Microstrip Yagi Antenna With Dual Aperture-Coupled Feed

NASA’s Jet Propulsion Laboratory, Pasadena, California

A proposed microstrip Yagi antenna would operate at a frequency of 8.4 GHz (which is in the X band) and would feature a mechanically simpler, more elegant design, relative to a prior L-band microstrip Yagi antenna. In general, the purpose of designing a microstrip Yagi antenna is to combine features of a Yagi antenna with those of a microstrip patch to obtain an antenna that can be manufactured at low cost, has a low profile, and radiates a directive beam that, as plotted on an elevation plane perpendicular to the antenna plane, appears tilted away from the broadside. Such antennas are suitable for flush mounting on surfaces of diverse objects, including spacecraft, aircraft, land vehicles, and computers.

This Simplified Exploded View depicts the main components of the three-element microstrip Yagiantenna featuring dual offset aperture-coupled feed for each patch.

The design of the original version of the prior L-band microstrip Yagi antenna utilized a dual coaxial probe feed to generate circularly polarized radiation. (In some other versions of the prior antenna, a single aperture-coupled feed has been used to obtain linear polarization, but this would be of no help in contemplated applications in which circular polarization would be required.) The coaxial feed in the original circularpolarization version introduces electrical and mechanical complexities and difficulties. Electrically, it is difficult to match the impedance of the coaxial cable to that of the antenna because of the parasitics involved in the coaxial through-feed connections. Mechanically, the geometry of the coaxial feed makes it difficult to impart a low profile and predominantly planar character to both the antenna and its feed structure. In contrast, in the proposed X-band microstrip Yagi antenna, a dual aperture-coupled feed would be used to obtain circular polarization, simplifying both the electrical and mechanical aspects of design and imparting a predominantly planar character to the overall shape.

Stated somewhat more precisely, what has been proposed is a microstrip antenna comprising an array of three Yagi elements. Each element would include four microstrip-patch Yagi subelements: one reflector patch, one driven patch, and two director patches. To obtain circular polarization, each driven patch would be fed by use of a dual offset aperture-coupled feed featuring bow-tie-shaped apertures (see figure). The selection of the dual offset bow-tie aperture geometry is supported by results found in published literature that show that this geometry would enable matching of the impedances of the driven patches to the 50-Ω impedance of the microstrip feedline while maintaining a desirably large front-to-back lobe ratio.

This work was done by Ronald Pogorzelski and Jaikrishna Venkatesan of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-41791, volume and number of this NASA Tech Briefs issue, and the page number.