Andrew R. Weily

A planar resonator antenna based on a woodpile EBG material

A resonator antenna made from a complex artificial surface and a metallic ground plane is described. The complex surface is realized using a woodpile electromagnetic bandgap (EBG) material, which is shown to have a frequency dependent reflection plane location. A highly directive radiation pattern is created due to the angle-dependent attenuation of the resonator antenna coupling to free space. The antenna has the advantages of low height, low loss, and low sidelobes. It is shown that the directivity can be varied over a fixed range by changing the aperture size of the device, with the maximum directivity determined by both the feed element and EBG material properties. The complete bandgap for the woodpile EBG material is confirmed from a band diagram, and its properties as a complex surface are investigated through transmission calculation and measurement. The design of the antenna is described, and two means of exciting the resonator, a microstrip patch and a double slot, are investigated. Theoretical results for these two antennas are calculated the using finite-difference time-domain and are shown to be in good agreement with measured results.

A Reconfigurable High-Gain Partially Reflecting Surface Antenna

A high-gain partially reflective surface (PRS) antenna with a reconfigurable operating frequency is presented. The operating frequency is electronically tuned by incorporating an array of phase agile reflection cells on a thin substrate above the ground plane of the resonator antenna, where the reflection phase of each cell is controlled by the bias voltage applied to a pair of varactor diodes. The new configuration enables continuous tuning of the antenna from 5.2 GHz to 5.95 GHz using commercially available varactor diodes, thus covering frequencies typically used for WLAN applications. Both the PRS and phase agile cell are analyzed, and theoretical and measured results for gain, tuning range, and radiation patterns of the reconfigurable antenna are described. The effect of the varactor diode series resistance on the performance of the antenna is also reported.

Polarization Reconfigurable U-Slot Patch Antenna

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 antennas 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.

Frequency Reconfigurable Quasi-Yagi Folded Dipole Antenna

A frequency reconfigurable planar quasi-Yagi antenna with a folded dipole driver element is presented. The center frequency of the antenna is electronically tuned by changing the effective electrical length of the folded dipole driver, which is achieved by employing either varactor diodes or PIN diodes. Two antenna prototypes are designed, fabricated and measured. The first antenna enables continuous tuning from 6 to 6.6 GHz using varactor diodes and the reflection coefficient bandwidth (≤ -10 dB) at each frequency is greater than 15%. The second antenna enables discrete tuning using PIN diodes to operate in either the 5.3-6.6 GHz band or the 6.4-8 GHz band. Similar end-fire radiation patterns with low cross-polarization levels are achieved across the entire tunable frequency range for the two antenna prototypes. Measured results on tuning range, radiation patterns and gain are provided, and these show good agreement with numerical simulations.

A novel absorb/transmit FSS for secure indoor wireless networks with reduced multipath fading

A novel absorb/transmit frequency selective surface (FSS) is presented for 5-GHz wireless local area network (WLAN) applications. The novelty of the design is that it is capable of absorbing, as opposed to reflecting, WLAN signals while passing mobile signals. The FSS consists of two layers, one with conventional conducting cross dipoles and the other with resistive cross dipoles. The absorption of the WLAN signal is important to reduce additional multipaths and resultant fading otherwise caused by the FSS. The structure has good transmission characteristics for 900/1800/1900-MHz mobile bands and performs well for both horizontal and vertical polarizations. The distance between the two layers is less than a quarter free-space wavelengths. Theoretical and experimental results are presented.

A Pattern Reconfigurable U-Slot Antenna and Its Applications in MIMO Systems

A new compact pattern reconfigurable U-slot antenna is presented. The antenna consists of a U-slot patch and eight shorting posts. Each edge of the square patch is connected to two shorting posts via PIN diodes. By switching between the different states of the PIN diodes, the proposed antenna can operate in either monopolar patch or normal patch mode in similar frequency ranges. Therefore, its radiation pattern can be switched between conical and boresight patterns electrically. In addition, the plane with the maximum power level of the conical pattern can be changed between two orthogonal planes. Owing to a novel design of the switch geometry, the antenna does not need dc bias lines. The measured overlapping impedance bandwidth (|S11| <; -10 dB) of the two modes is 6.6% with a center frequency of 5.32 GHz. The measured radiation patterns agree well with simulated results. The antennas are incorporated in a 2 × 2 multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system to demonstrate the improvement in system capacity. In the real-time MIMO-OFDM channel measurement, it is shown that compared to omnidirectional antennas, the pattern reconfigurable antennas can enhance the system capacity, with 17% improvement in a line-of-sight (LOS) scenario and 12% in a non-LOS (NLOS) scenario at a signal-to-noise ratio (SNR) of 10 dB.

Oblique Incidence Performance of a Novel Frequency Selective Surface Absorber

Oblique incidence performance of a novel two-layer absorb/transmit frequency selective surface (FSS) is investigated. The FSS has good frequency stability for both horizontally and vertically polarized waves incident normally or at oblique angles. Due to its transmission for 900/1800/1900 MHz mobile bands and good absorption for 5 GHz waves, it has the potential as a security wall or isolator for 5 GHz WLAN systems. The absorption in the stop band helps reduce additional WLAN multipath fading caused by conventional reflecting FSS designs. The first layer of the FSS consists of conventional conducting cross dipoles having a circular aperture in the centre, while the second layer uses resistive cross dipoles. Moreover, the conducting cross dipoles have been sandwiched between two dielectric sheets to achieve a stable response for different angles of incidence. The periodicity of both FSS layers is the same while the distance between the two layers is reduced to one eighth of the free-space wavelength. This reduction leads to a more compact design compared to the conventional Salisbury screen, while still achieving acceptable absorption in the stopband. Both theoretical and experimental results are presented to confirm the performance of the absorb/transmit FSS.

Multioctave Frequency Selective Surface Reflector for Ultrawideband Antennas

In this letter, we demonstrate the gain enhancement of an ultrawideband (UWB) antenna, achieved using an appropriately designed multioctave dual-layer frequency selective surface (FSS) reflector. The proposed novel FSS reflects effectively in phase over a bandwidth of about 120%. Hence, significant enhancement in antenna gain has been achieved with a low-profile configuration without compromising the impedance bandwidth of the UWB antenna. The proposed FSS reflector has a low transmission coefficient and linearly decreasing phase over an ultra-wide frequency band, which is the key requirement for providing an effectively in-phase reflection at the antenna plane. The composite structure is compact, with a total height of λ/4, where λ is the free-space wavelength at the lowest operating frequency of 3 GHz. Experimental results show an impedance bandwidth of 122%. The antenna gain is maintained around 7.5 dBi from 3 to 7 GHz. Between 7-14 GHz, the antenna is more directive with a gain of about 9 dBi with ±0.5 dB variation. Experimental measurements con firm the predicted wideband antenna performance and gain enhancement due to the FSS reflector.

Circularly Polarized Ellipse-Loaded Circular Slot Array for Millimeter-Wave WPAN Applications

A circularly polarized slot array suitable for millimeter wave WPAN applications is presented. The array uses a new type of circularly polarized element, comprising a circular slot loaded with an elliptical patch, and is fed by a microstrip line. The array is implemented on liquid crystal polymer (LCP), which is both low-loss and low-cost. A total of 8 array elements are used in a 4 times 2 configuration to achieve a maximum gain target of greater than 14 dBic. Sequential rotation of the elements within each of the 2 times 2 sub-arrays is used to increase the axial ratio bandwidth. Other novel features of the array are the finite ground plane, and open cavity structure used to support the LCP substrate of the slot array above a reflector plate. Measured and computed results presented for the packaged array confirm its wide impedance, axial ratio, and gain bandwidths. The effect of the packaged array back-short on the measured axial ratio is also discussed.

A Frequency-Reconfigurable Quasi-Yagi Dipole Antenna

A frequency reconfigurable quasi-Yagi dipole antenna is presented in this paper. Simulation results show that a 35% continuously tuning impedance bandwidth, from 1.85 to 2.5GHz, is obtained. Loaded with varactor diodes, the effective electrical lengths of the driver and director dipoles are varied by changing the biasing voltages. The length adjustable director allows the endfire pattern with relatively high gain being maintained over each tuning sub-band. Across the tuning bandwidth, the predicted gain variation is from 5.8dBi to 7.9dBi. The H-plane cross-polarization level is below −17dB while this value is below −32dB in the E-plane. The front-to-back ratio is better than 12dB.