Ali M. Mehrabani

Virtual array antenna with displaced phase centers for GMTI applications

A novel multi-phase center reflector antenna is introduced for the Ground Moving Target Indicator (GMTI) applications. A single antenna is used to virtually make it more than one antenna by controlling the excitation modes of the dual-mode primary feed. First, the multi-mode primary feed is analytically modeled to study the concept in general. Then, the results are presented for the dual-mode stacked circular microstrip patch antenna placed at the focal point of an offset reflector antenna.

Compact Dual Circularly Polarized Primary Feeds for Symmetric Parabolic Reflector Antennas

Four-arm Archimedean spiral antennas, capable of generating both senses of circular polarization with broadside radiation patterns, are investigated as a primary feed in symmetrical parabolic reflector antennas with circular rims. Unidirectional, center-fed, and compact spiral antennas are studied, backed by a planar reflector as a ground plane. Both right- and left-handed circularly polarized fields with broadside radiation patterns, according to the first and third modes of the spiral, are realized by confining the radiation zone to less than a three-wavelength circumference. The proposed feed exhibits excellent pattern symmetry and axial ratios well below 0.5 dB at the boresight direction for both senses of polarization. In this letter, the performance of the proposed spiral antenna is fully addressed as primary feeds in symmetrical parabolic reflector antennas. Different antenna parameters such as gain, cross polarization, and efficiency are studied for paraboloids, having different focal-length-to-diameter ratios, for both senses of circular polarization. Efficiencies of 70% and higher are obtained over a frequency band of 2.5-5.0 GHz.

Investigation of reduced height dual-polarized Archimedean spiral antennas with unidirectional radiation patterns

A compact dual-polarized Archimedean spiral antenna is further investigated in terms of its axial dimension. The spiral antenna is center-fed and capable of generating two orthogonal circularly polarized waves over a shared frequency bandwidth. Parametric studies are performed to examine the impact of the antenna heights on the frequency responses of the dual-polarized Archimedean spiral antennas.

Planar, self-complementary, and wideband antennas with polarization diversity

Summary form only given. With the ever-growing technology progress in wireless communication systems, from cell phones, wireless computer tablets, and multimode Global Positioning Systems (GPS) to modern satellite and radar applications, the channel capacity enhancement is in increasing demand during the recent decades. One remedy, that has long been used, is to transmit and or receive signals over the same band of frequency, well known as frequency reuse systems. In these systems, two orthogonal waves are utilized for each transmit and or receive channel, thereby leading to a significant increase in the channel capacity. The orthogonality can be realized by two perpendicularly linearly polarized waves or two circularly polarized ones with opposite senses of polarization. In radar applications, the circular polarization not only improves the radar cross section response of the objects, but also helps better clutter suppression due to the raindrops. Therefore, antennas with the capability of generating both senses of circular polarization (CP) are of great importance in polarization diversity applications. The planar self-complementary antenna structures, such as spirals and helices, are the best candidates. As it is well known, the sense of polarization in a spiral antenna is determined by the direction of its winding. Therefore, a given spiral geometry presumably produces CP waves with a right- or left-handed sense. In the literature, an orthogonal mode helical antenna with dual senses of polarization was reported by (H.P. Coleman and B.D. Wright, IEEE Trans. Antennas Propag., 414-415, 1984), when it was fed from its both arm ends. This, however, results in a complex feeding network and increases the overall antenna weight and volume, when applied to N-arm spiral or helical antennas.In this paper, a planar, self-complementary, four-arm Archimedean spiral antenna is investigated. The proposed antenna is only center-fed and produces both righthanded and left-handed CP waves over the same frequency band of nearly 13GHz, with the spiral winding preset to the right direction. Based on the band theory, the dominant mode produces a CP axial beam, whose sense of polarization accords with the winding direction of the spiral. The opposite sense of polarization is then generated using higher order modes. The higher order modes, however, have boresight-null beams. Therefore, the proposed spiral antenna is carefully designed to always radiate broadside beams for both senses of polarization. Moreover, linear polarization, either vertical or horizontal, can be achieved by signal processing technique from the resultant CP waves with two senses of polarization. All the corresponding results of such a reconfigurable antenna with polarization diversity will be discussed and presented in the conference.

Cavity backed circularly polarized spiral antennas

In this paper, a cavity backed spiral antenna is introduced. The antenna is a four-arm Archimedean spiral antenna. To effectively direct the radiation in the upper half space region, the spiral antenna is backed by a conducting cavity, which is peripherally filled by an absorber ring. Antenna radiates circularly polarized waves. The height of the surrounding cavity is only 7mm, which leads to an extremely low profile structure. The polarization purity and antenna gain are investigated over a wide frequency bandwidth of near 1 to 8GHz.

Effect of extending grounded substrate beyond AMC on the performance of spiral antennas

Development of artificial magnetic conductor (AMC) ground planes has resulted in low profile planar antennas. Essentially, AMC structures consist of a periodic array of arbitrarily shaped patches backed by a grounded dielectric slab. Among planar antennas, the spiral antenna is an excellent candidate for circular polarization applications. In [1–5], considerable efforts have been focused on decreasing the overall height of the curl and spiral antennas. Among which, a very compact and low profile spiral antenna was designed with a distance of only 0.02λο in [5], where λο is the free-space wavelength at 4.5GHz.

Parametric study of cavity backed spiral antennas

In this paper, an already designed low profile spiral antenna backed by a partially filled absorber cavity is numerically investigated. The cavity diameter and height were 144mm and 7mm, respectively. Herein, further attempts are made to make the cavity structure even smaller than the aforementioned values while retaining satisfactory CP characteristics. The effects of the cavity height and the width of the absorber ring on the CP performance of the proposed antenna are numerically studied.

Low profile offset-fed single arm spiral antenna over an AMC ground plane

Spiral antennas generate circularly polarized waves. When backed by a traditional perfect electric conductor (PEC) ground plane, the overall height of the antenna should be in the order of 0.25λο to obtain a reasonable axial ratio. In recent years, development of artificial magnetic conductors (AMC) has led to significant advancements in low profile antenna applications. Such structures provide high impedance surfaces with zero reflection phases over a specific frequency band. They can be used in filters, traveling wave amplifiers, and more importantly, as ground planes in low-profile antenna applications. Particularly, low profile curl and spiral antennas were extensively studied in [1–6]. Among which, a small distance of 0.02λο was obtained in [5, 6], where λο is the free-space wavelength at 4.5GHz. Conventionally, a curl antenna has three parts: the vertical feed probe, the radial linear and spiral parts. The radial linear section is normally used to feed the curl at its center. Ideally, this part should not contribute into the antenna radiation. However, it has an impact on the far-field radiation due to the close proximity of the antenna to an AMC ground plane.

Compact Rear-Radiating Circularly Polarized Feeds for Front-Fed Reflector Antennas

Unique backward radiation properties of compact four-arm Archimedean spiral antennas are explored and investigated. Conventionally, an active zone in the order of one wavelength is required to have the spiral antenna work in its first mode of operation, producing broadside radiation patterns. In this letter, it is shown that a reflector-backed four-arm spiral antenna with a diameter of much less than the conventional active zone is capable of effectively directing the same broadside radiation into the backward region with an opposite sense of polarization. The antenna such developed is well suited as a primary feed in small front-fed parabolic reflector antennas. The circular polarization performance of the proposed antenna is investigated including the circularly polarized gain, axial ratios, beamwidths, and cross polarization. Furthermore, the performance of the compact rear-radiating antenna as a primary feed in parabolic reflector antennas is investigated, proving the promising applications of the proposed compact feeds with backward radiation patterns.

Effect of spiral flare rate on CP performance of a dual circular-polarized spiral antenna

In this paper, a compact center-fed reconfigurable CP Archimedean spiral antenna, capable of generating two orthogonal CP waves, is presented. Dual circular-polarization performance of the four-arm antenna is investigated in terms of its flare rate variation. It is shown as spiral flare rate varies from 2mm/rad to 3mm/rad the high-end frequency considerably increases and dual-polarization performance widens. Specifically, the antenna bandwidth, shared by both left- and right-hand circularly polarized waves, improves by more than 30%.