Mohammad Reza Chaharmir

A Wideband Transmitarray Using Dual-Resonant Double Square Rings

A four-layer transmitarray operating at 30 GHz is designed using a dual-resonant double square ring as the unit cell element. The two resonances of the double ring are used to increase the per-layer phase variation while maintaining a wide transmission magnitude bandwidth of the unit cell. The design procedure for both the single-layer unit cell and the cascaded connection of four layers is described and it leads to a 50% increase in the -1 dB gain bandwidth over that of previous transmitarrays. Results of a 7.5% -1 dB gain bandwidth and 47% radiation efficiency are reported.

Design of Broadband, Single Layer Dual-Band Large Reflectarray Using Multi Open Loop Elements

A novel method is introduced to design a single layer, dual-band large printed reflectarray with open loop elements of variable size for both bands. The reflectarray is designed for two frequency bands: 11.4-12.8 GHz for receive and 13.7-14.5 GHz for transmit. Different classes of open cross loop elements were used in the design of the receive band elements. Noting the larger relative bandwidth at the lower band as compared to the upper band, the dimensions of these cross loops are adjusted, using an optimization technique to achieve required phase distribution at the center frequency and minimize frequency dispersion at extreme frequencies of the lower band. Double square open loop elements with variable loop length were used for the transmit band elements. The reflectarray consists of 3 × 3 panels of 40 cm × 40 cm, that are arranged side by side to construct the large 120 cm × 120 cm reflectarray. The flat configuration and modular nature of this reflectarray gives it an advantage from the installation point of view as compared to conventional dish antennas.

Novel photonically-controlled reflectarray antenna

A novel method for generating a reconfigurable reflectarray based on the creation of photoinduced plasma inside the semiconductor substrate of a reflectarray is presented in this paper. A reflectarray antenna prototype is fabricated and measured to demonstrate the validity of the design methodology. Measurements were carried out to characterize the bandwidth and the gain of the antenna in both the OFF and ON states of an optical flash lamp that was used to induce photoconductivity inside the silicon substrate.

Loss Reduction in Reflectarray Designs Using Sub-Wavelength Coupled-Resonant Elements

A novel design approach is proposed for designing high performance reflectarrays using low-cost, lossy substrates. Using sub-wavelength coupled-resonant elements, in particular sub-wavelength loops, one can dramatically reduce losses in reflectarrays and not incur significant gain drop due to the use of the lossy, low-cost substrate. It is further shown that the sub-wavelength loop achieves sufficient phase variation in a single layer design with a modest requirement in etching tolerance, making the loop superior to both the single and double-layer sub-wavelength patch elements.

Design of a Multilayer X-/Ka-Band Frequency-Selective Surface-Backed Reflectarray for Satellite Applications

A dual-band X-/Military Ka-band (MKa-band) single-aperture reflectarray structure that transmits and receives at both MKaand X-bands is introduced in this paper. Frequency selective surface (FSS) is used as the ground plane for the MKaband reflectarray. A cascade configuration of FSS-backed reflectarrays is designed for each of the receive and transmit bands of MKa-band to reduce the coupling between the elements of these two bands when they are etched on the same substrate. Additional FSSs are implemented to further enhance the isolation between the bands. The MKa-band elements are located on top of the X-band reflectarray that is composed of elements etched on a perfect electric conductor (PEC)-backed ground plane. The reflectarray elements also convert circular to linear polarization which results in a simplified feed structure.

A broadband transmitarray using double square ring elements

Dual-resonant double square ring elements are applied to the design of a four-layer transmitarray operating at 30 GHz. The design procedure is described and results of a 7.5% −1 dB gain bandwidth and 47% radiation efficiency are reported.

Novel approach for low-loss reflectarray designs

A novel design approach is proposed for designing high performance reflectarrays using low-cost, lossy substrates. Using subwavelength elements, in particular subwavelength loops, one can dramatically reduce losses in reflectarrays and not incur significant gain drop due to the use of the lossy low cost substrate. It is further shown that the subwavelength loop achieves sufficient phase variation in a single layer design with a modest requirement in etching tolerance, making the loop superior to the single and double-layer subwavelength patch element.

On Beam Squint in Offset-Fed Reflectarrays

A full-wave receive-mode analysis is used to show that it is the shift in the location of the focal point with frequency (caused by nonconstant path delays over the surface of the reflectarray) that is principally responsible for the beam squint that occurs in offset-fed reflectarray antennas, and not the element type used or the lattice size. A similar transmit-mode analysis confirms that it is this focal point shift that implies noncoincidence of the focal point and the feed phase center at off-center frequencies, which results in a phase distribution over the reflectarray aperture with a slope other than that required to have the main beam in the desired direction. We further demonstrate this fact (computationally and experimentally) by showing that if the feed is physically moved to the shifted focal point at some off-center frequency, the main beam pointing direction at this frequency is restored.

Derivation and Validation of the Basic Design Equations for Symmetric Sub-Reflectarrays

The basic design equations for symmetrical sub-reflectarrays are derived. These provide the required phase-distribution on the sub-reflectarray surface. Both ellipsoidal-type and hyperboloidal-type sub-reflectarrays are designed and fabricated using single-layer rectangular patch elements. Measured amplitude and phase patterns are shown for both types of sub-reflectarray, and compared to predictions. The behaviour of the ellipsoidal-type sub-reflectarray is furthermore compared to measured patterns of its solid subreflector counterpart. These experimental results show that the sub-reflectarrays indeed emulate the behaviour of their solid subreflector counterparts, and the validity of the basic design equations is thus established.

Reflectarray Design With Similarity-Shaped Fragmented Sub-Wavelength Elements

A new technique for synthesizing reflectarray antennas is presented. It utilizes fragmented elements in a manner that allows the elements of the reflectarray to be shaped-optimized so that a high degree of geometrical similarity is maintained between adjacent elements. The implication is that in a reflectarray of such elements each element will see an electromagnetic environment that more closely emulates the infinite periodic one used to compute the element properties. We show experimentally that this indeed results in aperture efficiencies closely approaching the upper bounds achievable for some selected feed system, and offers a significant improvement over that obtained with conventional reflectarrays (that is, those not using similarity-synthesized fragmented elements). It is also shown that a reflectarray surface that uses fragmented elements can be simultaneously patterned with a visual image while still closely maintaining the desired reflection phase from its surface to yield a high-gain antenna.