Mohammadreza Tayfeh Aligodarz

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.

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.

Photoresist-based dielectric resonator antenna fabrication and performance: A review

Some of the techniques that we have used to facilitate the fabrication and enhance the performance of photoresist-based dielectric resonator antennas are presented. These techniques are discussed under three different categories of material development, fabrication process, and antenna performance.

A fast and efficient permittivity estimation method for artificially engineered microwave materials

A simulation based method is proposed to estimate the effective permittivity of artificially engineered materials with controlled electromagnetic properties, that can increase the effective permittivity of low permittivity dielectrics. Details of the proposed method and the results for a sample with window-shaped inclusions are discussed. The method provides a systematic approach to study the influence of different parameters on the effective permittivity, which is necessary for using these materials in antenna design.

Fabrication of photoresist-based polymer resonator antennas using ultra-thick SU-8 based dry films

The purpose of this paper is to demonstrate fabrication of various photoresist-based polymer resonator antenna structures using ultra-thick SU-8 based dry films. First, a set of dielectric resonator antenna geometries with modified shapes was designed and included in an X-ray mask. Then the ultra-thick dry films were exposed through this mask to construct antennas. 2 mm tall structures with fine features down to 50 μm are achieved. The preliminary results show the existence of new modes in modified shapes which can be utilized for future antenna applications. Over 90% optically transparency makes these SU-8 based structures highly attractive for transparent antenna applications.

Investigations on Photoresist-Based Artificial Dielectrics With Tall-Embedded Metal Grids and Their Resonator Antenna Application

Novel photoresist-based artificial bulk dielectrics with tall-embedded metal grids are investigated and their radiation characteristics are studied in detail. The embedded metal grid substantially increases (around six times) the effective permittivity of the low permittivity base photoresist material, and enables excitation with a simple direct microstrip feed to radiate effectively at frequencies similar to a high-permittivity dielectric resonator antenna (DRA). Moreover, the grid changes the near fields inside the resonator and introduces novel modes that are completely different than those of usual DRAs. A set of artificial resonator antennas is designed, lithographically fabricated, and measured to demonstrate the new modes resonating at 17 and 19 GHz with 2% and 6% bandwidth and 7.6 and 6.6 dB broadside gain, respectively, with suitable cross-polarization levels of better than -20 dB. These new modes do not appear simultaneously, and can be excited separately by varying the length of the microstrip feed.