Efstratios Doumanis

Anisotropic Impedance Surfaces for Linear to Circular Polarization Conversion

Anisotropic impedance surfaces are employed as low-profile and broadband reflectors that convert orthogonal linear to right- and left-handed circular polarization respectively. By virtue of anisotropy, it is possible to independently control the reflection characteristics of two orthogonal linearly polarized incident plane waves and therefore achieve linear to circular polarization conversion. Equivalent circuits for anisotropic impedance surfaces with arbitrarily shaped elements are employed to demonstrate the operating principle and a design procedure is proposed. The proposed design procedure is demonstrated by means of an example involving a dipole array. A prototype is designed and its performance characteristics are evaluated. The 3-dB relative axial ratio bandwidth exceeds 60%, while low loss and angular stability are also reported. Numerical and experimental results on a fabricated prototype are presented to validate the synthesis and the performance.

Electronically Reconfigurable Liquid Crystal Based Mm-Wave Polarization Converter

An electronically tunable reflection polarizer which exploits the dielectric anisotropy of nematic liquid crystals (LC) has been designed, fabricated and measured in a frequency band centered at 130 GHz. The phase agile polarizing mirror converts an incident slant 45° signal upon reflection to right hand circular (RHCP), orthogonal linear (-45 °) or left hand circular (LHCP) polarization depending on the value of the voltage biasing the LC mixture. In the experimental set-up this is achieved by applying a low frequency bias voltage of 0 V, 40 V and 89 V respectively, across the cavity containing the LC material.

Inline Interdigital Pseudo-Elliptic Helical Resonator Filters

A new inline coupling topology for narrowband helical resonator filters is proposed that allows to introduce selectively located transmission zeros (TZs) in the stopband. We show that a pair of helical resonators arranged in an interdigital configuration can realize a large range of in-band coupling coefficient values and also selectively position a TZ in the stopband. The proposed technique dispenses the need for auxiliary elements, so that the size, complexity, power handling and insertion loss of the filter are not compromised. A second order prototype filter with dimensions of the order of 0.05λ, power handling capability up to 90 W, measured insertion loss of 0.18 dB and improved selectivity is presented.

Helical Resonator Filters With Improved Power Handling Capabilities for Space Applications

The power-handling capabilities of helical resonator filters for space applications are discussed. Emerging difficulties due to the multipaction effects are highlighted. A method is proposed to increase specified power handling without significantly sacrificing the size/quality factor. Experimental verification is attained by means of a fabricated prototype for which measured filter response and multipaction test results are obtained and presented.

Mm-wave low-profile reflection polarizer

This paper proposes a mm-wave low-profile reflection polarizer based on single layer anisotropic impedance surfaces. It is demonstrated that by virtue of anisotropy it is possible to independently control the reflection characteristics of the two orthogonal components of a linearly-polarized incident plane waves and therefore achieve polarization conversion. A W-band prototype has been designed and its performance characteristics are evaluated. The 1.5 dB axial ratio relative bandwidth exceeds 51 %, while low loss and better than previously reported angular stability are also demonstrated. The structure is compatible with traditional PCB etching techniques and does not require stacking of multiple layers.

Compact 60-GHz lens antenna with self-alignment feature for small cell backhaul

In this paper we present a highly compact and low-cost antenna solution based on a dielectric lens and a waveguide horn array feed for operation in the 57–64 GHz frequency band. The antenna is targeted for applications in point-to-point backhaul systems for Small Cell mobile access networks and features a beamforming capability providing self-alignment for ease of installation and improved link stability.

Dielectric powder loaded coaxial-cavity filters

In this paper we demonstrate the use of low-loss dielectric powders in the design and fabrication of RF filters. Initially, several different powders are tested for their RF performance — in particular their relative dielectric permittivity and loss tangent. Then, based on the information obtained in this way, the best performance powder is used to design a 5-pole Chebyshev filter operating at a centre frequency of 779 MHz, with a bandwidth of 17 MHz. Upon fabrication of the filter housing, the filter cavities are filled with the powder. Compaction of the powders is achieved in a systematic way and has resulted in stable performance.

A Technique to Suppress Harmonic Outputs in Coaxial Cavity Filters at the Excitation Port

A technique to improve the spurious performance of coaxial cavity filters is investigated. It involves incorporating elements/posts resonant at spurious frequencies within the coaxial input/output cavities. To perform suppression and control of harmonic outputs, the spurious elements are coupled directly to the excitation posts and prevent the excitation of harmonics at the fundamental elements. The spurious elements are physically significantly smaller than the fundamental elements that perform the basic filtering function (fundamental frequency); thus they impose only minimized degradation to the ohmic loss of the devices and miniscule additional design effort. Design guidelines of the proposed filters are given. Obtained results demonstrate significant improvement in the spurious performance, and experimental results verify the concept.