Richard C. Compton

Quasi-optical power combining using mutually synchronized oscillator arrays

A quasi-optical method for solid-state power combining with applications to high-power millimeter-wave generation is discussed. The approach uses two-dimensional planar arrays of weakly coupled oscillators. Limiting the strength of the coupling avoids multifrequency moding problems and simplifies the design. A radiating element is embedded in each oscillator so that the power combining is accomplished in free space. The concept which has been demonstrated with two prototype arrays, one using Gunn diodes and the other using MESFETs is discussed. A theoretical description of the coupled-oscillator arrays is presented for design purposes, and is used to investigate phasing problems and stability. Experiments are discussed which indicate that in-phase operation is facilitated by using a quasi-optical reflector element. which influences the operating frequency and coupling between the elements. Equivalent isotropic radiated powers of 22 W at 1% efficiency for a 16-element Gunn array and 10 W at 26% efficiency for a 16-element MESFET array which have been obtained at X-band are discussed. >

Bow-tie antennas on a dielectric half-space: Theory and experiment

A new formulation is discussed for the rigorous calculation of the radiation pattern of a bow-tie antenna of finite length and infinitesimal thickness, placed on a lossless dielectric substrate. The analysis is based on a representation of the current density on the metal surface of the antenna as a sum of an imposed (quasistatic) term and a set of current modes with unknown amplitudes. Free-space fields that are expressed in terms of continuous spectra of symmetrized plane waves are matched to the current modes using the method of moments. The resulting set of equations are solved for the unknown current amplitudes. The calculations show that for increasing bow length the antenna impedance spirals rapidly to a value predicted by transmission line theory. The theory also shows that the E -plane pattern of a two wavelength, 60\deg bow-tie antenna is dominated by low-loss current modes propagating at the dielectric wavenumber. As the bow tie narrows, the loss of the modes increases, and the dominant wavenumber tends to the quasistatic value. Pattern measurements made at 94 GHz are shown to agree well with theoretical predictions. Measurements for a long-wire antenna, a linear array of bow-tie elements, and a log-periodic antenna are also presented.

Improvement of microstrip patch antenna radiation patterns

Microstrip patch antennas typically have radiation patterns containing unwanted sidelobes or local minima caused by surface waves, especially when fabricated on high dielectric-constant substrates. The paper shows that removal of the substrate beneath the patch can greatly improve the pattern. A variation of this technique, compatible with monolithic fabrication, is applied to fabrication of a patch on GaAs and shows similar pattern improvement. >

A millimeter-wave vector network analyzer

A network analyzer for making accurate measurements in the 75-110-GHz band is presented. The analyzer operates by sampling forward and reverse signals using back-to-back directional couplers. The tapped signals are combined in a hybrid coupler which feeds the sum and difference into two detector diodes. An additional two measurements are obtained by switching a phase delay into one of the paths. These four measurements are manipulated using six-port theory to calculate the complex reflection coefficient. A 27-40-GHz prototype operating as a reflectometer is demonstrated. Calibrations and measurements are performed interactively with the aid of a Macintosh II personal computer equipped with a high-resolution color display.<<ETX>>

Electronic beamsteering of active arrays with phase-locked loops

A new electronic beamsteering technique for active arrays is presented along with experimental results at 10 GHz. The technique uses a single balanced diode mixer to phase-lock neighboring oscillators in an active array. Each oscillator has its own antenna that radiates energy into free space, so the phase difference between oscillators determines the direction of the main radiating beam. An offset voltage added to the phase-locked loop controls the phase difference and beam direction. Experimental results demonstrate over 100/spl deg/ of adjustable phase difference between neighboring oscillators. Because of its simplicity, this technique has significant advantages over traditional beamsteering arrays.<<ETX>>

Quasi-optical power-combining arrays

Semiconductor devices have limited power handling capabilities at high frequencies, particularly at millimeter-wave frequencies. A method is presented for overcoming this problem by combining the outputs of several devices quasi-optically in a resonator cavity. This method has been applied to a number of solid-state devices, including Gunn diodes and MESFETs. The devices do not require an external locking signal because they lock to a mode of the resonator cavity. Effective radiated powers of 22 W for a 4*4 array of Gunn diodes and 25 W for a 10*10 array of MESFETs have been achieved.<<ETX>>

Three-dimensional FDTD analysis of quasi-optical arrays using Floquet boundary conditions and Berenger's PML

Infinite periodic grid structures excited by normally incident beams are analyzed using finite-difference time-domain (FDTD), with Berengers PML (perfectly matched layer) absorbing boundary condition used to terminate the computation domain along the beam axis. Floquet boundary conditions are used to handle arbitrarily shaped unit cells. Restriction to normal incidence permits using a Gaussian pulsed excitation to generate the wideband frequency response. The technique is used to model a previously reported multilayer quasioptical rotator array, with excellent agreement to the measurements obtained in the 26.5-40 GHz hand in a lens-focused test setup.

Measurement and modelling of radiative coupling in oscillator arrays

Arrays of coupled oscillators can be used for power-combining at microwave and millimeter-wave frequencies, and have been successfully demonstrated with a variety of devices. Such arrays have also recently been mode-locked for pulse generation, and can be configured for phase-shifterless beam scanning. The nonlinear theory of coupled oscillator phase dynamics depends crucially on the parameters describing the coupled between oscillators. Methods for experimental characterization of these parameters are described here, and simple models which reproduce the measurements quite well are developed. The models apply to radiative coupling and the effects of external reflectors which are sometimes used for stabilization. The theory is verified with a two-oscillator system. >

Mode-locked oscillator arrays

The output characteristics of a linear array of oscillators are discussed. By arranging the output frequencies of each oscillator to give a spectrum of equally spaced components, a mode-locked system results, thereby producing short, high-power RF pulses. Promising results from a linear array of up to five elements are presented, with good theoretical agreement. The results presented suggest that it may be possible to generate high-power RF pulses with high-pulse repetition frequencies using planar arrays of devices. This quasi-optical array architecture is expected to scale up to the Terahertz frequency range using suitable devices.<<ETX>>

Broad-band characterization of millimeter-wave log-periodic antennas by photoconductive sampling

Results of photoconductive sampling measurements used to characterize millimeter-wave log-periodic antennas with continuous, simultaneous frequency coverage from 10 to 300 GHz are presented. Polarization properties are investigated employing wire-grid polarizers. This study reveals new information on structure resonances and antenna polarization. >