Mahmoud Niroo-Jazi

Design and Implementation of an Integrated UWB/Reconfigurable-Slot Antenna for Cognitive Radio Applications

A new incorporated planar ultrawideband (UWB)/reconfigurable-slot antenna is proposed for cognitive radio applications. A slot resonator is precisely embedded in the disc monopole radiator to achieve an individual narrowband antenna. A varactor diode is also deliberately inserted across the slot, providing a reconfigurable frequency function in the range of 5-6 GHz. The slot is fed by an off-centered microstrip line that creates the desired matching across the tunable frequency band. The measured antenna parameters are presented and discussed, confirming the simulation results.

Electronically Sweeping-Beam Antenna Using a New Cylindrical Frequency-Selective Surface

An electronically sweeping radiation-pattern antenna using a new active cylindrical frequency-selective surface (ACFSS) is presented for 2.45-GHz operating frequency. The ACFSS screen is constructed by a new reconfigurable hybrid unit cell, providing more freedom to control the antenna radiation characteristics. A design guideline based on the reflector antenna principles is also introduced to estimate the overall dimensions of the antenna geometry, and then the required number of unit cells for the ACFSS surface is calculated. Furthermore, to confirm the proposed design guideline, the performance of the designed antenna is compared to three conventional reflector antennas constructed with solid perfect electric conductors. The achieved simulation results for these antennas are carefully examined and the impact of the FSS surface on enhancing the ACFSS antenna gain is discussed. Moreover, parametric studies for the crucial dimensions of the unit cell are also presented to demonstrate the effect of each parameter on the antenna radiation characteristics. To completely examine the achieved results, the transmission coefficient response of the applied FSS sheet is experimentally assessed, and the antenna measured parameters are accordingly interpreted.

Z-Shaped Dielectric Resonator Antenna for Ultrawideband Applications

A new dielectric resonator antenna (DRA) is introduced for ultrawideband applications. Three methods of a Z-shaped dielectric resonator, a bevel feeding patch, and an air gap between the DR and the ground plane are used to obtain an ultrawideband impedance bandwidth. The effective dielectric constant and the -factor can be reduced by introducing the air gap. The bevel-shaped feeding mechanism provides a smooth impedance transition from one resonant mode to another. Furthermore, the results show that by applying a Z-shaped DRA, the electric field inside the dielectric is reformed over the desired bandwidth leading to improve the matching. A comprehensive parametric study is carried out using CST to optimize the bandwidth of the proposed antenna. The measurement results demonstrate that the proposed DRA provides an ultra bandwidth of about 120%, ranging from 2.5 GHz to 10.3 GHz for the reflection coefficient less than .

Experimental Investigations of a Novel Ultrawideband Dielectric Resonator Antenna With Rejection Band Using Hybrid Techniques

Practical investigations of a novel dielectric resonator antenna (DRA) are presented. The antenna is designed to create an ultrawide bandwidth with a stopband response centered at frequency 5.5 GHz. Hybrid techniques including multiresonator configuration, complex materials, DR shaping, and impedance matching are applied to obtain the desired objectives. A circular ring patch (CRP) with a monopole-like radiation pattern is integrated into the structure to control the position of the stopband and the radiation performance of the antenna over this bandwidth. Based on the examination of the electromagnetic near-field patterns, the physical explanation of the antenna radiation mechanism is provided at different frequencies across the achieved ultrawideband spectrum. The measured antenna characteristics are also exhibited, confirming the integrity of the proposed configuration.

A Hybrid Isolator to Reduce Electromagnetic Interactions Between Tx/Rx Antennas

A hybrid compact isolator with a cross section of 17 × 11 mm2 is proposed to reduce the electromagnetic coupling between two vertically polarized Tx/Rx microstrip antennas, both operating at 9.4 GHz. This isolator is built by embedding a piece of artificial material between two metallic walls. The obtained results using this hybrid isolator demonstrate that the coupling between the two antennas is significantly reduced across at least 55% of the matching band. To substantiate the proposed configuration, a prototype is fabricated and tested, and the corresponding measured results are presented. Furthermore, the discrepancies between the simulations and measurements are interpreted with the aid of the electromagnetic (EM) simulator.

A New Triple-Band Circular Ring Patch Antenna With Monopole-Like Radiation Pattern Using a Hybrid Technique

Investigation results of a new conical-shape radiation-pattern antenna based on a hybrid technique are presented. A circular-ring patch antenna excited with the dominant mode TM02 is integrated with a monopole antenna, offering two distinct operating bands. The coupling effect of the higher-order modes of the monopole and the circular-ring patch create a third operating broad band. Using this hybrid resonant configuration, the expected ripples in the E-plane pattern due to the monopole higher-order modes are alleviated over the third band, providing a good radiation performance. The measurement results performed for the proposed structure represent a good agreement with the simulated ones.

On Radiation Performances of Reflectarray Antennas Constructed With Subwavelength Unit Cells

Multi-element approach is utilized to increase the phase variation range of a square-ring subwavelength unit cell (UC) from 313° to more than 560°. Radiation characteristics of three thin reflectarray antennas (TRAs) constructed with the proposed UCs are investigated. This investigation demonstrates that to achieve a broadband highly efficient TRA, the employed λ/ 5 - UC must have reflection loss considerably less than 0.1 dB and its constructing elements should also scatter the incident fields in the same fashion, i. e., the current distributions of all parts of the multi-element UC are of the same shape and co-phased.

Reflectarray antennas using single layer polarization independent multi-resonant unit cells

The investigation results of a single layer reflectarray (RA) antenna are presented and discussed. The design frequency of the RA is set at 12 GHz, and it consists of 20×20 polarization independent resonant unit cells (UCs), which are illuminated by a circular horn antenna placed at F=200 mm to obtain F/D=0.77. The UCs are composed of three resonant elements, a cross loop surrounded by two concentric circular rings. The obtained results demonstrate that the proposed new UC provides a large linear phase variation range of at least 700°. The designed RA antenna offers a gain of at least 27.5 dB for an aperture area of 10Λ×10λ at the operating frequency.

Antenna beam shaping using conformal hybrid AMC/EBG reflectors

A conformal cylindrical hybrid reflector is used to increase the directivity of a dipole antenna and to control its radiation pattern at 2.45 GHz. The hybrid reflector is constructed with mushroom electromagnetic band gap (EBG) material and artificial magnetic conductor (AMC). The reflection phase responses of both EBG and AMC sections are precisely adjusted to broaden the beamwidth in the azimuth and decrease it in the elevation plane, while maintaining almost constant directivity compared to the case where only a conformal AMC reflector is used. The half power beamwidth (HPBW) in the H-and E-planes and realized gain of the proposed antenna are 112°, 35°, and 9 dB, respectively.

On the antenna gain enhancement using artificial materials

A single layer conventional mushroom-like electromagnetic bandgap structure is proposed to enhance the gain of a coaxial fed patch antenna. This EBG configuration is used in both E- and H-plane directions to investigate their effect on the antenna radiation pattern. The primary simulation results demonstrate that, without noticeably losing the matching bandwidth, more than two times gain enhancement is achieved by effectively converting the suppressed surface waves into the radiated ones.