A compact U-slot microstrip patch antenna with reconfigurable polarization is proposed for wireless local area network (WLAN) applications. PIN diodes are appropriately positioned to change the length of the U-slot arms, which alters the antenna's polarization state. Two antenna prototypes with identical dimensions are designed, fabricated and measured. The first antenna prototype enables switching between linear and circular polarization by using a PIN diode and a capacitor located on the U-slot. The second antenna prototype uses two PIN diodes to switch between the two circular polarization senses. A good impedance match (S11 ≤-10 dB) for both linear and circular polarization is achieved from 5.725 to 5.85 GHz, a band typically used for WLAN applications, and the 3 dB axial ratio bandwidth is greater than 2.8%. Details of the simulated and measured reflection coefficient, axial ratio, gain and radiation patterns are presented.

Polarization Reconfigurable U-Slot Patch Antenna

The basic geometry of a microstrip patch antenna (MPA) consists of a metallic patch printed on a grounded substrate. Three commonly used feeding methods are coaxial feed, stripline feed, and aperture-coupled feed. The patch antenna idea was first proposed in the early 1950s, but it was not until the late 1970s that this type of antenna attracted serious attention of the antenna community. The microstrip patch antenna offers the advantages of low profile, conformability to a shaped surface, ease of fabrication, and compatibility with integrated circuit technology, but the basic geometry suffers from narrow bandwidth. In the last three decades, extensive studies have been devoted to improve the performance of this antenna and the MPA has found numerous applications in both the military and the commercial sectors. This article begins with a brief description of the modeling techniques and basic characteristics of the MPA. Methods of broadbanding, dual and multiband designs, size-reduction techniques, and design for circular polarization are then reviewed. The paper ends with some concluding remarks.

Microstrip Patch Antennas—Basic Characteristics and Some Recent Advances

Deviations in effective electrical dimensions caused by slight variations in finished microstrip antenna length, variations in the relative permittivity of the substrate material or nonuniformity in the substrate thickness lead to discrepancies between the designed and actual resonant frequencies. This sort of discrepancy is especially significant because of the narrow bandwidth of such antennas. It is found that for low dielectric substrate microstrip antennas, the critical parameter affecting the change in resonant frequency is the error in antenna length while for high permittivity substrates, the tolerance in dielectric constant is the critical parameter.

Design Considerations in Microstrip Antenna Fabrication

A fixed frequency continuously beam-steerable leaky-wave antenna in microstrip is disclosed. The antenna's radiating strips are loaded with identical shunt-mounted variable-reactance elements, resulting in low reverse-bias-voltage requirements. By varying the reverse-bias voltage across the variable-reactance elements, the main beam of the antenna may be scanned continuously at fixed frequency. The antenna may consist of an array of radiating strips, wherein each strip includes a variable-reactance element. Changing the element's reactance value has a similar effect as changing the length of the radiating strips. This is accompanied by a change in the phase velocity of the electromagnetic wave traveling along the antenna, and results in continuous fixed-frequency main-beam steering. Alternatively, the antenna may consist of two long radiating strips separated by a small gap, wherein identical variable-reactance elements are mounted in shunt across the gap at regular intervals. A continuous change in the reactance value has a similar effect as changing continuously the width of the radiating strips. This results in a continuous change in the phase velocity of the electromagnetic wave traveling along the antenna, thereby achieving continuous fixed-frequency main-beam steering.

Fixed-frequency beam-steerable leaky-wave microstrip antenna

The design and analysis of a coplanar capacitive fed microstrip antenna suspended above the ground plane is presented. It is demonstrated that the proposed approach can be used for designing antennas with impedance bandwidth of about 50% and a good gain to operate in various microwave bands. The model of the antenna incorporates the capacitive feed strip which is fed by a coaxial probe using equivalent circuit approach, and matches simulation and experimental results. The capacitive feed strip used here is basically a rectangular microstrip capacitor formed from a truncated microstrip transmission line and all its open ends are represented by terminal or edge capacitances. The error analysis was carried out for validity of the model for different design parameters. The antenna configuration can be used where unidirectional radiation patterns are required over a wide bandwidth.

http://ap-s.ei.tuat.ac.jp/isapx/2009/pdf/1238.pdf

Coplanar Capacitively Coupled Probe Fed Microstrip Antennas for Wideband Applications

A variational method for the analysis of inhomogeneous broadside-coupled striplines is described. The data for even- and odd-mode characteristic impedances, effective dielectric constants, and mode phase velocity ratios are presented. It is found that the phase velocity ratio may be varied over the range 1.14 /spl les/ ( V/sub e/ / V/sub 0/) /spl les/ 3.6 for broadside-coupled suspended microstrip lines (BSML) and 0.36 /spl les/ ( V/sub e/ / V/sub 0/) /spl les/ 0.93 for broadside-coupled inverted microstrip lines (BIML) using materials with dielectric constant less than 16 and S/b /spl ges/ 0.05, W/b /spl les/ 2.0. The effect of nonzero strip thickness is also calculated. It is noticed that the effect of thickness is more pronounced for the odd-mode case than for the even mode. Losses are obtained using the incremental inductance rule of Wheeler. The odd-mode attenuation constant is always higher than the even-mode value.

Characteristics of Inhomogeneous Broadside-Coupled Striplines

Accurate design expressions for broadside-coupled striplines are presented. The expressions have general validity as long as W/S>0.35. Uncertainty analysis is described to calculate the effect of tolerances in parameters on coupling coefficient and input VSWR. The effect of tolerances in parameters on these characteristics increase as the coupling becomes tighter and tighter.

The Design of Broadside-Coupled Stripline Circuits

A novel approach for the design of leaky-wave antennas using artificial dielectrics at millimeter wave frequencies is discussed. The general radiation characteristics of leaky-wave structures of finite length are presented. The feasibility of frequency scanning and dielectric scanning (changing the direction of the beam by varying the relative permittivity of the electrically controlled liquid artificial dielectric medium) of a leaky-wave antenna using rodded artificial dielectric is investigated theoretically. Calculations shown that the beam angle changes from 20° to 50° off broadside when the frequency is changed from 31.1 to 35.4 GHz or the permittivity of the embedding medium of the artificial dielectric is changed from 1.6 to 2.04. Over a scan range of about 40° the beamwidth is almost constant. For large scan range, the beamwidth of a dielectric-scanned antenna (DSA) is about 15 percent less than the frequency-scanned antenna (FSA). The gain of a DSA is greater than the FSA and also has less variation over the scan range. The power efficiency is approximately the same for both the antenna types with worst case efficiency being about 85 percent.

Leaky-wave antennas using artificial dielectrics at millimeter wave frequencies

Aperture coupling between dielectric image lines and between shielded dielectric lines is analyzed to develop a design technique for directional couplers at millimeter wave frequencies.

I.J. Bahl

Papers

Design Considerations in Microstrip Antenna Fabrication

Characteristics of Inhomogeneous Broadside-Coupled Striplines

The Design of Broadside-Coupled Stripline Circuits

Leaky-wave antennas using artificial dielectrics at millimeter wave frequencies

Aperture Coupling Characteristics of Image and Shielded Dielectric Lines