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Microstrip antennas offer many benefits to high-frequency systems, including low profiles, light weight, small volume, and ease of integration with other RF/microwave components. Unfortunately, microstrip antennas have also been plagued by inherently narrow bandwidths, limiting their use in some applications. A number of approaches have been proposed to increase the operating bandwidths of these antennas, including increasing the substrate thickness, using different impedance-matching and feeding techniques, and using multiple resonators and slot antenna geometries.1-12 However, these methods tend to add to an antenna’s weight and volume, and can increase the fabrication cost.

Several newer, promising techniques have been proposed to enhance microstrip antenna bandwidth. U-slot rectangular patch antennas13,14—as well as U-shaped parasitic patch antennas15 with thick foam or substrate—have been found to provide wide bandwidths without enlarging antenna size. In addition, the use of shorting pins or shorting walls on the unequal arms of a U-shaped patch or L-probe feed antennas16,17 have helped achieve wideband impedance bandwidths while maintaining small antenna size. All of these microstrip antenna investigations have involved thick substrates, although low-profile broadband microstrip antennas are required in some applications, such as the conformal microstrip antenna or arrays. The current goal is to enhance the bandwidth of a microstrip antenna while using a thin substrate (less than 0.01λ0).

Microstrip Antenna Maintains Low Profile, Fig. 1

To meet this goal, a broadband low-profile microstrip patch antenna with a U-shaped parasitic element was investigated. This relatively compact antenna was fabricated on a substrate with thickness of 0.008λ0, where λ0 is the wavelength in air of the center frequency at 2.45 GHz. This new microstrip antenna was found to achieve a relatively larger impedance bandwidth than a conventional microstrip antenna while also featuring a low-profile mechanical design.

Figure 1(a) shows top and side views of the proposed antenna. For conformal applications, the antenna is constructed on a substrate with thickness (H = 1 mm) of 0.008λ0 and relative permittivity of 2.65. To achieve a wideband impedance match, two resonant modes should be excited simultaneously around the center frequency, with the return losses between the two resonant frequencies maintained below 10 dB.

Microstrip Antenna Maintains Low Profile, Fig. 2

The proposed antenna features a microstrip fed rectangular patch and a U-shaped parasitic element. The rectangular patch is designed to resonate at a lower frequency than the resonant modes. To maintain a desired resonant length with smaller size, the length of the main patch (LP is designed as 0.5λLg, with the width reduced to 0.25λLg, where λLg = the guided wavelength at the lower frequency. To produce another resonance at a higher frequency, a U-shaped parasitic element is added to surround the radiating and nonradiating edges of the main patch.

Microstrip Antenna Maintains Low Profile, Fig. 3

The resonant length of U-shaped patch can be controlled by adjusting its length (LU) and width (WU). Electromagnetic (EM) coupling between the main patch and parasitic patch is realized across the horizontal (GH) and vertical (GV) gaps. With a 50-Ω microstrip feed line, the low-profile antenna is convenient for integrating with other microwave components and conforming to the structure. In addition, the widths of the gaps (GH), GV, and GF)—together with the span of the feeding point, LF—have great impact on the impedance bandwidth.

Microstrip Antenna Maintains Low Profile, Fig. 4

This new antenna is designed to operate in the 2420-to-2484-MHz region. To cover this bandwidth, the positions of the two resonant frequencies should be close to the extreme frequency limits of the desired operating band. Therefore, the length and width of the main patch are designed to be close to 0.5λLg and 0.25λLg, respectively, at the lower resonant frequency (2430 MHz). Meanwhile the effective resonant length of the U-shaped patch—namely LU + 0.5WU - 0.5HU—is close to 0.5λHg at the higher resonant frequency of 2474 MHz.

Microstrip Antenna Maintains Low Profile, Fig. 5

The antenna has been optimized with the aid of the High Frequency Structure Simulator EM simulation software from Ansoft Corp. The final dimensions were set as LP = 38.5 mm; WP = 19.4 mm; LU = 33.6 mm; WU = 11.9 mm; HU = 1.7 mm; GV = 0.8 mm; GH = 1.8 mm; LF = 15.5 mm; WF = 2.73 mm; GF = 0.5 mm; and H = 1 mm. The ground and substrate size of the proposed antenna were defined as a length × width of 66 × 60 mm.

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