Patch Antennas With Loading of a Pair of Shorting Pins Toward Flexible Impedance Matching and Low Cross Polarization

Patch antennas with loading of a pair of shorting pins are proposed in this paper toward flexible impedance matching and low cross polarization. The shorting pins are introduced in the centerline of a square patch to strengthen the surface current density near the feeding point at edge. As these paired pins simultaneously move away from the center toward the two radiating edges of patch, the resonant input impedance of the microstrip-edge-fed patch antenna is increasingly reduced due to enlarged current density at the feeding point. Because of symmetric arrangement of these two shorting pins, surface current density on the patch is maintained as the odd-symmetric property with respect to the H-plane, thus tremendously degrading the cross-polarization level. Simulated and measured results are found in good agreement with each other in terms of input impedance and radiation pattern. They further demonstrate that co-polarization to cross-polarization ratio (CTCR) of the paired-pins-loaded patch antenna in H-plane is maintained at least 10 dB higher than that of its single-pin-loaded patch counterpart.

I. Introduction

Microstrip patch antennas have been intensively studied in the past decades. However, electrically thin edge-fed patch antennas usually suffer from high resonant input impedance, typically in a range of , which is difficult for direct matching with microstrip feeding line. It is well known that the input impedance of a resonant patch antenna is a cosine-square function of feeding position from the edge to the center of patch [1], [2] under the assumption of ideal magnetic walls in its surrounding periphery. For the probe-fed scheme as shown in Fig. 1(a), its impedance matching can be easily achieved by choosing a proper feeding point underneath the patch. However, to directly integrate the patch radiator with its microstrip feeding line at edge as highly demanded in many applications, an inset-fed scheme as shown in Fig. 1(b) has been often employed for impedance matching [3], [4]. Nevertheless, when the operating frequency is high and the characteristic impedance of feeding line is low, the notch becomes notably wide with respect to the width of patch, thus causing difficulty in implementation and fabrication. In addition, the symmetric geometry of the patch radiator is unfortunately destroyed, thus deteriorating the symmetrical radiation pattern in its E-plane and cross polarization in H-plane [5], [6]. Fig. 1.

Four distinctive feeding schemes of patch antennas toward input-impedance reduction at resonance. (a) Probe-fed patch. (b) Inset-fed patch. (c) and (d) Microstrip-edge-fed patches with loading of a single pin and a pair of symmetric pins, respectively.