Antenna fundamential properties
In the design and installation of wireless communication systems, it is necessary for
system design engineers, operators and many times, installers, to have a fundamental knowledge of antenna performance and RF propagation characteristics. This knowledge will assist these individuals with the proper selection of system antennas and their subsequent mounting location and orientation in an effort to ensure optimum system coverage and performance.
A properly selected antenna system has the capability of improving overall system
performance and may lead to a reduction in system cost if the overall number of stations or
access points can be reduced. Conversely, a poorly selected antenna system may degrade system performance and may lead to an increase in system cost.
The following sections will provide a discussion of fundamental antenna and RF
propagation properties and how these affect wireless system performance. These discussions
are intended to provide system engineers and operators with a basic knowledge of antenna
properties and antenna selection criteria.
In addition to antenna performance, other factors that influence antenna selection include
cost, size, and appearance. In the selection of an antenna system, there will always be tradeoffs
between these four issues.
I) FUNDAMENTAL ANTENNA PROPERTIES
The first concept to understand regarding antennas is that they are passive devices. To
operate, they require no supply voltage. They do not alter nor process RF signals and they do
not amplify RF energy. If they are 100% efficient, they radiate no more power than is delivered
to their input terminal.
The basic properties that are used to describe the performance of an antenna include
impedance and VSWR (Voltage Standing Wave Ratio), amplitude radiation patterns, 3 dB
beamwidth, directivity, gain, polarization and finally, bandwidth. These properties and their
impact on system performance are discussed in the following sections.
Impedance and VSWR
In order to achieve maximum energy transfer between a transmission line
and an antenna, the input impedance of the antenna must identically match the characteristic
impedance of the transmission line. If the two impedances do not match, a reflected wave will be generated at the antenna terminal and travel back towards the energy source. This reflection of energy results in a reduction in the overall system efficiency. This loss in efficiency will occur if the antenna is used to transmit or receive energy.
The reflection coefficient Γ is defined thus:
Γ is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases, when the imaginary part of Γ is zero, are:
- Γ = − 1: maximum negative reflection, when the line is short-circuited,
- Γ = 0: no reflection, when the line is perfectly matched,
- Γ = + 1: maximum positive reflection, when the line is open-circuited.
The resultant voltage wave on the transmission line is the combination of both the
incident (source) and reflected waves. The ratio between the maximum voltage and the minimum voltage along the transmission line is defined as the Voltage Standing Wave Ratio or VSWR.
For the calculation of VSWR, only the magnitude of Γ, denoted by ρ, is of interest. Therefore, we define
ρ = | Γ | .
At some points along the line the two waves interfere constructively, and the resulting amplitude Vmax is the sum of their amplitudes:
At other points, the waves interfere destructively, and the resulting amplitude Vmin is the difference between their amplitudes:
The voltage standing wave ratio is then equal to:
As ρ, the magnitude of Γ, always falls in the range [0,1], the VSWR is always ≥ +1.
The VSWR, which can be derived from the level of reflected and incident waves, is also an
indication of how closely or efficiently an antenna’s terminal input impedance is matched to the
characteristic impedance of the transmission line. An increase in VSWR indicates an increase in
the mismatch between the antenna and the transmission line.
Typically, most wireless communications systems operate with a 50 Ohm impedance and
therefore, the antenna must be designed with an impedance as close to 50 Ohms as possible. The antenna VSWR is then an indication of how close the antenna impedance is to 50 Ohms. A 1.0:1 VSWR would indicate an antenna impedance of exactly 50 Ohms. In many systems, the antenna is required to operate with a VSWR better than 1.5:1
To indicate how increased VSWR impacts overall system performance, Table 1 below
details the percentage of power reflected by the antenna, and the resultant overall transmission loss, for several typical VSWR values. For a 1.5:1 VSWR, the transmission loss is
approximately 0.2 dB or a 4.0% reduction in efficiency. It is also important to note that some
transmitter circuits decrease their output power with increasing antenna VSWR. This factor
varies with each transmitter and is not quantified in this discussion.
Table 1. Percent Reflected Power and Transmission Loss as a Function of VSWR.
Radiation Patterns and 3 dB beamwidth
The radiation patterns of an antenna provide the information that describes how the
antenna directs the energy it radiates. As stated earlier, an antenna cannot radiate more total
energy than is delivered to its input terminals. All antennas, if 100% efficient will radiate the
same total energy, for equal input power, regardless of pattern shape.
Antenna radiation patterns are typically presented in the form of a polar plot for a 360
degree angular pattern in one of two sweep planes. The most common angular sweep planes
used to describe antenna patterns are a horizontal or azimuth sweep plane and a vertical or
elevation (zenith) sweep plane. A graphical representation of these planes and a typical polar
pattern are presented in Figure 1. Radiation patterns are generally presented on a relative power dB scale.
Figure 1. Graphical Representation of the Horizontal and Vertical Sweep Planes and a
Typical Polar Pattern Plot.
In many cases, the convention of an E-plane and H-plane sweep or pattern is used in the
presentation of antenna pattern data. The E-plane is the plane that contains the antenna’s
radiated electric field potential while the H-plane is the plane that contains the antenna’s radiated magnetic field potential. These planes are always orthogonal. For dipole and Yagi antennas, the E-plane is always in the plane parallel to the linear antenna elements.
Once the antenna pattern information is detailed in a polar plot, some quantitative aspects
of the antenna pattern properties can be described. These quantitative aspects generally include the 3 dB beamwidth (1/2 power level), directivity, side lobe level and front to back ratio. To further understand these concepts, we first consider the fundamental reference antenna, the point source. A point source is an imaginary antenna that radiates energy equally in all directions such that the antenna pattern is a perfect sphere as shown in Figure 2. This antenna is said to be an omnidirectional isotropic radiator and has 0 dB directivity. In practice, when an antenna is said to be omnidirectional, it is inferred that this is referenced only to the horizontal or azimuth sweep plane.