Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Microstrip antennas offer many benefits, including small size and light weight, along with ease of integration with microwave-integrated-circuit (MIC) and monolithic-microwave-integrated-circuit (MMIC) devices.1 They are well suited for transferring data, voice, and video as part of wireless communications applications,2 but they lack one important property for longer-distance communications: higher gain.3 Fortunately, it is possible to develop a microwave antenna with coplanar-waveguide (CPW) feed that offers radiation patterns with enough gain to serve ultrawideband (UWB) communications applications.

The antenna design features a star-shaped radiation patch with wide impedance bandwidth of 3.0 to 11.7 GHz. Through the use of a three-layer frequency-selective surface (FSS), the antenna can be fabricated with higher gain, enhancing the gain by 4 to 5 dB while maintaining the wide impedance bandwidth. The use of the FSS even helps reduce the antenna’s backlobes.

The bandwidth of a patch antenna is limited if the patch is fabricated on a dielectric substrate that contributes to generation of surface waves, and in so doing reduces the bandwidth of the antenna.3 Numerous techniques are available to increase antenna bandwidth, such as incorporating different slot geometries in the ground planes, or the use of FSS screens, gap-coupled feeds4, and meandered ground planes.5 A UWB antenna with CPW feed can provide the good radiation patterns needed for broad bandwidth.

Antennas for UWB communications can support a wide range of applications, including for microwave imaging, impulse radio communications, and biomedical applications. Increased antenna signal-to-noise ratio is needed for improved gain, and this can be achieved by designing antennas with unidirectional or semi-unidirectional radiation patterns.6,7 The antenna gain can be improved in two ways: by increasing its efficiency or using a reflector surface. For increased efficiency, thinner substrates can be used to provide higher gain due to reduced surface-wave losses.

FSS surfaces are passive periodic structures of metallic or dielectric elements that can be used to control and manipulate the propagation of electromagnetic (EM) waves.8-10 FSS materials can be characterized by means of high surface impedance which retards the propagation of surface waves and supports in-phase reflections of waves striking the FSS surface, resulting in a high gain.11-15 For low-profile antennas, FSS designs should be compact, easy to fabricate, and commercially viable.

By using an FSS screen, it should be possible to improve antenna radiation pattern characteristics, directivity, polarization and, in many cases, bandwidth. FSS materials can be used as a filter, reflector, polarizer, propagation device, or as an artificial magnetic conductor (AMC).16,17 Typical FSS structures already reported in technical literature include square, ring, loop, dipole, and fractal-based shapes. The characteristics and behaviors of these different FSS materials depend on the size, shape, periodicity, and geometric structure of each FSS unit cell.18

To demonstrate the effectiveness of FSS materials and shapes, a wideband FSS form consisting of a rectangular ring element was used to enhance the gain of a star-shaped patch antenna operating from 3.0 to 11.7 GHz. The antenna design was simulated with the help of the CST Microwave Studio computer-aided-engineering (CAE) simulation software from CST, then fabricated and measured using commercial test equipment.

Antenna geometryFigure 1 shows the structural details of the antenna. It was printed on one side of 1.58-mm-thick FR-4 circuit laminate with relative permittivity (εr) of 4.3 in the z-axis of the material and loss tangent (tan δ), or dissipation factor of 0.019 in the z-axis of the material. Initially, a simple symmetric rectangular-shaped slot was etched on the ground plane. A double-stepped CPW feed line with widths of Wlf and Wuf, and terminated on a star-shaped patch, was used to excite the slot. A double-stepped CPW feed was used for better impedance matching. Figure 1 also shows the formation of the star-shaped patch, which was formed by inverting and adding two isosceles triangles having base width Tb and height Th, resulting in the star-shaped structure.

To increase antenna bandwidth, the slot was made asymmetric by adding a rectangular section on one side (Fig. 1). The slot’s asymmetric nature provides an inductive characteristic to counter the capacitive effects of the patch (produced by the fringing effect). The bandwidth can also be enhanced by changing the size and shape of the slot. The antenna was optimized further for miniaturization, with optimized parameters shown in the table.

Antenna parameter dimensions

The return loss of the fabricated antenna was measured using a model R&S ZVA-40 vector network analyzer (VNA) from Rohde & Schwarz. Figure 2 compares measured and simulated return loss for the antenna. The measured impedance bandwidth of the antenna is 8.7 GHz (3.0 to 11.7 GHz). From Fig. 2, it can be seen that measured and simulated results are in close agreement except at certain frequencies, where deviations may be due to substrate impurities or deviations because of SMA connectors.

Reflection coefficients

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.