Direct Energy, 2018a

74 4.3 Antenna Components and Denitions
Antennas may be used individually or as part of an array. Arrays may
also be driven or parasitic. In a driven array, all elements are connected
to the power supply or receiving circuitry [50]. In a parasitic array, oneor
more of the elements are parasitic and not connected [50]. Arrays are also
classied based on the direction of radiation compared to the axis of the
array. In a broadside array, radiation is mostly perpendicular to the axis of
the array while in an end re array, radiation is mostly along the direction
of the axis of the array [50].
A transmission line is a pair of conductors which is used to transmit
a signal and which is very long compared to the wavelength of the signal
being sent. Communications engineers and power systems engineers both
use the term transmission line, but they make dierent assumptions. To
a communications engineer, it is a long pair of conductors over which a
signal is sent. To a power systems engineer, it is a cable that is part of the
power grid. The communications denition will be used in this text. The
conductors of a transmission line may be a pair of parallel wires, they may
be a waveguide formed by a pair of parallel plates, they may be a coax
cable, or they may have another geometry. Coax cable is formed by a wire
and cylindrical tube separated by an insulator, both with the same axis,
so they are coaxial. For example, a coax cable connecting a transmitter
operating at a frequency of f =88MHz on the rst oor of a building and
an antenna on the top of the tenth oor of the building is a transmission
line because the length of the cable is long compared to the wavelength of
λ =3.4 m. As another example, a pair of wires connecting a transmitting
circuit operating at f =4GHz on one end of a printed circuit board and
an antenna on the other end 25 cm away is also a transmission line because
the length of the wires is long compared to the wavelength of λ =7.5 cm.
Some antennas have a balun. Balun is a contraction for balanced/ unbalanced.
It is used between balanced loads and unbalanced transmission
lines [15, p. 406] [50]. A typical transmission line, made up of a coax
cable, is constructed from an inner conductor and an outer conductor.
These conductors have dierent radii, so they have dierent impedances.
The transmission line is called unbalanced due to this impedance dierence.
Suppose that this transmission line is connected to a dipole antenna
formed from two symmetric conductors. The impedance of the two arms
of the dipole are equal, so we say that it is a balanced load. A balun can
used in this type of situation when a balanced antenna is connected to an
unbalanced transmission line. By properly choosing the impedance of a
balun, reections at the interface between the antenna and transmission
line can be reduced so that more energy gets to or from the antenna and
less remains stored in the transmission line.