2.6 Microstrip antennas – Radiation mechanism – Application

A microstrip antenna, also known as a patch antenna, consists of a metal patch on a substrate on a ground plane, as shown in the figure below. Different feed configurations, including microstrip line feed and coaxial feed, are also shown in the figure.

The patch can take various forms to meet different design requirements. Typical shapes are rectangular, square, circular and circular ring. The microstrip antenna is low-profile, conformable to planar and nonplanar surfaces, simple and cheap to manufacture using modern printed-circuit technology, and mechanically robust. In addition, it is very versatile in terms of resonant frequency, input impedance, radiation pattern and polarization. All these have made it an extremely popular modern antenna for frequencies above 300 MHz (from the UHF band). The major disadvantages of this type of antenna are: low efficiency (conducting, dielectric and especially surface wave losses), low power-handling capability (not suitable for high-power applications), poor polarization purity, and relatively narrow frequency bandwidth.

In this section a rectangular patch antenna is chosen as an example for investigation, since it is the most popular printed antenna. We are going to examine the operational principles, major characteristics and design procedures along with a design example.

Microstrip antennas and their feeds (a) a microstrip antenna with its coordinates; (b) two feeding configurations: microstrip feed and coaxial feed

Operational Principles :

The rectangular antenna dimensions and coordinates are displayed in figure(a). Usually, the patch length L is between λ0/3 and λ0/2 and its width W is smaller than λ0 while the substrate thickness d is very small. To be a resonant antenna, the length L should be around half of the wavelength.

In this case, the antenna can be considered a λ/2 transmission line resonant cavity with two open ends where the fringing fields from the patch to the ground are exposed to the upper half space (z>0) and are responsible for the radiation. This radiation mechanism is the same as the slot line, thus there are two radiating slots on a patch antenna, as indicated in figure (a). This is why the microstrip antenna can be considered an aperture-type antenna.

The fringing fields at the ends are separated by λ/2, which means that they are 180 degrees out of phase but are equal in magnitude. Viewed from the top of the antenna, both fields are actually in phase for the x components, which leads to a broadside radiation with a maximum in the z direction.

Analysis and Design :

As a resonant cavity, there are many possible modes (like waveguides), thus a patch antenna is multimode and may have many resonant frequencies. The fundamental and dominant mode is TM100 (a half wave change along the x-axis and no changes along the other two axes).

Radiation Pattern and Directivity:

The radiation comes from the fringing fields at the two open ends, which is equivalent to two slot antennas separated by a distance L. It can be proved that the far-field electric field can be expressed as:

where β is the free space wave number.

The first factor is the pattern factor for a uniform line source of width W in the y direction and the second factor is the array factor for the two-element slots separated by L in the x direction. For both components, the peak is at θ=0, which corresponds to the z direction. It has a broadside unidirectional pattern.

The radiation patterns in the two principal planes are

The typical radiation patterns in the E- and H-planes are shown in the figure below.

Typical radiation patterns of a resonant rectangular patch antenna

The larger the width, the larger the directivity.

Advantages

The advantages of microstrip antenna are:

1.Easy to fabricate (use etching and photolithography).

2.Easy to feed (coaxial cable, Microstrip line, etc.) .

3.Easy to use in an array or incorporate with other Microstrip circuit elements.

4.Patterns are somewhat hemispherical, with a moderate directivity (about 6-8 dB is typical).

5.Light weight, smaller size and lesser volume.

Disadvantages :

1.Low bandwidth

2.Low efficiency

3.Low gain Long wire antenna

Applications:

1.Mobile and satellite communication application

2.Global Positioning System applications

3.Radio Frequency Identification (RFID)

4.Worldwide Interoperability for Microwave Access (WiMax)

5.Radar Application

6.Rectenna Application

7.Telemedicine Application

8.Medicinal applications

2.7 Numerical tool for antenna analysis

Computational electromagnetics deals with the art and science of solving Maxwell's equations numerically or with the numerical simulation of electromagnetic fields.

It has become an indispensable tool for antenna analysis because of the predictive power of Maxwell's equations: If these equations are solved correctly, the solution can predict experimental outcomes and design performances. Because of their high predictive power and capability of dealing with complex structures, numerical simulation tools can support a wide variety of engineering applications, such as designing antennas analytically and predicting the impact of platforms on antenna performance and address more complex applications, including calibration of antenna systems, estimating cosite interference of multiple antenna systems on a platform and predicting scattering from low-observable antenna installations.

In addition to the capability of analyzing complex antennas, numerical simulation has four more distinctive advantages over traditional antenna design by experiment. The first advantage is low cost. When an antenna can be designed, analyzed, and optimized on a computer, its design cost is reduced significantly compared to that of constructing a prototype physically and measuring it in an anechoic chamber.

The second advantage is the short design cycle. It typically takes far less time to simulate an antenna on a computer than to actually build one and measure it in a laboratory. The third advantage is the full exploration of the design space. Because of the low cost and short design cycle, the designer can evaluate a large variety of design parameters systematically to come up with an optimal design through numerical simulation, which is simply impossible with laboratory experiments.

The last but not the least advantage of numerical simulation is the enormous amount of physical insight it provides. With a numerical solution to Maxwell's equations, the designer can now use a computer visualization tool to "see" the current flow on an antenna and field distributions around the antenna. Such a capability is extremely useful because it can help to pinpoint the source of design deficiency, such as the source of mutual coupling between antennas and the source of interference for antennas mounted on a platform. All these advantages become much more pronounced when dealing with more complex antennas involving many design parameters. Indeed, in many cases numerical simulation coupled with an appropriate set of validating measurements is the best practical solution to an antenna design problem.

Unfortunately, the great advantage of numerical simulation are also accompanied by a series of challenges. The main challenge is due to improper use of a numerical simulation, such as insufficient discretization and use of a method outside its bounds. Such improper use would yield either a poor or a completely erroneous design while wasting time and resources. Therefore, it is very important to understand the basic principles, solution technologies and applicability and capabilities of numerical methods behind the numerical simulation tools.

Such knowledge can not only reduce the possibility of improper use of a method, but also help in choosing from a suite of tools the technique best suited for a specific problem, thus increasing the designer's productivity.

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