Aug 1, 2008 12:00 PM, By Jay M. Jacobsmeyer, P.E.

Often abused and neglected, portable radio antennas are indispensable

Land mobile radio antennas come in two basic types: the base station or repeater site antenna and the user radio antenna. This article focuses on user radio antennas, with an emphasis on the portable radio antenna.

The land mobile radio antenna is a transducer that converts voltage and current on a coaxial cable to an electromagnetic wave traveling in free space. The principle of antenna reciprocity ensures that the antenna has the identical pattern whether it is used to transmit or receive. This property is helpful because most mobile radio antennas are duplexed, meaning that a single antenna is used for both purposes.

Antenna performance is characterized by several parameters. The antenna parameters of greatest interest to us are the radiation pattern, gain, impedance, efficiency, polarization and bandwidth. Let's briefly look at each.

Radiation pattern

The radiation pattern is a three-dimensional description of the angular dependence of radiation from (or to) the antenna. Because three-dimensional patterns are difficult to view on two-dimensional surfaces, it is common to describe the radiation pattern in two parts: the azimuth pattern and the vertical pattern. Note that an omnidirectional antenna radiates energy equally in all directions in the azimuth plane, but it can be quite directional in the vertical plane. In other words, omnidirectional does not mean isotropic. Desirable patterns for portable radio antennas are omnidirectional with relatively wide vertical patterns to cover the range of possible look angles between the user and the repeater site. Figure 1 is the radiation pattern for a half-wavelength dipole antenna.

Gain

The isotropic gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction divided by the intensity radiated at the same distance by a hypothetical isotropic antenna. Isotropic gain is usually specified in terms of decibels relative to isotropic, or dBi. In mobile radio, it is common to specify gain relative to a half-wavelength dipole, which is dBd. Because a dipole has a gain of 1.64 or 2.15 dBi, the gain of an antenna in dBi is always 2.15 dB greater than the gain in dBd.

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