Zahra Allahgholi Pour

A Simplified Feed Model for Investigating the Cross Polarization Reduction in Circular- and Elliptical-Rim Offset Reflector Antennas

An analytic model for the primary feed of offset reflector antennas is presented that facilitates a more general investigation of the cross polarization reduction in both asymmetric and inter-cardinal planes. It includes both first and second order azimuthal modes, primarily the TE11 and TE21 type modes, and allows selection of symmetric and asymmetrical patterns, with different field tapers in the principal planes. Using this model first the influence of the second order TE21 mode on the cross polarization of an offset reflector with a circular rim is investigated. Then, the problem for an offset reflector with an elliptic rim is also studied. It is shown that minimizing the cross polarization in the asymmetric plane does not necessarily reduce it globally, as in the inter-cardinal plane it remains high. The procedure for reduction of the cross polarization globally and its effects on the sidelobe levels of the reflector antenna are also investigated. With the proposed feed model, the aperture efficiency of the reflector antenna is improved compared with a standard Gaussian feed with the same edge taper. Based on this study, a dual-mode circular waveguide horn is also proposed and designed. Its performance on an offset reflector antenna is studied and shown that the cross polarization levels are reduced quite satisfactorily within a respectable frequency band.

An Ultra Wideband (UWB) Mixer with 0.18UM RF CMOS Technology

In this paper a CMOS down-conversion mixer for UWB applications is presented. The mixer circuit, designed using a standard 0.18mum RF CMOS technology, is an integral part of an entire UWB transceiver chip, working at the 3.1-10.6 GHz frequency range. The core of the mixer has been designed based on Gilbert cell architecture and uses a current injection method for increasing the linearity. For matching of RF and LO inputs to 50Omega a wideband matching circuit, using passive on-chip components, has been designed which provides a good match over the entire frequency range. A simple buffer is incorporated at the IF output. The simulated conversion gain of the mixer is greater than 10dB. The 1dB compression point, referred to input, is -10dBm and the IIP3 point is 4dBm. The RF return-loss is well below -10dB. Also, the LO-IF isolation is greater than -30dB, while the NF is about 10dB. The DC power consumption of the mixer is only 10mW. All characteristics are achieved for the entire frequency range of 3.1-10.6 GHz and meet the UWB standard requirements. In the paper it is shown that this mixer, which uses only a 1.8V supply, has considerable advantages over previously published mixers designed for this purpose

A Novel Dual-Mode Dual-Polarized Circular Waveguide Feed Excited by Concentrically Shorted Ring Patches

There are a number of applications where an adaptive dual-mode dual-polarized reflector antenna feed is required, such that the mode excitation and polarization can be controlled externally. This paper presents one such feed. The modes are excited by two concentric shorted microstrip ring patches placed at the bottom end of an open-ended circular waveguide. It is shown that the proposed feed is capable of producing two perpendicular polarizations within a single hardware with excellent isolation between the excitation ports. A prototype antenna is fabricated and tested. The measured results satisfactorily validate the numerical ones.

Investigation of Virtual Array Antennas With Adaptive Element Locations and Polarization Using Parabolic Reflector Antennas

A novel multi-phase center parabolic reflector antenna is investigated that utilizes a single hardware system to displace the antenna phase center location electronically in any direction, while generating identical broadside secondary radiation patterns at the far-field region to convert the reflector antenna into a virtual array. The primary feed is a dual-mode circular waveguide operating at the TE11 and TE21 type modes. The combination of these modes with different polarizations, amplitude ratios, and phase differences are considered. It is shown that the phase center location of the antenna can be displaced from the physical center of its geometry by changing the excitation amplitude and phase of each mode, and more importantly by employing different mode orientations. This results in a virtual array antenna with element locations that can be displaced in any direction, depending on the polarization of each mode, by a simple signal processing procedure without mechanically moving the antenna itself. The antenna developed using this technique has potential applications as a transceiver antenna in precise positioning systems, radars with moving platforms, and virtual smart antennas. The operating frequency is 10 GHz. The proposed multi-phase center virtual antenna concept is verified experimentally exhibiting excellent agreement with the numerical results.

A Corrugated Printed Dipole Antenna With Equal Beamwidths

A corrugated printed dipole antenna with equal beamwidths in the principal planes is proposed, designed, and evaluated. Orthogonal stubs are added to the dipole arms to equalize its radiation patterns. They also reduce the size of the dipole arms. The antenna is designed to operate at 3 GHz, and has dimension of 36 × 80 × 1.58 mm3. The -10 dB S 11 bandwidth of the antenna is from 2.75 to 3.62 GHz, i.e., 27.3%. The E- and H-plane radiation patterns of the antenna are nearly equal up to 135 ° from the boresight. To confirm the simulation results, the antenna is fabricated and tested, showing good agreement between simulation and measurement results. Its performance as a feed on a deep reflector, with f/D = 0.25 and D = 45 cm is also studied, where f is the focal length of the reflector and D is its diameter. It provides an enhanced bandwidth of 29.6%, equal E- and H-plane radiation patterns of the reflector, and a gain of 21.4 dBi or an efficiency of 69%. The reflector is only 4.5 λ0 in diameter and would suffer significant blockage losses with conventional waveguide feeds. The proposed feed causes negligible blockage, and provides a respectable efficiency. It is a suitable feed for small symmetric reflectors.

Dielectric loaded circular waveguide feeds

In this paper, dielectric loaded circular waveguide feeds are designed and investigated for radiation pattern equalization. The small diameter circular waveguide feed has unequal radiation patterns in the principal planes, which are not suitable for prime focus reflectors. To make the pattern equalization, the dielectric rod is used inside the waveguide. It further reduces the overall diameter of the waveguide. The antenna is designed at 10 GHz, with dielectric constant εr = 2.5 and loss tangent 0.002. The numerical result shows that the radiation patterns are symmetrical in the principal planes with 10-dB beamwidths of 126°, cross-polarization level is -27.6 dB, and back lobe level is -31.4 dB, when the dielectric rod is extended by a 0.5 λ0 from the aperture with a rod radius a = 0.3 λ0. The S11 of the antenna is also investigated and it is below -10 dB within the frequency band of 8 to 12 GHz.

A Practical Approach to Locate Offset Reflector Focal Point and Antenna Misalignment Using Vectorial Representation of Far-Field Radiation Patterns

A direct and unambiguous approach to determine the focal point of a single offset reflector is presented. The proposed approach makes use of the vectorial form of far-field radiation patterns, both amplitude and phase. The method is based on iteratively defocusing the primary feed laterally and axially. First, mathematical expressions of the offset reflector with a small lateral and axial defocused feed are reviewed. Then, the numerical results are presented. It is shown how the corresponding far-field phase pattern information can be utilized to optimally locate the focal point of an offset parabolic reflector. The method can also be used to place the phase center of an unknown feed on the reflector focal point, and align the reflector on a test tower.

Improved Cross-Polarization Performance of a Multi-Phase-Center Parabolic Reflector Antenna

This letter addresses the cross-polarization properties of electronically controlled multi-phase-center parabolic reflector antennas, illuminated by over-moded primary feeds. Due to the presence of the higher-order modes in the primary feed, the secondary cross-polarization levels deteriorate in the case of symmetric reflector antennas. As for the offset reflectors, the primary multimode feed required to displace the phase-center location may produce a significant crosstalk in the orthogonal polarization. It is shown how one can improve the cross-polarization performances of aforementioned adaptive antennas, which are capable of providing axial beams while their phase-center locations are displaced. The conventional array technique with a simple signal processing is used for the symmetric case, whereas a trimode primary feed is employed for the offset case.

8-Element sequentially-rotated array for cicular polarization applications

In this paper, circular polarization performance of an 8-element circular array with sequentially-rotated elements is investigated. Each array element is a center-fed infinitesimal dipole. The computed results are presented and compared with a 4-element counterpart and crossed infinitesimal dipoles. The results are obtained by closed-form equations.

Analytical Models of Dual-Polarized Primary Matched Feeds for Offset Reflector Antennas With Low Cross-Polarization Properties at Both Asymmetry and Diagonal Planes

Conjugate-matched primary feeds for offset reflector antennas are reviewed and investigated for both xand y-polarized cases. Tapered dual-mode circular waveguide feeds are analytically modeled to support a TE11-type mode with unequal beamwidths and the TE21-type mode for the two orthogonal polarizations. First, required mode content factors and tapering numbers of the TE11 mode are addressed for the y-polarized case in offset reflectors with different focal length-to-diameter ratios. Then, the model is appropriately tailored for the x-polarized primary feeds. It is shown that when the polarization orientation changes, the corresponding beamwidths of the TE11 mode should be swapped in the principal Eand H-planes. This is particularly important to simultaneously realize low cross-polarization levels at both asymmetry and diagonal planes, when a dual-polarized matched feed is utilized in offset reflector antennas.