A small, segmented microstrip patch antenna integrated with an X-band feedback oscillator on a high- permittivity substrate has been built and tested (see Figure 1). The oscillator antenna is powered by commercial solar photovoltaic cells mounted nearby or on the same substrate. This oscillator antenna is a prototype for demonstrating the feasibility of such devices as compact, low-power-consumption building blocks of advanced, lightweight, phased antenna arrays that would generate steerable beams for communication and remote-sensing applications.
The solar-powered oscillator antenna includes a commercially available super-low-noise, high- frequency field-effect transistor integrated into the center of the segmented microstrip-patch antenna. Along with bias lines, a feedback loop, and other conductors that are parts of the oscillator antenna circuitry, the patch antenna was formed by etching the corresponding pattern out of a surface metal layer on the substrate, which is a commercial microwave laminate having a relative permittivity of 10.2 and a thickness of 0.635 mm. The oscillator antenna occupies an area of 5 by 6 mm on the substrate. The oscillator feedback path extends between the drain and gate terminals of the transistor and includes a 1.2-pF capacitor that passes the oscillation signal while providing DC isolation between the drain and the gate.
A comparison between simulated and experimental performance data confirmed that the oscillation frequency is controlled mainly by the length of the feedback path. The RF signal radiated at the fundamental frequency from the solar powered antenna is shown in Figure 2(a). The magnetic field radiation pattern of the solar powered antenna is shown in Figure 2(b). The solarpowered oscillator antenna radiates a power of 1.8 mW at a frequency of about 11.2 GHz with a directivity of 5.25 relative to an isotropic radiation pattern. The power in the second harmonic at 22.4 GHz is 20 dB below the fundamental signal level. It has been found that a current of about 120 mA is needed to initiate oscillation, but thereafter the device can continue to oscillate at a current <20 mA. One of the drawbacks of using commercial solar cells is its large size due to low efficiency. Ways to minimize solar-cell area could include: (1) powering the oscillator antenna from a rechargeable battery that would be recharged by a single, smaller solar cell, (2) the use of more-efficient solar cells, and/or (3) the use of a capacitor to supply the high current needed to start oscillation.
One notable advantage of using high-permittivity substrates is the inherent reduction in the sizes of resonant antenna elements. In addition to compactness, the designs of this oscillator antenna feature simple geometry, and the radiated power levels and radiation efficiency are comparable to those typical of oscillator antennas on lower-permittivity, thicker substrates.
Another motivation for the use of high-permittivity substrates is the prospect of developing solar oscillator array antenna as fully integrated circuits. Some essential components of such a development have already been accomplished: High-mobility transistor structures are readily available on insulating substrates having dielectric properties similar to those of the microwave laminate material mentioned above. Dielectric substrate materials including gallium arsenide and sapphire that support high-frequency active electronic devices have relative permittivities between 10 and 14.
This work was done by Richard Q. Lee, Félix A. Miranda, Eric B. Clark, and David M. Wilt of Glenn Research Center and Carl H. Mueller, Carol L. Kory, and Kevin M. Lambert of Analex Corp.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18114-1.