In the course of antenna building and designing, we run into an interesting phenomenon involving balanced antennas and unbalanced cables. This can in turn lead to problems with the cable radiating, which is why when testing antennas ferrite is often added to the coax.
A balanced antenna and a balanced cable
To understand what is happening with a balanced antenna, consider a correctly designed dipole. The dipole is a very simple antenna, consisting of two arms in a straight line, with the arms almost touching in the middle. The signal is taken at the point where the two arms almost meet.
To simplify what happens, it is close enough to say that the RF signal flows down one arm, through the receiver attached at the center, and out the other arm. This makes the dipole a balanced antenna. After all, at the center point you have the two equal signals induced on both arms flowing in opposite directions; one is entering the receiver, the other leaving. The two currents then balance.
If we attach this dipole to the receiver via a twin-lead line, this symmetry is maintained, for the two parallel wires of the twin-lead line have current flowing in opposite directions through them. Yes, these wires can and do radiate like an antenna, but since the radiating currents are equal and flowing in opposite directions, this radiation is canceled out, and the cable acts like a cable – not an antenna.
A balanced antenna and an unbalanced cable
But what happens if we connect the dipole to an unbalanced cable? Doing this rather drastically alters the scenario. While what is coming out of the dipole by itself is indeed balanced, connecting it to a coax (that is an unbalanced cable) rather changes things.
A coax consists of a center conductor surrounded by a conductive shield. Both conductors are actively used to carry the signal. If we connect this to the dipole, something happens. The center conductor cannot radiate, any radiation being blocked by the shield. The shield, however, has nothing preventing it from radiating, so it does so. The net result is that one arm of the dipole – the one connected to the center conductor – acts the same as always, but the shield of the cable acts as an extension to the arm of the dipole connected to it. This means that the cable is radiating, and the antenna is now grossly imbalanced, and acts rather like a lopsided dipole, which performance is usually highly undesirable.
An unbalanced antenna and an unbalanced cable
Before we go any further, it is important to note that a coax can work very well with some antennas.
The monopole, for instance, consists of a vertical wire in conjunction with a large ground plane. This is an unbalanced antenna, for the ground plane is typically much larger than the antenna proper. If we connect the center conductor of a coax to the monopole wire, and the coax shield to the ground plane, we find that nothing untoward happens. The shield simply acts like an extension of the already large ground plane, and things work well.
The main thing that can go wrong here is when you try to test a monopole with a too-small ground plane. Then, the test coax will act like part of the ground plane, leading to erroneous data by making the ground plane look bigger and better than it truly is.
The solution: Balun
So what do you do to allow a balanced antenna to be used with an unbalanced cable? The solution is a balun. Balun stands for BALenced to UNbalanced, and it does just what you might expect from its name. There are many kinds of baluns, but they all have the same purpose: to allow for a smooth transition between a balanced antenna and an unbalanced cable (or vice versa).
One simple type of balun is a transformer. This effectively isolates the antenna from the cable, preventing the cable from greatly affecting the symmetry of the antenna.
Another kind of balun is a common mode choke. A common mode choke is a rather clever device consisting of two chokes wound in the same direction on the same core. Each lead of the dipole goes through one of the chokes. This would seem like it would tend to attenuate the RF, but it actually works well. As the dipole is balanced, two equal but opposite currents are set up in the chokes. As the chokes share the same core, these currents cancel, effectively taking the choke out of the circuit as far as the dipole is concerned.
However, if presented with an unbalanced signal, the choke acts just like a choke should and attenuates this unbalanced signal, there being no longer complete cancellation of the induced fields within the choke. In this fashion, the choke prevents any unbalanced signals from permeating the dipole while allowing balanced signals to pass unattenuated.
Another simple form of balun is to wind the coax connecting to the balanced antenna (like our dipole example) around a ferrite core. While crude, this has the effect of adding RF-attenuating inductance to the shield of the coax, preventing the coax from radiating. An equivalent (though less pronounced) effect is obtained by sliding ferrite around the coax, as is often done in antenna testing for this very reason.
There are other types of baluns; however, all baluns serve the same function: adapting a balanced system to an unbalanced one.
ARSI 
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