Z. Allahgholi Pour

A Ring Choke Excited Compact Dual-Mode Circular Waveguide Feed for Offset Reflector Antennas

A simple ring choke excited compact dual-mode circular waveguide feed, for offset reflector antennas, is proposed. It consists of a central round waveguide with a single longitudinal slot opening to a surrounding single choke of circular cross-section. It generates the TE11 mode inside the waveguide and the higher order TE21 mode inside the ring choke, with a quadrature phase difference. The proposed design, therefore, separates the two modes spatially, and isolates the input waveguide from the detrimental effects of the higher order TE21 mode. It thus provides a wide impedance bandwidth, without the need for a matching network. The measured results of the return loss and radiation patterns on a fabricated prototype show excellent agreement with the numerical data. The antenna structure is very compact and when used as a primary feed in offset reflectors it reduces the cross polarization.

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.

A Novel Impedance Matched Mode Generator for Excitation of the TE

A compact dual-mode circular waveguide horn antenna is presented that is fed by the dominant TE11 mode and generates the higher-order TE21 mode in order to reduce the cross polarization of offset reflectors. To excite the second mode, two blocks and a small discontinuity step are used. These blocks are tangent to the aperture of the second waveguide, as opposed to the traditional case where they are usually located right at the discontinuity step, which results in high reflections. An analytic model is then de ployed to extract the mode content factor along with the field tapers of each mode in the principal planes. Other than far-field radiation patterns, emphasis is placed on the scattering parameters of the antenna, and they are compared to the ones for the same antenna with no blocks as well as the traditional case when the blocks are placed right at the discontinuity of the stepped waveguide. It is shown that the proposed feed provides a good impedance match, over a wide frequency band, similar to its counterpart for the single-mode antenna with no blocks. To verify the concept, a prototype antenna is fabricated and tested. There is an excellent agreement between the simulated and measured results.

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In this paper, the problem is investigated in a more general form, involving phase center location and polarization of multimode, multilayer, circular microstrip antennas. It is shown that, these parameters can be controlled by amplitude and phase excitations of the modes, in particular, when the first three modes are being excited, which are the TM11, TM21 and TM02. It is also shown that, the phase center of such antennas can be changed, while maintaining the main beam direction constant, thus, introducing the concept of virtual antennas

Mode in Compact Dual-Mode Circular Waveguide Feeds

Horn antennas have been used in diverse applications in communication systems, electromagnetic sensing, radio frequency heating, and antenna measurement, as well as feeds in lens and reflector antennas [1]. The application of pure-mode horn antennas is limited to array elements because of their poor electrical characteristics. In symmetric reflectors, an optimum feed must provide symmetric field patterns for both the field amplitude and phase to optimize the gain performance [2]. The corrugated conical horn is an example of this optimum feed [3]. Another example is the multimode horn antenna. Potter [4] designed a dualmode horn antenna to provide low sidelobe levels and symmetric patterns. Koch designed a multimode coaxial feed to optimize the spillover and the aperture efficiency of reflector antennas [5]. The applications of multi-mode horns are not limited to feeds in symmetric reflectors as the higher order modes can be appropriately combined to illuminate an offset reflector antenna to cancel or decrease the crosspolarization level, as reported in [6–8].

Control of phase center and polarization in circular microstrip antennas

Knowledge of the phase centre location of the antenna is of great importance in various applications, such as global positioning system (GPS), remote sensing, radars, and virtual arrays [1]. In parabolic reflectors, the phase centre is located at the center of the aperture plane when it is illuminated by a prime-focus point source feed having an axially symmetric pattern [2–3]. It is clear that if one displaces the antenna phase centre, the apparent location of the antenna will move resulting in a virtual antenna. The virtual array antenna can be thought of as an antenna having multiple identical beams, with multiple displaced phase centre locations. Such a property is desirable in remote sensing applications, to allow multiple antenna representations. Multiple phase centre reflector antennas were studied in [4–6] using a dual-mode feed horn as a primary feed. In this paper, the impact of the feed polarization on the phase centre location of offset reflector antennas will be addressed. The feed is a dual mode circular waveguide antenna operating at its fundamental mode, TE11, and the higher order mode TE21. It will be shown that the phase centre displacement can be controlled by simply changing the excitation amplitude and phase of each mode, as well as employing different mode orientations. In particular, the direction of the phase centre movement will depend on the polarization of each mode.

A novel dual mode circular waveguide horn antenna

An analytic dual-mode primary feed is first modeled to illuminate offset reflector antennas to reduce their cross polarization. The feed is linearly polarized and includes the dominant TE11 and higher order TE21 modes. The phase error is then applied to the primary feed to investigate its impact on the cross polarization of offset reflector antennas. Two cases are studied in terms of which of the above-mentioned modes will be affected by the phase errors. The phase errors applying to the resultant dual-mode feed are studied as the first case. The second case includes the effect of phase error only on the higher order TE21 mode resulting in different phase center locations for each mode. Asymmetric right-left phase patterns are used to conduct the study. Both linear and quadratic phase variations are studied. A broad range of focal-length-to-diameter (f/D) ratios from 0.5 to 1.1 are considered to investigate the reduced cross polarization properties in the presence of phase errors. It is shown that the reflector cross polarization increases drastically in the presence of the phase errors. In particular, the boresight-null cross polarization will no longer exist when the phase center location of the second mode is displaced from that of the dominant mode.

Effect of primary feed polarization on phase centre location of parabolic reflector antennas

This paper presents the formation of an adaptive virtual array antenna in a parabolic reflector illuminated by a tri-mode circular waveguide feed. The modes of interest are the TE11, TE21, and TM01 type modes. By appropriately exciting these modes in the primary feed, the effective source of radiation is displaced within the reflector aperture while the resulting secondary patterns remain axial.

Investigation of Asymmetric Phase Errors of an Optimized Dual-Mode Primary Feed on the Cross Polarization of Offset Reflector Antennas

The effect of a dual-mode circular horn antenna, as a feed in offset reflector antennas, on the crosspolarization field is studied using the physical optics, geometrical optics, and geometrical theory of diffraction software GRASP. It will be shown that, one can reduce the crosspolarization of an offset reflector antenna by properly exciting two modes in circular horn antenna. These two modes are the fundamental mode, TE11, and the higher order mode, TE21. The results for some offset reflectors, with different f/D ratios, will be presented to investigate the above property.

Applications of trimode waveguide feeds in adaptive virtual array antennas

Summary form only given. In certain applications, such as radars, remote sensing, and precise global positioning systems, the phase reference of the radio frequency (RF) system is of great importance as it affects the system accuracy, such that even small uncertainty could lead to big errors. In wireless communications, this phase reference is dominated by the antennas, the last components of the RF front-end parts, which radiate the Electromagnetic waves. Thus, knowledge of the antenna phase reference, well known as the phase centre, is vital in aforementioned applications. The phase centre location of an antenna is the effective source of radiation providing a uniform phase pattern at the far-field zone over a finite angular range in space, which is normally around the main beam. Therefore, any phase centre displacement will move the phase reference of the communication system. Particularly, in moving target indicator radars, where the antenna part is mounted on a moving platform, the forward motion of the moving platform changes the effective phase centre location of the operating antenna. This could result in errors as significant as missing low velocity objects. One of the techniques to overcome this problem is the displaced phase centre antenna (DPCA) processing technique [M. I. Skolnik, Radar Handbook, McGraw-Hill, 1990]. Traditionally, DPCA technique exploits two or more identical aperture antennas, which provide separate phase centre locations with identical secondary radiation patterns. This will, however, increase the hardware and complexity of the antenna systems. In this paper, an electronically DPCA technique is reviewed through the use of a single adaptive aperture antenna. The emphasis is placed on parabolic reflector antennas, both symmetrical-cut and offset geometries, illuminated by over-moded primary feeds in order to adaptively change the aperture distribution. Both dual and tri-mode waveguide primary feeds are investigated. It is shown that the phase centre location of the antenna can be electronically displaced over a single aperture by controlling the mode content factors as well as the mode alignments in the primary feed, while the associated secondary radiation patterns are axial at the far-field region. Such adaptive operation can be readily performed by signal processing algorithms. The proposed DPCA technique is a smart alternative to the mechanical DPCA, as it lessens the complexity and volume of the antenna system. All corresponding numerical and measured results will be presented and discussed in the conference.