Oscillations have been obtained at frequencies from 100 to 712 GHz in InAs/AlSb double-barrier resonant-tunneling diodes at room temperature. The measured power density at 360 GHz was 90 W cm-2, which is 50 times that generated by GaAs/AlAs diodes at essentially the same frequency. The oscillation at 712 GHz represents the highest frequency reported to date from a solid-state electronic oscillator at room temperature.

Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne converter or photomixer, to generate coherent continuous-wave output radiation from microwave frequencies up to 3.8 THz. The photomixer consists of an epitaxial layer of LTG GaAs with interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes, and power is radiated into free space through a three-turn self-complementary spiral antenna. In a photomixer having a 0.27-ps electron-hole lifetime and small electrode capacitance, the output power is practically flat up to about 300 GHz and then rolls off at a rate of approximately 12 dB/oct.

open, and the other cells are closed. In configuration 2, the MEMS switches in PBG cells 2, 4, 6, and 8 are open, and the other cells are closed. In fact, configurations 1 and 2 are symmetrical configurations, except for the feed point. NUMERICAL AND MEASURED RESULTSAnsoft HFSS 8.0 is used to analyze the antenna. The antenna operates at around 10 GHz. The size of the substrate is 50 ϫ 30 mm, and the metal ground 60 ϫ 40 mm. In this work, the open or closed states of a switch are simulated by the absence or presence of a metal pad of size 0.4 ϫ 0.4 mm, which is approximately the size of a MEMS switch [5]. According to the simulated antenna, three prototypes are fabricated and measured.Figures 2(a), 3(a), and 4(a) show the simulated and measured return losses of configurations 0, 1, and 2, respectively. As can be seen, the simulated results are in good agreement with the measured results, except for the operating frequency shifting downward slightly. According to the measured results, 9.65 GHz is selected as the radiation-pattern measurement frequency. At this frequency, antenna configurations 0, 1, and 2 have the return losses of Ϫ11.2, Ϫ11.2, and Ϫ11.5 dB, respectively.The measured far-field radiation patterns of configurations 0, 1, and 2 are shown in Figures 2(b), 3(b), and 4(b), respectively. In Figure 2(b), the main beam in the E-plane directs to Ϫ24°, and the 3-dB beamwidth covers a range from Ϫ52°to 25°. In Figure 3(b), direction of the main beam in the E-plane is 34.5°and the range of 3-dB beamwidth is from 16°to 63°. The side-lobe level is less than Ϫ7 dB. In Figure 4(b), direction of the main lobe in the E-plane points at Ϫ35°and the 3-dB beamwidth ranges from Ϫ66°to Ϫ18°. The side-lobe level is less than Ϫ7 dB. The measured results are in good agreement with the simulation. The simulated directivities of configurations 0, 1, and 2 are 8.87, 10.08, and 9.71 dB, respectively. All three configurations of the antenna have low cross-polarization of less than Ϫ21 dB. CONCLUSIONA pattern reconfigurable quasi-Yagi microstrip antenna has been presented and three prototypes have been measured. By using switch-controlled PBG structure, the pattern-reconfigurable antenna scans from Ϫ66°to ϩ63°in the upper-half space while maintaining the operating frequency around 9.65 GHz. The PBG structure also increased the antenna gain. NONUNIFORM MEANDERED AND FORK-TYPE GROUNDED ANTENNA DESIGN