This common-mode current has three disadvantages. First, it radiates a signal that can distort the
radiation pattern of the antenna. For example, the symmetrical “figure eight” pattern of a dipole
can be distorted into a highly asymmetric blob who’s performance is difficult or impossible to
predict. Second, this radiation subtracts from the useful RF energy radiated to the receiving
station. Third, the RF on the outside of the shield can be conducted back into the transmitter,
causing problems such as distorted transmit audio.
Normal dipole
“figure eight”
radiation pattern
Distorted radiation
pattern due to
common-mode
coax radiation
0
90
180
270
0
90
180
270
The term “balun” stands for “balanced-unbalanced”. A balun is a matching transformer designed
to interface between unbalanced transmission lines and balanced antennas, or between the
unbalanced output section of a transmitter or tuner and a balanced transmission line. Some baluns
also act as an impedance matching transformer, converting the lower impedance of the transmitter
to the higher impedance of certain transmission lines. In principal, you should use a balun at the
feedpoint of a dipole where the coax is attached. However, in practice, the diameter of the coax is
usually such a small fraction of the feedline length that the effects can be largely ignored
A more significant problem is using ladder line to feed a high-impedance antenna such as a
folded dipole or loop. Ladder line has several advantages over coax that recommend it in some
applications. However, the transition from the unbalanced output section of the transmitter or
tuner to the balanced transmission line can create significant problems. A balun must be used in
this case, both to make the unbalanced-to-balanced transition, and also to match the higher
impedance if needed.
7
2
It is common to address this problem by making a choke at the feedpoint by coiling the coax.
