balun makes use of a transmission line wound around a ferrite core. This type of balun presents a high impedance (e.g. > 1000 Ohms) to common mode currents across a wide range of frequencies - easily across a decade (e.g. 3 MHz to 30 MHz). It gives predictable and repeatable performance, both highly desirable characteristics.

Making a balun is straightforward. The Yankee Clipper Contest Club has a useful Powerpoint that shows the process of making a high power balun of the type found on professional HF antennas.

Want to know more? The classic paper on baluns by Roy Lewellan is here.

Baluns: what are they and do you need one?

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