Active integrated antennas (AIAs), which integrate one or more active solid-state devices and circuits within an antenna element, are highly relevant to NASA applications. They could be the foundation for highly efficient, beam-steerable phased-array antenna systems that would be tailored to meet a wide range of mission requirements. For optimal direct-current-to-radiofrequency (RF) efficiency, the active solid-state device is a transistor mounted close to or directly into the radiating structure. This minimizes the conductor losses that are inherent in circuits where the active devices and radiating elements are spatially separated. In an AIA, there is no distinction between the microwave circuit and the antenna--the antenna serves both as a load and a radiator for the active device--and the integrated solid-state device may serve as a local oscillator for a self-oscillating antenna.
AIA radiating element powered by a single 1.5-V battery.
To demonstrate the concept, researchers at the NASA Glenn Research Center fabricated and tested AIA elements on a high-dielectric-constant( εr = 10.2) microwave laminate that was 0.635 mm thick. A gallium arsenide (GaAs) transistor was mounted inside the radiating element, and the entire device occupied a 5- by 6-mm2 area (see the preceding photograph). The AIA demonstrated very stable oscillations and excellent radiation patterns at the X-band (8- to 12-GHz) frequencies.
Radiated power from single radiating antenna element, with the detector located 132 cm away from the AIA. The transistor was biased without a gate connection, with a drain voltage of 1.5 V, an open gate voltage, and a drain current of 17.7 mA. Data taken over a wide frequency range
show that the fundamental signal at 8.48 GHz dominated the spectrum. For the inset, the frequency was 8.477 GHz and the maximum power was -35.4 dBm.
Comparison between simulated and experimental data confirmed that the oscillation frequency was controlled by the length of the feedback loop between the drain and gate terminals and that radiated power and radiation efficiency were maximized when the feedback length was one wavelength long. With the device biased solely with a 1.5-V battery and with the gate terminal open, the effective isotropic radiated power (EIRP) was 13.2 mW. The RF power generated by the AIA was 2.8 mW, and the antenna directivity was 4.72. The RF power spectrum of the AIA at these bias conditions is shown in the preceding graph. When the AIA was biased for higher power levels, with the drain and gate voltages at 3.5 and -0.6 V, respectively, the EIRP increased to 52.2 mW. Under these bias conditions, the RF power generated by the AIA was 9.4 mW, and the antenna directivity was 5.55.
The electric and magnetic field radiation patterns are shown on the left and right graphs, respectively, in the following figure. The cross-polarization levels in the electric field scan did not exceed -10 dB. At broadside (0°), there was sharp null in the cross-polarization power level, indicating that the circuit was balanced and symmetric and that the pattern was free of perturbations that could cause higher order modes to propagate. In the magnetic field scan, cross-polarization levels did not exceed -15 dB. The radiation pattern at higher bias levels was identical to that shown in the preceding graph. The results obtained thus far suggest that this design can be scaled to multielement phased-array antennas, and we expect that these AIAs will serve as the foundation for phased-array antennas, wherein the active and passive circuitry is fully integrated onto a single substrate.
Field data from a single AIA element. Power was supplied via a single 1.5-V battery connected between the source and drain electrodes, the gate terminal was open, and the EIRP of the AIA was 13.2 mW. Frequency, 8.48 GHz; maximum power, -46.28 dBm. Left: Electric field data. Right: Magnetic field data.
Glenn contacts:
Dr. Richard Q. Lee, 216-433-3489, Richard.Q.Lee@nasa.gov
Dr. Robert R. Romanofsky, 216-433-3507, Robert.R.Romanofsky@nasa.gov
Dr. Félix A. Miranda, 216-433-6589, Félix.A.Miranda@nasa.gov
Authors:
Dr. Carl H. Mueller, Dr. Carol L. Kory, and Kevin M. Lambert
Programs/projects:
Space Communications, Glenn Independent Research and Development
Last updated: December 14, 2007
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