Page 439 - Complete Wireless Design

Wireless Issues

438 Chapter Ten

As the electromagnetic energy travels through space after leaving the trans-
mitting antenna, it will eventually cut the elements of a receiving antenna.
This will induce a tiny voltage, which must then be heavily amplified and fil-
tered by the receiver to obtain the desired demodulated signal information at
the required amplitude and SNR.
The following is some common antenna terminology:
Beamwidth—The number of degrees, in the horizontal, of the main beam at
its 3-dB-down points. Intimately linked with antenna gain, and measured in
degrees.
Directivity—The power in the main beam of an antenna compared to an
isotropic. Measured as a ratio.
G/T ratio—A figure of merit for microwave satellite receivers, G being the
gain of the receiver’s antenna and T being the system’s noise temperature, or
the ratio of ground (noise) temperature to antenna gain. Minimizing
sidelobes maximizes this G/T ratio.
Gain—The gain supplied by an antenna over that of a (typically) isotropic
antenna, and is measured in dBi. High gain, high directivity, and large
electrical size (to wavelength) are interdependent. It is impossible to have a
high-gain omnidirectional antenna, or a high-gain antenna that is
electrically small. As the gain of an antenna is increased, the beamwidth
must decrease; a general rule is that doubling the antenna’s physical
elements will double the antenna’s gain, and halve its beamwidth. (However,
losses within the antenna and feed network will soon reach a point where
adding more elements to an antenna increases gain very little.)
Isotropic—A theoretical omnidirectional antenna that radiates equally well
in all directions. Used as a reference to compare a real antenna’s true gain
specifications.
Main beam—The dominant lobe of a directional antenna where the majority
of the output power radiates.
Polarization—Polarization is the orientation of the electric field of the
electromagnetic wave as it travels through space. When an antenna’s
elements are parallel with the ground, it is referred to as a horizontally
polarized antenna. Such an antenna can receive an electromagnetic wave
only from a vertically polarized source through the small shift in polarization
that takes place over distance. Indeed, if it were not for this slight EM wave
change, a perfect horizontal antenna would not be able to induce a voltage
into a perfect vertical antenna, and vice versa.
Sidelobes—Antennas will unfortunately emit electromagnetic radiation at
other directions than that in the main lobe. These are normally wasted and
undesired emission areas, and are referred to as the sidelobes of an antenna.
In fact, high-powered outputs from the antenna may contain dangerous levels
of EM radiation within these sidelobes. Minimizing sidelobes will increase

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