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 (Eφ-horizontal or Eθ-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.
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.
Antenna Design and Simulation
The initial design of an antenna can arise from a set of dimensional formulas based on closed-form electromagnetic relations. In practice, these antennas require some empirical adjustment/tuning steps to arrive at a final design. Secondly, the electromagnetic relations associated with most antennas are not closed form and therefore do not yield dimensional synthesis equations. In order to design and validate an antenna prior to fabrication, it is beneficial to simulate the antenna using a electromagnetic field solver that can predict the behavior of radiating systems.
One such solver, CST Microwave Studio®, offers many methods of solution that can simulate open-boundary, radiating structures. One example shows the relative utility of the simulation tool. Presented in Figure 2 is the input page for a 2.4-GHz sleeve dipole antenna. The input page contains the dimensional and material parameter inputs required to carry out the simulation.
Upon completion of the electromagnetic simulation, the radiation pattern of the electric field is available as a 3D plot and as conical cuts. Further, the simulator predicts the input reflection coefficient and represents it as a scattering parameter (Ss). The simulator provides all of the essential information about the antenna prior to its physical realization in order to pre-validate the design approach.
Antenna Design Validation and Measurement
With the antenna synthesized and realized, the design must be validated through measurement. The first necessary measurement is to measure the reflection coefficient of the antenna input port or driving point. The reflection coefficient and associated driving point impedance is measured with a Vector Network Analyzer (VNA). Care must be taken during this measurement to ensure that the antenna is radiating and not being disturbed by and any surrounding objects. Ideally, this measurement is performed in an anechoic chamber. However, with sufficient separation between the antenna and any perturbing obstructions, this measurement can typically be performed within a normal laboratory environment.
In order to initially validate the antenna design, the reflection coefficient and associated driving point impedance must be matched to the system impedance (generally 50 ohms). Once it has been established that the antenna is matched to the system impedance, the radiation pattern must be measured to compete the final steps of design validation. The measurements are performed in an anechoic chamber by exciting the antenna under test with a known transmit source power and measuring the received power, received voltage, or electric field intensity at a fixed distance. A photograph of an antenna under test in a 3-meter anechoic chamber is presented in Figure 3.
The antenna is swept through a series of conical cuts in an effort to compare them to simulated results or to build a set of cuts to assemble into a 3D gain pattern. The absolute received signal is normalized either by the conducted power applied to the antenna or compared to a known reference such as a half-wave dipole. Both polarization cases are measured. With the set of pattern data at hand, the measurements can be also examined against the system requirements in terms of minimum, maximum, and average gain, or against gain distribution requirements, if applicable.
Antennas provide the primary interface between the radio and the propagation environment. The antenna requires special considerations in terms of performance requirements, design constraints, design, and realization. Specification of the antenna gain and relating those requirements to the system performance in terms of range and system link gain is a foundation for the design goals of the antenna. During the antenna topology/ structure selection process, consider packaging constraints in terms of the size, location, and possible obstructions. Be prepared to compromise performance versus package conformance.
Ideally, one should use a simulation tool to assess the performance of the antenna prior to realization, not only to gauge the fundamental performance of the antenna, but also to check the effects of antenna compaction, obstructions, and other compromised parameters. The final physical realization and consequent measurement of input terminal reflection/impedance and antenna gain complete the design process. Often times, the measurement results require that antenna structure be modified to empirically optimize its performance.
This article was written by Brian Petted, Chief Technology Officer of L.S. Research, LLC, Cedarburg, WI. For more information, Click Here