Seven-Element Circularly Polarized Patch Antenna

John H. Glenn Research Center, Cleveland, Ohio

A seven-element microstrip patch antenna is designed to operate with circular polarization and high gain at a frequency of 2.2875 GHz. The antenna was developed in the early 1990s as a potential replacement for a helical antenna aboard the Advanced Tracking and Data Relay Satellite. Recently, the Federal Communications Commission allocated frequencies around 2.4 GHz for commercial communication systems. The present antenna is well suited for these systems, not only because it provides high gain in the desired frequency range, but also because like other patch antennas, it is lightweight, can be fabricated easily, can be mounted with a low profile, and is inexpensive. In addition, its circular polarization is especially advantageous in cellular-telephone and mobile communications, in that circular polarization can mitigate multipath fading, which is severe in urban environments.

The seven patch antenna elements are arranged in a hexagonal pattern (see figure). The patches are fed in equal amplitude and phase by a multilayer feed network that consists largely of coplanar-waveguide (CPW) and balanced stripline components fabricated at low cost by printed-circuit techniques.

Radio-frequency power from a transmitter is fed to the antenna through a coaxial cable that terminates in a coaxial connector. The coaxial connector is orthogonal to the plane of a seven-way radial CPW power divider. The inner conductor of the coaxial connector meets the junction formed by the middle strip conductors of the CPWs. The outer conductor of the coaxial connector is slotted and meets the ground-plane conductors of the CPW.

This Seven-Element Patch Antenna generates a circularly polarized beam with high gain. Its characteristics are well matched with requirements for commercial communication systems that operate at frequencies around 2.4 GHz.

Each of the seven output ports of the power divider is connected, via a post coupler, to one of the ports of a balanced stripline line stretcher, which equalizes the signal-propagation path lengths to the seven patch antenna elements. Each port of the line stretcher is connected to one of the patch antenna elements by a post coupler that also serves as a probe feed.

The measured on-axis axial ratio (that is, the ratio between axes of the polarization ellipse) is 1.5 dB; in other words, the polarization is nearly circular as intended. The measured beam widths are 36° in two orthogonal meridional planes, and the gain of the antenna as determined from the beam widths is 13 dB. The measured return loss at the coaxial connector is more than 15 dB at the design frequency.

This work was done by R. Q. Lee of Glenn Research Center, G.R. Lindamood of the University of Akron, and R. N. Simons of Sverdrup Technology, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category.

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