Qing Han

Mutual Impedance Extraction and Varactor Calibration Technique for ESPAR Antenna Characterization

An extraction and calibration technique is proposed that allows electronically steerable parasitic array radiator (ESPAR) antennas to achieve high efficiency and precision in performance characterization. The proposed technique is based on a reactive-near-field measurement or direct acquisition of RF current. We present a simple formulation that requires minimal current measurements to successfully extract the mutual impedance. Currents are predicted for an arbitrary combination of bias voltages that are used to control the reactance of varactor diodes integrated with the antennas parasitic elements. These predicted currents are compared by conducting an experiment with a seven-element ESPAR. For the bias voltage combinations that have reasonably good symmetry, the differences in relative gain, beam direction and half power beam width (HPBW) actually fall within 0.5 dB, 4deg and 9deg, respectively. Directivity patterns obtained by calculating the predicted currents are verified with those obtained by calculating the directly measured currents. Furthermore, the calculated patterns are verified through comparison with far-field measurements in a conventional anechoic chamber. The differences in beam direction and HPBW are also found to be acceptable. Using the proposed technique, the required number of measurements is reduced from 85,766,121 to just 149 for a seven-element array

A Compact Espar Antenna with Planar Parasitic Elements on a Dielectric Cylinder

This paper presents a technique for designing a dielectric Electronically Steerable Parasitic Array Radiator (Espar) antenna to achieve miniaturization of the conventional Espar antenna. The antennas size is reduced by immersing the central active element in a dielectric cylinder, mounting the surrounding planar parasitic elements at the circumference of the cylinder, and decreasing the radius of the ground skirt to that of the parasitic elements. An example of a polycarbonate (∈ r = 2.9 + j0.006) Espar antenna operating at 2.484 GHz is optimised by using a genetic algorithm in conjunction with an FEM-based cost function. The designed antenna generates a half-power beam width of 78° and a main lobe that elevates at an angle of only 5° from the horizontal plane. The designed antenna is also fabricated and measured. Good agreement between the measurement and simulation results is obtained. We reduce the size of the designed Espar antenna to 1/8 the size of its conventional counterpart while achieving a 12° improvement in half-power beam width.

Evaluation of the Adaptive Beamforming Capability of an ESPAR Antenna Using the Genetic Algorithm

The electrically steerable parasitic array radiator (ESPAR) antenna, which is expected to be a key component in wireless ad hoc networking, is a single-output array. Because it is not possible to observe the output signal of each element of the antenna, it is impractical to directly apply most of the algorithms designed for conventional array antennas. Therefore, to evaluate the adaptive beamforming capability of the ESPAR antenna, the genetic algorithm (GA) is applied and the results are compared with those obtained by applying the steepest-gradient algorithm (SGA) to the ESPAR antenna. The GA results reveal that the ESPAR antenna possesses the ability to form a beam and null at the same time, even when the angular difference between the desired and interference signal is less than 60 degrees

Evanescent Reactive-Near-Field Measurement for ESPAR Antenna Characterization

A technique to efficiently characterize the azimuth radiation patterns of monopole arrays based on evanescent reactive-near-field measurement in a compact anechoic box is presented. We describe a desktop-size measurement setup for determining the azimuth far-field pattern of an electronically steerable parasitic array radiator (ESPAR) at 2.5 GHz, and present a thorough statistical evaluation of the ESPAR antenna characterization technique. In addition, the sensitivity to probe position and the errors caused by possible fluctuations in the probe position are investigated. Furthermore, comparisons among patterns obtained by the proposed method, simulations and conventional far-field measurements are carried out, and a good agreement among them is obtained. We also provide a statistical evaluation of the proposed desktop-size measurement setup using forty ESPAR antennas and conclude that the proposed setup can be an inexpensive candidate to replace the conventional measuring system

Tapered Radial Fed Circuit and Varactor-Mounted Waveguide Switch for Millimeter-Wave Switched Sector-Beam Antenna

This paper describes the simulated designs of an improved radial fed circuit and a varactor-mounted waveguide switch, which are expected to play important roles in the realization of a millimeter-wave switched sector-beam antenna. The aim of this antennas development is to realize an ultra-high-speed gigabit-rate wireless LAN. In this paper, an eight-sector beam antenna is described. The radial structure is designed so that the input power from a unique RF port at the structures center can be sent to any one of the selected sectors with maximum power under the required bandwidth. The simulation results of the proposed tapered radial structure show that the bandwidth of S21, with an insertion loss of -1.0 dB in the frequency range of 59-66 GHz, is 5.4 GHz, which is the three times that achieved in our previous work. Basic study is also carried out on factors such as choke structure, switch position, and the losses caused by the dielectric and metallic materials as well as the parasitic components of the varactor of the proposed waveguide switch

A GaAs monolithic anti-series varactor pair for voltage-controlled capacitance with reduced RF nonlinearity

This paper presents a trial fabricated GaAs monolithic anti-series varactor pair. Its second-order and third-order harmonic distortions, which are generated from the nonlinearity of capacitance-voltage characteristics, are investigated experimentally and numerically. Compared to the case of a single varactor, the distortions of measurement and simulation are suppressed by about 25 dB and 45 dB for the second-order harmonic, and 3 dB and 10 dB for the third-order harmonic, respectively.

Statistical evaluation of ESPAR antennas and adaptive beamforming performance prediction using extracted parameters of an ESPAR antenna

Dozens of electrically steerable parasitic array radiator (ESPAR) antennas were measured in a conventional far-field measurement system. The measured directivity results were evaluated statistically. The results show that the ESPAR antennas are useful in many applications. A technique to predict the adaptive beamforming behavior of a 7-element ESPAR antenna is presented. Instead of using simulated reactances, practical reactances which were extracted from an ESPAR antenna are used in the simulation of the adaptive beamforming algorithm to make the antenna steer its main lobe to any desired direction and place null in the direction of an interference signal. The predicted beams and nulls show good agreements with those of experiments.