A Primer on Antennas

Figure 13.1 reminds us of how a radio wave travels through free space with an electric
field component and a magnetic field component. The distance from trough to trough
is the wavelength; the number of waves passing in Hz (cycles per second) is the frequency.
Antennas are used to transmit or receive these waves. Antennas are passive
components dimensioned to resonate at a particular frequency or band of frequencies.
In Chapter 1 we showed how wavelength decreases with frequency (see Table 1.1).
We normally design handset and base station antennas to resonate at fractions of a
wavelength. Antennas therefore become more compact as frequency increases, but
they also become less efficient and more subject to localized effects such as coupling
between antennas on a mast or between antennas and the mast or (in handsets) capacitive
coupling effects (the effect of our hand on the outside of the phone).
On handsets, fashion now determines either internal antennas or external stub
antennas, which may or may not be 1/4 wave or 1/8 wave. These are inefficient lossy
devices. A number of companies have developed proprietary techniques for improving
handset performance, for example, by using polarization diversity (capturing both vertical and horizontal plane energy) or spatial diversity (an antenna either end of the
handset). However, fundamental space constraints mean handset antennas are a serious
compromise in terms of performance.
As antennas are passive devices, they can only radiate the same amount of energy
that is supplied to them. The fundamental reference antenna is the isotropic radiator—a
theoretical antenna that radiates a total sphere of energy. An antenna is said to have
gain if it is dimensioned to focus or concentrate this energy into a specific pattern,
direction, or beam. The gain is the ratio of the field strength that would be received at
a specific point from the isotropic radiator to the field strength that is received at the
same point from the directional antenna. The gain is dimensioned in dBi (dB isotropic).
A practical reference antenna is the quarter-wave dipole, and the gain of a directional
antenna may also be expressed in dBd (dB referenced to a quarter-wave dipole). Effective
isotropic radiated power (EIRP) is the product of the transmitter power and the
gain of the transmit antenna. It is expressed in dBW, where 0 dB = 1 W.
We have said there is not much we can do to improve handset antenna performance.
Typically, handset antennas show a negative gain—a loss of 1 or 2 dB, or more. Base
station antennas, however, give us much more potential for improvement or, rather,
producing gain where we need it. Remember: We are also particularly interested in
being able to null out unwanted interference that adds unnecessarily to the noise floor
of our Node B receiver.
Antenna design principles have not changed much in 70 years. Let’s just review
some of the basics. 299