Atabak Rashidian

On the Two Segmented and High Aspect Ratio Rectangular Dielectric Resonator Antennas for Bandwidth Enhancement and Miniaturization

The goal of this communication is to maintain wide impedance bandwidth while miniaturizing a microstrip-fed dielectric resonator antenna (DRA). The approach of using high aspect ratio structures is proposed, discussed, and compared with the two segmented structures approach. Finite element method parametric study of high permittivity high aspect ratio structures is used to demonstrate how two appropriate resonant modes can be merged to obtain miniaturized antennas with broadband operation, stable radiation patterns, and low cross polarization levels. To show the effectiveness of the approach, a set of microstrip-fed two segmented and high aspect ratio DRAs is fabricated, characterized, and the results are compared with Ansoft HFSS simulations. A high aspect ratio DRA with cross section as small as 0.08lambda by 0.12lambda has better than 11 percent bandwidth and is especially attractive for compact array applications.

Compact Wideband Multimode Dielectric Resonator Antennas Fed With Parallel Standing Strips

As the number of resonances increases, it becomes difficult to improve and maintain the performance of dielectric resonator antennas (DRAs), over the expanded impedance bandwidth. To remove unwanted modes, adjust the frequency distance between individual modes, reduce antenna size and cross polarization, and preserve radiation patterns in a wideband configuration, a dielectric resonator antenna fed with parallel standing strips is proposed in this paper. The use of parallel standing strips provides several degrees of freedom in the design procedure to enhance the DRA characteristics. To validate the effectiveness of this approach, two DRAs with parallel standing strips were fabricated using different procedures. The antennas were tested and characterized. The measured results are in good agreement with simulation ones. A 46% size reduction was achieved for the multimode DRA. The impedance bandwidth of the proposed DRA of simple rectangular shape was over 60% with a measured gain ranging from 5.5 to 9.5 dBi. Broadside radiation patterns with fairly low cross polarizations can be maintained over the impedance bandwidth. The simulated radiation efficiency is more than 96% within the frequency band.

Dielectric characterization of materials using a modified microstrip ring resonator technique

The goal of this study is to present a simple model based on the ring-resonator technique to measure nondestructively the permittivity and loss tangent of dielectric materials. The proposed measurement model utilizes a modified ring-resonator technique in one-layer and two-layer microstrip configurations. This method eliminates the requirement to metalize the samples and enables characterization of permittivity and dielectric loss from 2 to 40 GHz. The effects of conductor and radiation losses that may introduce significant errors in the calculation of the loss tangent, especially at very high frequencies, are minimized. The measurement precision is evaluated by comparing the results with those obtained by using two well-known standard techniques. Uncertainties associated with the proposed model are addressed.

Deep x-ray lithography processing for batch fabrication of thick polymer-based antenna structures

Deep x-ray lithography is applied for the first time to fabricate polymer-based antenna structures with different portions of ceramic contents. To produce successful and viable antenna structures, three different methods are proposed using positive and negative tone resists. In the first method the structures are lithographically fabricated avoiding an intermediate molding step using SU-8 as a photosensitive resist filled with fine ceramic powder with particles in the submicron range. In the second and third methods a polymethylmethacrylate (PMMA) mold is first fabricated by x-ray lithography, and then SU-8/MMA mixed with the high ceramic powder content is injected into the mold. In these methods a final step of crosslinking for SU-8 and polymerization for MMA is also required. Optimized fabrication parameters allow the production of high quality antenna structures as thick as 2.3 mm. X-ray lithography capabilities in fabrication of antennas and other passive microwave components with special features reinforce the idea of fabricating integrated passive microwave circuits along with active circuits using this emerging technology.

On the Matching of Microstrip-Fed Dielectric Resonator Antennas

As the permittivity of dielectric resonators decreases, it becomes difficult to feed the dielectric resonator antennas (DRAs) using direct microstrip lines. The variation of the resonant input resistance with the feed location becomes smaller, while the maximum achievable peak resistance dramatically drops to lower than 50 Ω in most cases. To satisfy the impedance matching, field matching, improve coupling to low-permittivity dielectric resonators, and further increase the antenna bandwidth associated with the dominant mode, without disturbing far-field properties, tapered microstrip line-fed DRAs are proposed, designed, fabricated and evaluated in this communication. Both measurements and simulation investigations are presented and the results are compared with other forms of microstrip feed lines. The impedance bandwidth can be 75% larger than the bandwidth achieved by the step-shape microstrip-fed DRA. Symmetrical radiation patterns with low cross-polarization levels (lower than -22 dB) and a gain ranging from 4.9 to 6.8 dBi, within the impedance bandwidth of the antenna, are observed in the measurements. It is also shown that the proximity coupled tapered microstrip line is an ideal feeding for high-permittivity DRAs in situations where the microstrip line cannot be positioned underneath the dielectric resonator. To verify, one such antenna is designed, simulated and experimentally investigated, obtaining satisfactory results.

Development of Polymer-Based Dielectric Resonator Antennas for Millimeter-Wave Applications

The goal of this paper is to use polymer-based materials (instead of hard ceramics) in fabrication of dielectric resonator antennas at millimeter-wave frequencies. The soft nature of polymers facilitates machining of antennas, while the low permittivity of polymers naturally enhances the bandwidth. More importantly, advantageous properties (e.g., ∞exibility and photosensitivity) of some polymers introduce special capabilities which can not be achieved by ceramics. A photosensitive polymer is utilized in this paper to fabricate polymer-based resonator antennas. As a result, deep X-ray lithography is enabled to produce high quality antenna structures. The proposed dielectric resonator antennas which inherently have very low relative permittivity (usually in a range from 3 to 5) are excited efiectively using a slot-coupled feeding method and analyzed in both the frequency and time domains. Impedance and radiation properties are compared with higher permittivity ceramic antennas. Impedance bandwidths up

Photoresist-Based Polymer Resonator Antennas: Lithography Fabrication, Strip-Fed Excitation, and Multimode Operation

Artificially modified materials are becoming increasingly important in antenna design. Attractive features make polymer composites very promising materials for improving the fabrication process and antenna performance. In this study, a photosensitive polymer composite is utilized to fabricate precise dielectric-resonator antenna structures using deep-X-ray lithography. The multimode operation and miniaturization aspects of strip-fed composite antennas with very low permittivity (εr <; 5 ) are investigated for the first time. The prototype antenna offers a -10 dB impedance bandwidth of 48%, from 18.8 GHz to 30.7 GHz, and gain in the range of 5 dBi. The nonradiating modes are removed by the special boundary conditions enforced by the vertical strip. Stable radiation patterns and low cross-polarization levels over the entire impedance bandwidth are therefore preserved. Further improvements in impedance bandwidth are presented, and the antenna performance and fabrication processes are discussed.

Microwave performance of photoresist-alumina microcomposites for batch fabrication of thick polymer-based dielectric structures

The goal of this paper is to investigate the electrical properties of photoresist-alumina microcomposites with different portions of ceramic content. Substrates of photoresist-alumina microcomposites are fabricated and a comprehensive analysis is performed to characterize their dielectric constant and dielectric loss tangent at microwave frequencies up to 40 GHz. To evaluate the performance of these materials for microwave applications, the properties of various lithographically fabricated antenna elements are examined and analysed based on the measured electrical properties. The experimental results show that the electrical properties of the photoresist composite are nonlinearly affected by ceramic content and also a minimum percentage of ceramic portion is required to improve the electrical properties of the photoresist composite. For instance, comparison of 0 wt% with 23 wt% SU8-alumina shows that no reduction is achieved for the dielectric loss tangent. Comparison of 38 wt% with 48 wt% SU8-alumina microcomposite shows that the dielectric loss tangent is improved from 0.03 to 0.01 and the dielectric constant is increased from 3.8 to 5.0 at 25 GHz. These improvements can result in superior performance for the photoresist-based microwave components.

SU-8 resonator antenna

Originally developed and patented by IBM in 1989 and commercially introduced by MicroChem Corporation in 1996, SU-8 rapidly found wide use in UV and X-ray lithography as a negative tone photoresist for microsystem technology applications [1]. Its very low optical absorption in UV and X-ray ranges leads to uniform exposure conditions for thicknesses up to a few millimetres resulting in high structural quality and excellent sidewall verticality. Cured SU-8 is highly resistant to solvents, acids and bases and has excellent thermal stability, making it attractive for applications in which the SU-8 is considered as a permanent part of the device. For instance, it is used in fabrication of X-ray lenses [2], metamaterials [3], single-mode and multi-mode optical waveguides [4], [5], and micro channels for bio applications [6].

A modified microstrip line for excitation of wide-band dielectric resonator antennas

A novel approach for effectively feeding a wide-band dielectric resonator antenna (DRA) of very low-permittivity is presented in this paper. It is shown that a tapered microstrip line with optimized length and width can provide excellent impedance matching. The impedance bandwidth of the antenna (S11 <; -10 dB) associated with single resonance mode is over 35%. A minimum reflection coefficient of -60 dB is observed in the simulations for a dielectric resonator with dielectric constant of εr = 5. The broadside radiation pattern of the antenna is stable over the impedance bandwidth. Low cross polarization levels (<; -20 dB) across the frequency range 21-30 GHz are noticeable.