D. N. P. Thalakotuna

Controlling the beam scanning limits of a microstrip leaky-wave antenna

An electronically controlled half-width microstrip leaky-wave antenna (HW-MLWA) that can scan the main beam at a fixed frequency is presented. A technique is described to change the upper and lower limits of the beam direction without changing the scanning range itself. By varying the reactance profile at the free edge, the main beam of the proposed antenna can scan a range of 23° in discrete steps. There are 30 stubs positioned at the free edge of the antenna that gives the freedom to the designer to change the upper and lower scanning limits of the antenna without changing the scanning range itself.

Active Switching Devices in a Tunable EBG Structure: Placement Strategies and Modelling

Different placement strategies for Field-Effect-Transistor (FET) active switching devices in an electromagnetic band gap (EBG) periodic structure are investigated. Two possible placements for the switches are considered: (a) switches on top of a substrate and (b) switches at the ground-plane level. Their advantages and drawbacks are analyzed and discussed in relation to fabrication, extension to 2-D and ground plane design. The transmission results and band gaps of the EBG structure are presented for both switch placements. When determining the transmission of the structure, the FET switches are modeled in two different ways, first as an ideal conductor and then using a more accurate model consisting of embedded scattering parameters. Significant differences have been observed between the results predicted using simple and accurate models. Experimental characterization of the switch reveals technological issues related to their biasing.

Effect of active device insertion losses on the electromagnetic bandgap characteristics of a tunable 1D periodic structure in the S band

Effect on the electromagnetic bandgap (EBG) of an active, tunable, EBG structure, due to the insertion loss and other scattering parameters of active switching devices, is investigated. Active FET switches are modeled two ways, first as perfect conductors with the switch packaging and secondly as a black box with embedded scattering parameters provided by the switch manufacturer. The results obtained from the simulations for the transmission characteristics of the EBG structure shows a significant downshift in the resultant electromagnetic bandgap due to the additional transmission line effects introduced by the switches.

Modelling PIN diode switches in reconfigurable leaky-wave antenna design

The effect of PIN diodes on the performance of a reconfigurable leaky-wave antenna (LWA) is studied. The PIN diode is modelled in three different ways. It is shown that the non-ideal behaviour of the PIN diodes changes the predicted return loss and radiation characteristics of the antenna compared to ideal switches.

Investigation on FET switch integration techniques for a tunable microwave periodic structure

A recently proposed tunable microwave periodic structure requires many switches to dynamically change its microwave propagation and radiation characteristics. The performance of a discrete Field Effect Transistor (FET) switch package, suitable for the structure, is investigated in two potential configurations. In one configuration, the switch is co-planar with the ground plane and in the other it is on a different surface. Measurements made on two test boards confirm that when the switches are turned “on” the switch insertion losses are not significantly different between the two configurations. However when the switches are turned “off”, locating the switches on the ground plane provide better isolation between RF ports.

EQUIVALENT-CIRCUIT MODELS FOR EFFICIENT TRANSMISSION AND DISPERSION ANALYSES OF MULTI-STATE PERIODIC STRUCTURES

An equivalent-circuit model for a reconfigurable unit cell is proposed. This circuit model facilitates fast prediction of scattering parameters and dispersion analyses of a reconfigurable periodic structure. The cutoff frequencies obtained using equivalent-circuit models are in excellent agreement with those from measurements and full-wave numerical simulations. The proposed circuit model is then modified to include non-ideal, commercial RF FET switches. The effect of such a switch in each state, On or Off, is modeled by a frequency-dependant impedance, derived from the scattering parameters of the switch. The proposed technique can be used to analyze a reconfigurable periodic structure with any type of switches. For the structure with 24 unit cells considered here, the equivalent circuit model is about five orders of magnitude faster than full-wave simulations.

A transmission polarizer based on width-modulated lines and slots

In this paper a novel printed Frequency Selective Surface (FSS) transmission polarizer is presented. A width-modulated microstrip unit cell is used as the reference element, loaded by a capacitive tuning slot, to achieve the desired axial ratio (AR) performance for the transmitted electromagnetic wave. The dimensions of the unit cell are λ0/2×λ0/2 at 30 GHz. This polarizer provides an axial ratio of 0.99 at the resonance and a cross polarization lower than -20 dB in a more than 5% bandwidth. Theoretical analysis and results of numerical investigations related to the new structure are presented.

A technique to extract dispersion characteristics of one-dimensional periodic structures

A method is proposed to extract the phase constant β and attenuation constant α of bounded waves in a finite periodic structure using scattering parameters. The proposed method accounts for the mutual coupling between the unitcells when computing the wavenumbers of the propagating modes. It is useful when the unit cell configuration is complex and typical eigen-mode solvers fail.

Wide-band matching of a tunable periodic structure in GaAs technology

Wide-band matching of a tunable 1D periodic microwave structure, designed for fabrication in GaAs technology, is investigated. The two-layer unit cell of the periodic structure consists of a microstrip line loaded by multiple rectangular patches, which can be selectively short-circuited. The wide variation of the position of the bandgap, obtained by electrically controlled switches, induces a large variation of the effective dielectric constant in the considered wide frequency range. This effect is further reflected as a significant variation of the characteristic impedance, and hence a potentially high mismatch loss. A method for wide-band minimization of the mismatch (with respect to a 50Ω reference impedance), without compromising the performance of the device, is presented.