Farhad Bayatpur

Single-Layer High-Order Miniaturized-Element Frequency-Selective Surfaces

A new miniaturized-element frequency-selective surface is presented in this paper. This frequency-selective surface is made up of a 2-D periodic array of metallic loop and a wire grid of the same period printed on either side of a very thin substrate. Unique features of the new design include localized frequency-selective properties, high-order frequency response achieved by a single substrate, lack of passband harmonics in the frequency response, and very low frequency response sensitivity to the incidence angle. High-order frequency response is accomplished through the application of a thin substrate that allows considerable couplings between the elements on the two sides of the substrate. The layers couplings in conjunction with each layer characteristics are designed to produce a high-Q bandpass frequency response, in addition to a transmission zero. It is shown that by inserting variable capacitors in the gap between the metallic loops, the center frequency of the passband can be tuned over nearly an octave. In addition, using a cluster of loops as the unit cell and modifying the parameters of the loops within the cluster, a dual-band characteristic from a single-layer miniaturized-element frequency-selective surface can be achieved. A prototype sample of the miniaturized-element frequency-selective surface, whose unit cell can be as small as lambda0 /12, is fabricated to verify the design performance through a standard free-space measurement setup. The transmission characteristic of the structure is measured and compared with numerical simulation results.

A Tunable Metamaterial Frequency-Selective Surface With Variable Modes of Operation

A reconfigurable miniaturized-element frequency-selective surface (FSS) is presented in this paper. A standard waveguide measurement setup is used to evaluate the performance of the design. The proposed FSS consists of two periodic arrays of metallic loops, with the same periodicity, on either side of a very thin dielectric substrate. The tuning of the reconfigurable surface is shown numerically to be possible by incorporating tuning varactors into the structure. Using varactors on both layers, a reconfigurable frequency response is achieved, which has two modes of operation: bandstop and bandpass. In addition to two completely different modes of operation, the center frequency, as well as the bandwidth of the response can be tuned independently. Frequency tunability with a constant bandwidth over 3-3.5 GHz is shown. A bandwidth tuning at a fixed center frequency is also demonstrated. Simulation results are verified experimentally by fabricating prototypes of the design at S-band loaded with lumped capacitors. To demonstrate the tunability, different pairs of fixed-valued capacitors, as opposed to varactors, are used in a waveguide measurement setup to avoid difficulties associated with biasing varactors in the waveguide.

Multipole Spatial Filters Using Metamaterial-Based Miniaturized-Element Frequency-Selective Surfaces

This paper presents a novel multipole miniaturized-element frequency-selective surface (FSS) having a very low thickness and a desired multipole frequency response. For this design, new miniaturized elements for the FSS are developed to achieve low thickness solution and improved functionality. The proposed FSS enables implementation of higher order spatial filters over low-profile conformal antenna arrays. First, design of a very thin-layer modified miniaturized-element FSS producing a single-pole bandpass response in addition to a transmission zero is presented. The modified design is just a single-sided circuit board with a particular unit cell consisting of a loop centered within a wire grid. Next, using a similar metallic pattern on the other side of a very thin substrate, a dual-bandpass frequency response is produced. This response is achieved by choosing proper dimensions for the loops and wire of each layer and by appropriately positioning the layers with respect to each other. To establish a benchmark, dual-pole FSSs using cascaded layers of a previously designed miniaturized-element FSS are considered. In comparison, the modified dual-bandpass design has only two metal layers, instead of four, and a single substrate, instead of three. The proposed multipole FSS has a thickness of lambda/300 which is six times thinner than the benchmark structures. Moreover, the frequency response of the new FSS shows higher out-of-band rejection values. Performance of the multipole screens is tested by fabricating FSSs with maximally flat and dual-bandpass responses and measuring their frequency responses using a standard measurement setup in a free-space environment.

Tuning Performance of Metamaterial-Based Frequency Selective Surfaces

A bandpass, miniaturized-element frequency selective surface with relatively high out-band rejection was recently reported whose unit cell dimensions are smaller than lambda 0 /10. The new FSS is made up of a metallic, square loop array and a metallic grid on either side of a thin dielectric substrate. Further analysis on this structure reveals that, a very wide frequency tuning can be accomplished by inserting lumped, variable capacitors in the gap between the loops. This paper presents the tunability performance of the miniaturized-element FSS. For experimental verification, a waveguide measurement approach is chosen to lower the fabrication cost. The FSS unit cell dimensions are modified appropriately to fit within a WR90 waveguide aperture and appear as a perfect periodic surface once image theory is applied. Prototypes of the structure in the form of a waveguide flange at X-band are fabricated and then loaded with fixed-valued, chip capacitors of different values. Each waveguide sample contains eighteen capacitors mounted on it. The measurement results show that a wide tuning range, with acceptable performance, from 8.49 to 11.48 GHz can be accomplished by altering the capacitance from 0.2 to 0.05 pF.

Design and Analysis of a Tunable Miniaturized-Element Frequency-Selective Surface Without Bias Network

A novel, tunable miniaturized-element frequency-selective surface that does not require additional bias networks is presented. This spatial filter is composed of two wire-grids printed on opposite sides of a substrate and connected to each other with an array of varactors using plated via holes. Varactor diodes are positioned between the grids. Via sections and metallic pads are fabricated and create a dc path for biasing the varactors with the grids themselves. This configuration eliminates the need for any additional network, and therefore resolves the design difficulties associated with the spurious response of the bias network. An equivalent circuit model is developed to facilitate the design procedure. Full-wave numerical simulations are used to validate the results based on the circuit model. Simulations show that by altering the capacitance of the varactors from 0.1 to 1 pF, a frequency tunability from 8 to 10 GHz with an almost constant bandwidth can be achieved.

Miniaturized FSS and Patch Antenna Array Coupling for Angle-Independent, High-Order Spatial Filtering

A new filter-antenna array design is presented in this article. This design approach can be employed to simplify the vertical integration of array beamformers. Basically, by placing a high-order filter, whose response is not sensitive to angle of arrival, in front of the array elements, the need for integrating bulky RF filters behind each element is eliminated. A new method for design of such phased-arrays is provided here in which the bandpass filters are removed, and instead, a metamaterial-based frequency-selective surface is placed directly over the antenna to perform the necessary filtering. The small spacing between the frequency-selective surface and the antenna, which is as low as ~ ¿/10, results in creation of a compact filter-antenna design. The close proximity of the surface and the antenna is utilized to achieve a proper coupling between the selective surface and the array for high-order filtering without adversely affecting the gain or scan characteristics of the array. To test the performance of this approach, a 9 × 9-element patch-array is fabricated and measured at X-band. The array is then loaded with a 0.003-thick, single-pole frequency-selective layer at a close distance on top. The measured received power as a function of frequency exhibits an improved frequency selectivity (better than the frequency-selective surface or the array alone). An improvement of about 50% reduction in the bandwidth and significantly higher frequency roll-off rate is observed once the array is covered with the metamaterial surface.

Advances in nonlinear measurement & modeling of bulk acoustic wave resonators (invited)

Microwave acoustic filters operated at high power levels generate nonlinear mixing products necessitating improved nonlinear modeling. The latest communications protocols place strict limits on such mixing products including the generation of intermodulation distortion products, which desensitize a receiver, as well as second harmonic emissions. We extend the conventional nonlinear acoustic theory and practice into the microwave frequency range. We have developed a methodology for modeling the linear and nonlinear response of a resonator comprised of arbitrary piezoelectric and metal electrode layers and thicknesses. In this work we assume that the piezoelectric film is the dominant nonlinear source. An extended set of piezoelectric constitutive equations containing arbitrary nonlinear terms representing the behavior of the piezoelectric film is mapped into a general purpose nonlinear circuit, which is an extension to Masons original linear model of a resonator. A harmonic balance circuit simulator is used to solve this extended set of constitutive equations. We have fitted our model to a set of nonlinear two-tone resonator measurements by identifying four dominant nonlinear terms, by adjusting their corresponding scaling parameters such that the in- and out-of-band 2nd harmonic emissions and inter-modulation-3 products generated by the model is consistent with our measurements.

Experimental Characterization of Chiral Uniaxial Bianisotropic Composites at Microwave Frequencies

This paper presents an experimental procedure for retrieving the effective constitutive parameters of chiral materials. Unlike past research that primarily deals with isotropic materials, this study considers a lossy uniaxial bianisotropic slab with a nonzero chirality along its axial direction. First, plane-wave scattering off the uniaxial slab in a free-space environment is studied analytically. This forward analysis gives insight into the problem and the choice of proper independent measurements required for the inverse process, i.e., retrieving the slab constitutive parameters from its S-parameters. Based on this analysis, three sets of co-polarized and cross-polarized S-parameters are required, including both the transmission and reflection coefficients of the slab. Given the measured scattering data, the complex permittivity, permeability, and chirality tensors are determined numerically using the results of the analytic study. To test the performance of the new retrieval method, an array of 2 × 56 long, metallic helices is designed and fabricated for operation at C-band. Having the same handedness, the helices are closely spaced and held in parallel to each other in a wooden frame in order to create an effective uni-axial chiral medium. A conventional transmission/reflection setup measures the array scattering parameters, which are fed into the retrieval process to obtain the effective parameters. The measured parameters well model the array scattering response, exhibiting a significant averaged chirality of ~0.4 over 5.5-8.7 GHz and a plasmonic behavior at ~7.1GHz.

A tunable, band-pass, miniaturized-element frequency selective surface: Design and measurement

Frequency selective surface (FSS) structures, with resonant unit cells whose dimensions are comparable to half of a wavelength, have been studied for a variety of applications like radar and satellite communications. FSSs are usually 2D planar, periodic structures consisting of one or more metallic patterns, each backed by a dielectric substrate. The frequency response of an FSS is entirely described by the geometry of the structure in one period called a unit cell. In traditional FSS design, the frequency selective properties result from mutual interactions of the periodic FSS elements.

Composites with mechanically tunable plasmon frequency

This paper summarizes our efforts to create a composite material with a mechanically tunable plasmon frequency at the microwave band. The permittivity of the composite changes sign at the plasmon frequency. Such composites, therefore, can be used as electromagnetic filters. Theoretically, an array of non-magnetic, metallic wire coils has been shown to have a plasmon behavior that is dependent on the wire thickness, coil inner diameter, pitch and coil spacing. Here, a material is made out of an array of coils placed within a non-metallic frame, and the material plasmon frequency is tuned through altering the pitch. The coils are arranged with alternating handedness to create an effective, non-chiral medium. A transmit/receive setup is used to characterize the electromagnetic behavior of the composite. The setup consists of a vector network analyzer and two horn antennas, which are used to measure the scattering parameters of the material. These parameters are then used to calculate the permittivity. The results show an increase in the plasmon frequency with increase in the pitch. Increasing the pitch 30%, from 3 to 3.9 mm, results in a corresponding increase from 6.3 to 7.5 GHz in the frequency. (Some figures in this article are in colour only in the electronic version)