Theory of operation:
Virtually all amateur transmitters and transceivers in use today, and all LDG tuners, are designed
with an unbalanced output. This means that the output signal appears on a single conductor
referenced to a grounded second conductor. Most amateur transmitters use a “pi network” output,
and LDG tuners employ a “switched-L” network, both of which feature an unbalance output.
These output sections are designed to be used with coaxial cable, an unbalanced transmission
line. The signal is carried on the center conductor, and the reference ground is the
inside
surface
; the two signals are equal in amplitude but 180 degrees out of phase. These two out-
of-phase signals cancel each other out along the length of the coax, preventing it from radiating,
and delivering almost all of the RF energy to the antenna feedpoint.
Many commonly used amateur antennas are balanced antennas; examples include a dipole, and
the driven element of a Yagi-Uda array (a “Yagi” antenna). A balanced antenna features two
equal electrical elements driven 180 degrees out of phase. It is common amateur practice to feed
such antennas directly with coaxial cable, an unbalanced transmission line. At the interface
between the unbalanced transmission line and the balanced antenna, an extraneous current flow is
created on the
outside
of the coax shield. This is called a “common-mode” current, as it is in the
same direction as the signal on the center conductor.
~
Transmitter
Coax
1/2 wavelength dipole
Center conductor current
Inside shield current
Outside “common-mode”
shield current due to
balanced-to-unbalanced
transition
6
1
The so-called “skin effect” confines RF currents to the surface of conductors.
