Thermography at millimeter wavelengths

Radiometer measurements show that the human body emits thermal signals at millimeter wavelengths which can produce a thermographic image potentially useful for detection of "hot spots." The spatial resolution which can be obtained at a frequency of 45 GHz is in the order of 1.3 cm, and the temperature resolution is 0.1 K.     

Astronomical refraction at millimeter wavelengths

Measurements of the difference between radio and optical astronomical refraction atlambda=8.6, 4.3, 3.1, and 2.2 mm are described. The measurement technique utilized solar limb crossing times observed with a 16-foot radio telescope. Results show the expected dependence upon atmospheric water vapor belowlambda= 3mm, but radio refraction tends to approach optical refraction at the shortest wavelength.     

Solar observations at millimeter wavelengths

Solar radiations at millimeter wavelengths consist of a basic component of thermal radiation from an undisturbed sun, a slowly varying component originating from active regions on the sun, and a solar burst component. Eclipse measurements of the brightness distribution have led to a quiet sun model of the chromosphere containing small jet-like spicules. High-resolution millimeter-wave measurements of the solar disk show that the slowly varying component is enhanced radiation from bright spots correlating with plage areas. An increase in brightness of a plage area is observed as early as one day before a plage region develops a flare. Both gradual-rise-and-fall bursts and outbursts have been observed at 4.3 mm. The large outbursts occur in the flash phase of a solar flare. Some of these outbursts had higher flux at 4.3 mm than at longer wavelengths of 3.15 cm and 9.4 cm.     

Cloud attenuation at millimeter wavelengths

Total atmospheric attenuation under conditions of complete and partial cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. The attenuations were actually inferred from extinction measurements using the Sun as a source. Measurements were made at 29 elevation angles from 1° to 20°, and the angular dependence of the attenuation was examined. For most cloud conditions the attenuation was found to be proportional to the slant path distance through the absorbing atmosphere. For elevation angles above about 8°, a flat Earth approximation is valid and the slant path distance is proportional to the cosecant of the elevation angle. For low elevation angles the slant path distance is a function of the effective earth radius and the effective height of the attenuating atmosphere, in addition to the elevation angle. A statistical technique for determining the radius and height is described. A zenith attenuation was extrapolated from each set of data. The humidity and frequency dependence of the attenuation was examined. An algorithm for estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived     

High power sources at millimeter wavelengths

Design considerations for high power millimeter-wave oscillators and amplifiers are discussed. Typical structures, electron gun and beam systems, electrode configurations, and interaction processes are described, particularly with regard to their applications in the millimeter-wave portion of the spectrum. The advantages of the linear beam approach in the design of these sources become evident. Typical operating characteristics for general classes of amplifiers and oscillators are reviewed. A survey is presented of the various individual sources which are commercially available or have reached a practical prototype status.     

Rain backscatter measurements at millimeter wavelengths

The US Army Ballistic Research Laboratory (BRL) conducted an experiment in 1973 to measure the properties of radar backscatter from rain at millimeter wavelengths. Rain backscatter and attenuation were measured with pulse radars operating simultaneously at 9.375, 35, 70, and 95 GHz over a wide range of rain intensities while continuous measurements of raindrop size and rainfall rate were made. This report describes the measurement technique, details of the instrumentation, the data analysis procedure, and the rain backscatter data obtained from A-scope photographs and video tapes     

Radar astronomy at millimeter and submillimeter wavelengths

The scant two-decade existence of radar astronomy has seen important contributions by this new technique to positional astronomy, to the metrics of the solar system, and to the study of lunar and planetary surfaces. The role of millimeter wavelengths in this new astronomy is discussed and the problems unique to these short wavelengths defined. Brief mention is made of the importance of correlation of active and passive measurements and important new millimeter wave component requirements are listed. The implementation of an eight-millimeter radar capable of detecting the moon is described. At the eight-millimeter wavelength, the moon is found to be generally rough and has a radar cross section of about seven percent of its projected geometrical area. Methods of improving these measurements and their feasibility are outlined. Future equipment configurations capable of extending millimeter wave measurements can be implemented, given critical component development success.     

Atmospheric effects on propagation at millimeter wavelengths

In order to meet future earth-to-space communication needs, new regions of the electromagnetic spectrum must be utilized. This article discusses the feasibility of using the millimeter-wave region. It is shown, for example, that for clear-sky conditions the effects of the atmosphere on propagation at frequencies of 15 and 35 GHz are very small. However, the attenuation increases significantly under conditions of heavy cloud cover and precipitation. On the basis of these results it is evident that atmospheric attenuation is sufficiently low in selected regions of the millimeter-wavelength spectrum to permit wideband earth-to-space communications with moderate reliability. For high reliability the high attenuation due to heavy rainfall must be overcome either by the use of space diversity techniques or by locating the ground terminal in a dry climate.     

Fresnel zone plate antennas at millimeter wavelengths

The performance characteristics of Fresnel zone plates are described with emphasis on applications at millimeter wavelengths. Recent results are included along with a summary from numerous earlier investigations. Parameters described include design information, efficiency, bandwidth, focal characteristics, off-axis performance, axial intensity dependence, aberrations, and far-field pattern. Both transmission and reflection configurations are discussed, mostly for cases where the focal length and diameter are comparable. This type of zone plate has advantages (compared to a lens or paraboloid) of reduced loss, weight, volume, and cost, as well as simple planar construction, with similar diffraction-limited beamwidth and major sidelobe performance, but lower efficiency.     

Application of multilayer structures at millimeter wavelengths

The paper presents results of theoretical analyses and experimental investigations of the optical characteristics of devices based on multilayer dielectric structures and intended for the millimeter wave band. As is shown, multilayer dielectric structures can prove useful in devices with variable optical characteristics representing coupling elements for quasioptical open resonators, adjustable reflection coefficient mirrors in narrow-band interference Fabry-Perot filters or adjustable multilayer polarizers.     

Thermal detection radiometry at short millimeter wavelengths

Details concerning the design, construction, and operation of thermal detection radiometers for wavelengths near one mm are given. The sensitivity of these systems greatly exceeds that of more conventional radiometers at short millimeter wavelengths. Although the present designs are shown to be far from optimum, they have made possible a considerable number of astronomical observations that were previously beyond the state of the art. Measurements of atmospheric attenuation in the one-mm window as a function of total precipitable water vapor are presented, and methods of reducing the effects of varying sky background are discussed.     

Characterization and measurements of laterally shielded coplanarwaveguide at millimeter wavelengths

A laterally shielded coplanar waveguide consisting of a coplanar waveguide of finite width placed vertically in a cutoff parallel-plate waveguide is proposed for millimeter-wave applications. Advantages of the waveguide over the conventional coplanar waveguide are weak dispersion and low loss characteristics due to elimination of undesired surface waves and radiation. Simple but accurate analytical formulas for circuit design are derived and semi-empirical formulas for the weakly dispersive characteristics are also examined. Transitions to coaxial line are devised and the propagation constant is measured at millimeter wavelengths     

Theoretical performance of novel multipliers at millimeter and submillimeter wavelengths

A theoretical comparison of various low and high order multipliers for 200 GHz and 1 THz has been carried out. Novel diodes including single barrier varactors, barrier-intrinsic-n⁺ diodes and high electron mobility varactors are shown to have excellent theoretical performance, comparable or better than the conventional Schottky varactors for single and double diode frequency multipliers at millimeter and submillimeter wavelengths, whereas quantum well diodes, since they suffer from high resistive losses, are shown to be less attractive. In comparison to the conventional Schottky varactor, these new diodes have some potential advantages in their characteristics such as nonlinearity or a special symmetry. For future optimization some general comments on these advantages as well as other factors affecting multiplication is given.     

Continuous oscillations at millimeter wavelengths with silicon avalanche diodes

CW operation of an abrupt-junction p-n silicon avalanche diode has produced 370 mW at 36.5 GHz with 6 percent efficiency. Another unit produced 320 mW at 39 GHz with 7 percent efficiency. Higher efficiency with present diodes requires power densities exceeding 3 × 10⁵W/cm².     

Small signal characteristics of ferroelectric ceramics at millimeter wavelengths

A method is described for measuring dielectric properties at wavelengths near 1 mm. The particular material tested was a ferroelectric ceramic composition of 73 per cent barium titanate-27 per cent strontium titanate. Material properties were determined from transmission measurements through an interferometer, and the radiation was generated by a magnetic undulator. It was found that there was a marked dispersion in dielectric constant for this material in the range from 50-100 kMc.     

Flexible circular waveguides at millimeter wavelengths frommetallized Teflon tubing

Flexible waveguides for use at millimeter wavelengths have been fabricated by deposition of metallic film onto the composite-modified inside surface of Teflon tubing. The attenuation characteristics in the range 80 to 115 GHz show losses on the order of 0.1 dB/cm. Bending, twisting, and rotating to the limit of plastic mechanical stability (curvature radius typically >8 cm) have a negligible effect on the attenuation, and bend angles ⩽45° produce relatively small changes in the insertion phase     

Automatic measurement of the complex permittivity at millimeter wavelengths

A measuring system including an oversized cavity resonator operating in the TE₀₁ mode for the determination of the complex permittivity ε^′- jε^″ of low-loss liquids at frequencies of about 36 GHz is described. While ε^′ is obtained by wavelength measurements in the filled and the empty resonator, ε^″ is determined from the variation of the Q factor of the filled resonator with the length of the dielectric sample. The Q factors of values of about 2·10⁴ to 10⁵ can be measured automatically by means of a desk calculator which controls the frequency and collects the digitized values of the detector output voltage. By means of the calculator, the Q factors of the resonator are determined by fitting analytical (Lorentzian) resonance curves to the measurement data. ε^″ and the Q factor for zero sample length are calculated according to Göttmann's methods.     

Polarization properties of some materials at near millimeter wavelengths

The degree of polarization of some materials widely used as optical components in the submillimeter and millimeter region has been measured in the wavelength interval 0.5÷3 mm. The procedure used in the measurements allows to determine degress of polarization of the order of 10^?5.