An ultra-high-frequency microstrip-patch antenna has been built for use in airborne synthetic-aperture radar (SAR). The antenna design satisfies requirements specific to the GeoSAR program, which is dedicated to the development of a terrain-mapping SAR system that can provide information on geology, seismicity, vegetation, and other terrain-related topics. One of the requirements is for ultra-wide-band performance: the antenna must be capable of operating with dual linear polarization in the frequency range of 350 ± 80 MHz, with a peak gain of 10 dB at the middle frequency of 350 MHz and a gain of at least 8 dB at the upper and lower ends (270 and 430 MHz) of the band. Another requirement is compactness: the antenna must fit in the wing-tip pod of a Gulfstream II airplane.

The antenna includes a linear array of microstrip-patch radiating elements supported over square cavities. Each patch is square (except for small corner cuts) and has a small square hole at its center. Figure 1 shows the layout and principal dimensions of the cavities and microstrip patches. Wide-band performance is made possible by the relatively large cavity depth.

Each patch is fed by four identical probes positioned symmetrically on the orthogonal patch axes. To obtain either or both of two orthogonal polarizations, the antenna is fed through either or both of two orthogonal ports. A high degree of isolation between the ports is achieved in the following way: the two probes on opposite sides of the center on same axis are fed 180° out of phase with each other. The electromagnetic fields from these probes travel to the orthogonal probes, but they result in little or no coupling to the orthogonal probes because they cancel each other by virtue of the 180° phase relationship.

Figure 1. A Linear Array of Microstrip Patches supported over square cavities is shaped and dimensioned to provide the desired gain and broad-band frequency response with dual linear polarization.
As is usual for microstrip devices with thick dielectric substrates (in this case, the cavities are the dielectric substrates) each microstrip patch in this antenna presents undesired inductance at its feed points. With the help of empirical tuning, the feed probes (see Figure 2) are uniquely designed to provide enough capacitance to cancel this inductance. Each feed probe includes an outer metal cylinder plus an inner metal cylinder with a cone at one end. The upper end of the feed probe is separated from the microstrip patch by a 2-mm-thick polytetrafluoroethylene disk. The polytetrafluoroethylene-filled gaps and the cone at one end of the inner cylinder provide the needed capacitance.

In a test, the antenna exhibited gains of 8, 10, and 11 dB at 270, 350, and 430 MHz, respectively. The 10-dB gain at the middle frequency is associated with an aperture efficiency of 80 percent. This high aperture efficiency is expected because in the 270-to-430-MHz frequency band, one expects low insertion losses in a power divider, cables, and hybrids that are parts of the antenna feed. Also determined in the tests was the level of isolation between orthogonal ports; this level was found to be 37 dB across the frequency band.

This work was done by Robert F. Thomas and John Huang of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Computers/Electronics category.

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