1590 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015 A Novel Matched Feed Structure for Achieving Wide Cross-polar Bandwidth for an Offset Parabolic Reﬂector Antenna System Rajib Jana and Ratnajit Bhattacharjee, Member, IEEE Abstract—This letter presents the details of a novel matched feed structure which promises improved cross-polar performance for an offset parabolic reﬂector antenna. Feed aperture formed by an intersection of circular and rectangular waveguides provides wide cross-polar bandwidth due to the cutoff wave numbers being close for the two modes used in conjugate matching. The mode coefﬁcients and relative phase difference are estimated using particle swarm optimization (PSO) technique for the speciﬁed antenna system. Physical optics (PO) is used to evaluate the reﬂector pattern; feed radiation pattern is obtained using mode ﬁeld patterns and cutoff wave numbers, which are computed using 2-D ﬁnite element method (FEM). The higher order mode which is combined with the main operating mode of the waveguide for conjugate matching is generated using two posts placed behind the aperture. The antenna system performance is evaluated using HFSS and CST Microwave Studio. Index Terms—2-D ﬁnite element method (FEM), cross-polar bandwidth, matched feed, offset parabolic reﬂector, particle swarm optimization (PSO), physical optics (PO). I. INTRODUCTION O FFSET reﬂector antennas offer features such as reduced aperture blockage, low side lobe levels, and isolation between feed and reﬂector as compared to their axisymmetric counterpart. Such features have been made use of in various applications like satellite communication, telemetry, remote sensing, mono-pulse tracking radar, etc. [1]. In spite of these advantages, such reﬂector antennas often restrict the systems in exploiting the beneﬁt of frequency reuse and affect the accuracy of the signal tracking by exhibiting high cross-po- larization level when illuminated with linearly polarized feed. Over the years, several techniques like placement of dielectric lens in front of feed [2], making the reﬂector using polarization selective grid [3], etc. have been developed. However, such methods also have their own limitations, e.g., use of dielectric lens raises noise temperature and reduces gain, system cost and complexity increases if polarization grid is used. Conjugate matching is an effective technique introduced for cross-polar reduction by Rudge et al. [1]. Such technique is Manuscript received December 29, 2014; revised February 23, 2015; accepted March 12, 2015. Date of publication March 16, 2015; date of current version August 13, 2015. The authors are with the Department of EEE, IIT Guwahati, Guwahati 781039, India (e-mail: r.jana@iitg.ernet.in; ratnajit@iitg.ernet.in). Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LAWP.2015.2413837 widely used nowadays for the design of feed as a multi-mode horn or antenna array. Various matched feed structures have been reported in litera- ture [4]–[7] and in the majority of the cases, the cross-polar per- formance is presented only at the design frequency. The cross- polar and impedance bandwidth, which are two signiﬁcant pa- rameters for practical applications, have been dealt with ade- quately in very few literatures. In order to generate higher order modes that are to be used for conjugate matching, obstacles such as post or septum are gener- ally kept inside the waveguide at appropriate locations. Usually, such junctions provide constant relative phase difference of the higher order mode with respect to main operating mode in their vicinity. However, it becomes a challenging task to maintain the required relative phase difference of the higher order mode at feed aperture for a wide range of frequencies, as needed in case of wideband conjugate matching. Individual modes propagate with different propagation constant within the waveguide and also they further combine with reﬂected waves which ultimately change the relative phase difference at feed aperture. However, efforts have been made in recent works to achieve the wideband conjugate matching using circular waveguide. The cross-polar and impedance bandwidth could be enhanced by keeping the two posts in front of aperture using the quarter wave length matching in case of mode [6]. A ring choke excited com- pact dual-mode circular waveguide feed using a slot is intro- duced in [7] and such design could provide a higher degree of freedom in choosing the dimensions for the wideband operation. For designing a wideband feed, a possible solution may be to use a proper feed aperture for which cutoff wave numbers of main operating mode and desired higher order mode (used for cross-polar suppression) are very close. In order to achieve the same, we propose a new kind of aperture that consists of in- tersection of a rectangular and circular geometry. The shape of aperture is decided on a trial-and-error basis using a 2-D ﬁnite element method (FEM) for ﬁnding the mode solution. For the proposed dual mode feed, the two modes used have close cutoff wave numbers, and the main operating mode has pattern close to mode of circular waveguide, while the other mode pattern is close to mix of mode of rectangular and mode of circular waveguide. The required mode coefﬁcients and rel- ative phase difference are estimated using particle swarm opti- mization (PSO) technique [8] for considered system speciﬁca- tion. Physical optics (PO) is used to calculate the reﬂector radi- ation pattern illuminated with the proposed feed conﬁguration. Far ﬁeld pattern of the feed is evaluated using a semi-analytical 1536-1225 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.