Recent advances in the development of fast-wave, or periodic-beam, devices operating at millimeter- and submillimeter-wavelengths are not the only signs of resurgence in the field of RF vacuum electronics. Significant progress is being achieved in areas of well-established slow-wave power tube technology. In addition, the field is being stimulated by the introduction of techniques (CADD/CAM and microelectronics fabrication) and technologies (improved magnetics) from other disciplines. By any single-parameter measure -- peak or average power, efficiency, bandwidth noise reduction -- or by more complex figures-of-merit -- power-bandwidth product, Pf2 -- vacuum RF power device performance is advancing, not incrementally, but at "breakthrough" rates. This dramatic progress could presage a period of growth in RF vacuum electronics unequaled since the postwar period of achievement in magnetron and klystron development.
The planar orotron is a promising new device which is capable of producing millimeter and sub-millimeter radiation of moderate to high power. The orotron resonator is a slow wave structure consisting of a rectangular metal grating which is opposed by a planar metal boundary. When an electron beam grazes the grating surface, it can couple to travelling waves which then amplify and leave the resonator in the direction of the beam. For a given grating design, a unique and continuous set of frequencies may be represented by the travelling waves, making the planar orotron a remarkably tunable source. Details of the theory of operation are presented, as are experimental results of one design which has produced up to 2 kW of power in the 30 to 40 GHz range using beam voltages under 20 kV.
The far-infrared free-electron laser operating at the University of California at Santa Barbara is described The free-electron laser (FEL) at the University of California at Santa Barbara has been in operation since August 1984. It is a far-infrared laser driven by long pulses of electron beam originating in an electrostatic accelerator with a beam recovery system. The main thrust and funding for this project have been motivated, so far, by the need for a tunable, high-power, coherent source in this region in areas such as condensed-matter physics, biology, chemistry and medicine as well as the development of an electrostatic-accelerator driven FEL. It is now being used routinely for research in these fields.
Linear Colliders operating in the 0.3 TeV to 1 TeV energy range may be optimally driven at a microwave frequency near 10 GHz. Assuming the use of 16 X pulse compression circuits, the required microwave amplifer tubes would need to have pulse duration ~ 2 μs, and peak power rating of 25 to 50 MW for each collider port (100 to 200 MW if one tube is to be used for four ports). To test the feasibility of meeting these requirements, a study of a 36 MW, 10 GHz, TE°01 mode, 4-cavity gyroklystron is in progress. The tube employs a 1.5 μs, 160 A, 500 kV magnetron injection gun which has been designed for relatively modest values both of cathode loading (5.6 A/cm2) and of field at the electrodes (< 91 kV/cm). The self-consistent circuit analysis considers all important effects including: fringing fields in the drift spaces, electron velocity spread, and space charge depression of electron energy. The values of cavity Q factors, and the dimensions of the cavities and the drift spaces are optimized for stability and for maximum efficiency. It is calculated that the amplifier will have saturated efficiency of 45.5% and saturated gain of 63 dB. Modulator construction and cavity cold testing have been completed, and a laboratory demonstration of this amplifier is expected in calendar year 1987. Finally, prospects for scaling X-band gyroklystrons to peak power levels of 100 to 200 MW, and phase stability of gyroklystrons are discussed.
This paper highlights some of the development initiatives in microwave magnetrons stimulated by materials and techniques breakthroughs of the past decade. The paper concentrates on the performance achievements and prospects for miniature, rugged magnetrons, designed for rapid switch on with highly refined operational parameters such as rapid r.f. build-up, low jitter, and high frequency and/or pulse stability. Recent development trends towards higher and higher mean to peak power ratios are discussed, along with phase locking for high coherence and frequency control. The traditional image of the magnetron as a simple fixed frequency pulse transmitter for low duty ratio applications gives scant recognition to the diversity of designs and range of performance of modern magnetrons. The magnetron remains one of the most widely used microwave power sources, employed in some of the most sophisticated military systems, especially those that are airborne or otherwise environmentally punishing. Consistent characteristics of magnetrons are high efficiency, small size and weight, and ease of manufacture. Furthermore, the relatively low operating voltage of magnetrons means that power supplies and modulators can be designed with the minimum of complexity, and packaged within tight volume constraints.
Experimental cathode-driven (circuit-on-emitting-sole) crossed-field amplifiers have been designed and built at S-band under a Navy program (Contract No. N00039-79-C-0295). Peak and average power levels achieved were approximately one uegawatt and 20 kilowatts, respectively. Gain was nominally 30 dB at efficiencies as high as 807. Operating voltages were typically near 30 kV. Cathode-anode (circuit) isolation was better than 20 dB. Noise measurements indicated superior noise performance. CDCFAs were operated reliably for hundreds of hours. There are good prospects that efficiency, gain, and cathode-anode isolation can be iuproved even further with this device.
Quasi-optical gyrotrons appear promising as multi-megawattt sources of cw radiation at frequencies to 300 GHz. In this paper, the theory of the quasi-optical gyrotron as well as recent experimental results are briefly reviewed, and comparisons made with conventional microwave cavity gyrotrons. The parameters of a 5 MW, 300 GHz device, based on scaling from simulations of lower power gyrotrons, are presented.
A number of high-power linear beam millimeter-wave TWTs have been developed at Varian Associates, in recent years. The development of these tubes required the design of extremely high-power density electron beams using cathodes capable of delivering current densitites of 10 A/cm . These electron beams incorporate confined flow focusing, using both solenoid and PPM focusing techniques. New innovative ladder circuit designs have been developed to provide high-thermal capacity, making the generation of high power at millimeter-wavelengths possible. Further, the ladder circuits afford reliable and repeatable fabrication of circuits requiring stringent dimensional tolerances with high manufacturing yields. These high-power millimeter-wave TWTs have been named (MILLITRONS) and have elevated the power output capability of millimeter-wave TWTs to their present levels of performance. The expected limits of power output for these devices are discussed.
An offset Cassegrain modified focal-pivot-scan monopulse antenna was designed for use with a 94 GHz search and track radar system which would be mounted on an armored vehicle. The focal-pivot-scan design provides for scanning of the main reflector about its focal point while the feed horn and subreflector remain fixed. This permits rapid beam scanning in azimuth for target acquisition while minimizing the mass that must be scanned (and the required scan motor capacity) and precludes the necessity for rotary joints with their attendant losses and possible mechanical failure. The elevation search function and tracking in both azimuth and elevation are accomplished using separate drive motors.
Abstract: At millimeter and submillimeter wavelengths, integrated-circuit antennas are often mounted on a substrate lens to eliminate losses due to substrate modes. This approach takes advantage of the fact that antennas on dielectrics are more sensitive to radiation from the substrate side. However, coupling efficiencies for these antennas have been limited to about 25% because of poor patterns and dielectric losses. We have solved these problems by fabricating the antennas on 1-μm thick silicon-oxynitride membranes (below left). The membranes are fabricated by depositing a 1-μm silicon-oxynitride layer on both sides of a <100> silicon wafer by plasma-enhanced chemical vapor deposition. An opening is defined on the back of the wafer by patterning the silicon oxynitride with photoresist and etching it in a buffered-HF solution. Then the silicon is etched in an ethylenediamine-pyrocatechol solution until the transparent membrane is exposed. The membrane is much thinner than a wavelength, so that the antenna effectively radiates in free space. This approach eliminates the substrate lens and thus greatly reduces the dielectric losses, and allows the use of free-space antenna designs and techniques. The antenna patterns are bidirectional, so a reflector is needed to make the patterns unidirectional. The peak of the pattern is normal to the wafer.
Inter-injection-locked oscillators have been proposed as a simple and efficient alternative to conven-tional methods of driving millimeter-wave phased antenna arrays. In this concept, each radiating element is driven by its own oscillator, and phase control is achieved through inter-oscillator coupling networks. This paper examines the possibility of using free-space radiation coupling to synchronize inter-injection-locked oscillators. A discussion of the two-oscillator case leads to a simple model which predicts the behavior of the two possible modes. Predictions of this analysis are verified by an experimental study using an X-band planar Gunn oscillator, and implications for more sophisticated arrays are discussed.
A quasi-optical power combiner was designed to couple millimeter wave radiation from a number of IMPATT diodes to produce a single power source. Phase coupling at 35 GHz has been accomplished and rf power exceeded the sum of the individual diode outputs. Millimeter waves of high spectral purity, less than 50 kHz bandwidth, were produced. Although problems of coupling loop impedance matching have not been fully solved, the results of this experiment should encourage the production of large arrays of millimeter wave diodes in monolithic integrated circuits.
The computer-aided analysis and design of various open and shielded microstrip discontinuities is discussed in detail. The analytical method employed in these algorithms efficiently takes into account the effect of radiation and surface wave propagation and accurately evaluates the related losses. The technique can be applied to a wide variety of discontinuities in the millimeter-wave frequency range. Comparison with available experimental data is very good.
The requirement for accurate measurements of radar cross section (RCS) at millimeter wave frequencies can only be met by careful calibration of the radars used in collecting these data. Georgia Tech has developed millimeter wave instrumentation radars specifically for the purpose of measuring RCS. Two of these radars operating at 35 and 95 GHz have sufficiently high power levels to allow RCS measurements of small targets at ranges up to several km. A balloon lofted sphere was used to calibrate these radars during a test at White Sands Missile Range. This paper describes the results of an experiment to determine the effect of interference between the sphere and the balloon.
The performance of millimeter-wave (MMW) communications techniques for air-to-air applications is reviewed. The anti-jam (AJ) and low-probability-of-intercept (LPI) performance of a communications system resulting from the attenuative characteristics of the atmosphere in the 50-60 GHz band are characterized. The objective of the communications system design is to avoid traditional AJ and LPI techniques such as spread-spectrum modulation and narrow-beam antennas, resulting in a simple system with a robust and secure communications capability.
A review of millimeter wave radiometric measurement programs performed for various external sponsoring agencies is the subject of this paper. Over the past ten years radiometric data has been gathered in the following areas: atmospheric water vapor, high altitude severe storm activity including hurricanes and tornadoes, Arctic ice and marginal ice zone signatures, detailed passive target signature data such as armored vehicles, and the detection of ice accumulation on the Space Shuttle external tank. The frequency region covered by the various measurement programs was from 35 GHz to 220 GHz.
A significant program is currently underway in the U.S. to investigate, develop and produce a variety of GaAs analog circuits for use in microwave and millimeter wave sensors and systems. This represents a "new wave" of RF technology which promises to significantly change system engineering thinking relative to RF Architectures. At millimeter wave frequencies, we look forward to a relatively high level of critical component integration based on MESFET and HEMT device implementations. These designs will spawn more compact RF front ends with colocated antenna/transceiver functions and innovative packaging concepts which will survive and function in a typical military operational environment which includes challenging temperature, shock and special handling requirements.
A tactically sized 35 GHz millimeter wave (MMW) radar sensor has been developed by Martin Marietta Orlando Aerospace to demonstrate functional performance in a compact package. Due to the increased emphasis on packaging, we designed this sensor to meet all required performance parameters and still fit within the seeker envelope of most modern-guided weapons. An envelope of 7 inches diameter X 17 inches long was achieved by employing a planar array mono-pulse antenna and microwave integrated circuits (MIC) for the transmitter and receiver. The radar sensor is fully coherent, dual circular polarized, and includes a two channel monopulse receiver. A nontactically configured programmable digital signal processor was developed for use with the radar sensor to form a complete testbed radar seeker.
A 35 GHz dual Passive/Active Mode (PAM) sensor has been developed for a wide variety of air-to-ground imaging applications. The sensor measures radiometric temperature with a sensitivity of 2°K in the passive mode and terrain altitude to a resolution of 20 feet in the active mode. The sensor consists of a small 25 cubic inch ruggedized RF transceiver packaged on the back side of a 12 inch parabolic antenna providing a 2° one-way beamwidth. The system is designed to scan an angle of ±20° at 160°/sec about nadir in the cross range axis to the platform. In the active mode, terrain altitude is measured from 3,000 ft. to 30,000 ft. In the passive mode, radiometric temperature imaging has been accomplished up to altitudes of 54,000 ft. This paper presents an overview of the fundamentals of operation, a description of the advanced PAM sensor, signal-to-noise ratio performance and system measurement accuracy. Examples of airborne test data are included.
Recent advances in millimeter-wave monolithic integrated circuits (MiCs) are presented. Performances of several MICs are summarized. These include a GHinLi oscillator at 68 GHz with an output power of 1 mW, a W-band balanced mixer with a minimum conversion loss of 6.0 dB and a W-band PIN switch with an isolation greater than 11 dB over a 6 GHz bandwidth.
Many present and future military and commercial systems operating at millimeter wave frequencies require the use of sophisticated electronically controllable antennas for maximum capability and flexibility. Electronic control of the antenna pattern is provided by electronically switchable phase control of each radiating element such as that achieved in phased array antennas or via electronically reconfigurable antenna feeds referred to as beam forming networks (BFN). Multibeam antennas provided by BFN'S can be realized using switches, variable power dividers (VPD), and phase shifters. Ferrite materials and associated application technology are being utilized to achieve these switchable RF control components at millimeter wave frequencies. The performance achievable in ferrite switchable circulators, variable power dividers and phase shifters in the frequency region from 20 to 100 GHz is discussed.
The development, design and performance of 94 GHz Microstrip Integrated Circuit (MIC) Gunn VCOs, down-converters, IMPATT injection-locked amplifiers, and low loss circulators are described in this paper. These components have subsequently been integrated into an all integrated circuit short pulse transceiver. Test results of the transceiver verifies their superior performance working as a subsystem. This work clearly establishes MIC technology as a viable approach toward achieving compact and lightweight 94 GHz transceiver design in planar construction, which is a necessary precursor to full monolithic implementation.
A broadband power sensor covering the 18 to 40 GHz band instantaneously with 60 dB dynamic range is discussed. The sensor utilizes an anti-parallel pair of Schottky barrier diodes mounted in double-ridge waveguide. The sensor's calibration procedure is presented along with an accuracy analysis and measured performance (linearity and dynamic range as a function of frequency).
The relative spectral radiance of a C. P. Clare & Co. noise tube, model TN-167, designed for the frequency range 90-140 GHz (3.3mm to 2.1mm) was compared to that from a Hanovia 200 watt high pressure HgXe arc lamp over the wavelength region from 0.5 to about 5mm. A Michelson Fourier transform spectrometer and a lamellar grating instrument were used in conjunction with liquid helium cooled bolometers of NEP 10-12 to 10-14 watt/(Hz)1/2 to measure relative spectral radiant power. With this instrumental arrangement, the noise tube exhibited a very sharp low frequency cutoff at about 2.2cm-1. The HgXe arc lamp emitted more radiant power than the noise tube in the wavelength region below 3mm (100GHz) down to 0.5mm. Above 3mm, the noise tube had a stronger output. The noise tube spectral radiance shifted to lower frequencies when the input current was lowered from 125mA to 50mA.