Antenna Design Considerations

With the advent of prolific wireless communications applications, system designers are in a position to consider the placement and performance of an antenna system. The first step in establishing antenna requirements is to determine the desired communication range and terminal characteristics of the radio system (transmit power, minimum receiver sensitivity level). Given those parameters, one can ascertain the amount of gain or loss required to maintain the communication range.

Antenna gain (or loss) must be part of a trade-off study between performance and the physical realization considerations of size, placement, and clearance (distance from obstructions). Ideally, there should exist a free-space clearance zone around the antenna. For example, if the largest dimension of the antenna is half of a wavelength, the minimum clearance zone is a half-wavelength. This serves as a basic guideline; however, in many physical realizations, this clearance zone is compromised and the effects must be determined through simulation or empirical measurement. Antenna gain is defined as the ratio of radiated power intensity relative to the radiated power intensity of an isotropic (omni-directional) radiator. Power intensity is the amount of radiated power per unit solid angle measured in steradians [sr].

Ideally, antenna patterns are displayed as a three-dimensional plot as shown in Figure 1. This 3D plot is often constructed from multiple cross-sections known as conical cuts. A typical conical cut is formed by holding the elevation angle, θ, constant, and measuring the pattern over a complete revolution of the azimuthal angle, φ. Secondly, a separate plot is generally made for each component of the electric field or polarization (E_φ-horizontal or E_θ-vertical).

There may be an interest in determining the distribution of communication ranges and system gains, given the nonuniform nature of a directional antenna that is used in an omni-directional application. In those cases, probability density functions (pdfs) can be associated with antenna patterns, both conical cuts and 3D patterns. Even though the directional antenna patterns are deterministic, the fact that their application is omni-directional with a random link axis angle makes the antenna gain a random variable with respect to communication range.

Antenna Topologies

There exist many possible topologies or structures for an antenna. An interesting set of structures is those that evolve from the basic half-wave dipole. Starting with the half-wave dipole, the lower element of the dipole can be realized by a reflected image of the upper element onto a ground plane (using electric field boundary conditions and/or image theory). The monopole can be folded over, however, with degradation in impedance match and gain. The degradation due to matching can be recovered by feeding the antenna at a different point along the resonant length of the antenna (recall the impedance variation of a transmission line with a standing wave present). This results in the inverted “F” antenna. The elements may be extruded from the wire form to a planar for to realize an increase in impedance and gain bandwidth, but with a small degradation in gain.