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Pulsed Quantities.-Additional pulsed power services were offered from 300 to 500 MHz. Pulse voltage, or base-band pulses, are now calibrated for nanosecond rise times, from 5 to 1000 volts.

Increased Immittance Services.-The large increases in accuracy announced last year have been extended to new frequencies, and to new values of capacitance, inductance, and resistance. NBS capabilities are now being better used in that new standards, which use the new precision connectors, are now being received for calibration. Uncertainties can be reduced up to 100 times on such standards.

Heat Flow.-A theoretical study was made of the de-rf substitution error of a Wollaston wire bolometer power meter. The study resulted in the solution of the nonlinear differential equation which describes the heat flow by all significant transport mechanisms. Application of this theoretical study to millimeter wave equipment is expected to lead to the use of millimeter wave barretters as power standards.

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To improve measurements of power at millimeter wave frequencies, this largescale model of a Wallaston wire bolometer and related components was made approximately 100 times normal size. (In cross section the model is 16" by 8".)

Low Attenuation Measurements.-In the process of selecting lengths of waveguide of uniform specific resistance, microwave low-attenuation measurement capability has been extended to microbel precision. The stability necessary for such measurements was achieved through refinement of existing technology and inclusion of some innovations such as regulation of power output through servo-control of the klystron beam voltage. For eventual use in the system, and other systems, a rotary vane attenuator with microbel sensitivity and repeatability has been developed. An optical readout and improved rf shielding are features of the attenuator.

Reflection Coefficient Studies.-A computer program has been written to enable preparation of tables and graphs to facilitate calculation of reflection coefficients of quarter wavelength, short-circuited waveguide impedence standards. The tables will cover 38 standard sizes of waveguide and three sizes of 50-ohm coaxial line. This will enable anyone to construct his own standards of reflection coefficient for frequencies up through 325 GHz.

Small Attenuations of Waveguide Joints. Data have been obtained on the small attenuations of waveguide joints versus frequency. Little or no data have previously been available on the frequency dependence of these losses, and these data should reveal more about their nature. The series and shunt losses of waveguide windows have been measured over the frequency range of the waveguide. The method which has been developed is especially convenient and accurate, but not quite as sensitive as cavity reasonance techniques.

Microwave Calibrations

Power Calibration Systems.-A new calibration service was established to fill a strongly needed measurement gap in the area of microwave power measurements for coaxial devices. The service provides continuous frequency coverage from 4 to 10 GHz for low-value bolometric power measurements on interlaboratory standards terminated with Type N coaxial connectors and from 4 to 8.5 GHz for the newer 14-mm precision coaxial connectors. The service is based on existing, recently developed microwave power calibration systems in rectangular waveguide and upon a waveguide-to-coax "adaptor" technique. Microwave power calibration systems were also developed for the measurement of low-value CW power in WR284 waveguide (2.63.95 GHz) utilizing the impedance variation method of power measurement and in WR42 waveguide (18.0-26.5 GHz) using the reflectometer method.

Calibration Accuracy of Bolometer Units.-A significant improvement in the calibration accuracy applied to bolometer units was

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achieved. This was accomplished by experimental and theoretical verification of the magnitude of an important error, usually referred to as the "rf-de" substitution error, which must be accounted for in bolometric measurements. The reduction in the rf-dc substitution error by 0.7 percent permits an equivalent improvement in several calibration accuracies applied to both waveguide and coaxial bolometric powermeasuring devices.

Reflection Coefficient Measurements.-Microwave calibration systems were developed for the measurement of reflection coefficient magnitudes in WR284 waveguide (2.6-3.95 GHz) and WR42 waveguide (18.0-26.5 GHz) using a modified reflectometer method of measurement. The development of the WR284 calibration system required the construction of an unusually large section of precision waveguide. Weighing more than 300 pounds, this component was constructed in NBS machine shops with specially-developed techniques and tolerances on the internal dimensions better than 100 microinches throughout its 24-inch length.

Phase Shift Calibration System.—A microwave phase shift calibration system has been developed for measurements on variable phase shifters in WR90 waveguide (8.2–12.4 GHz) utilizing a modulated subcarrier method of measurement. The estimated limits of the total measurement error for this new service are reported as approximately ±0.25 degree including the error contribution from the reflection coefficient magnitude of the phase shifter being calibrated.

Electromagnetic Field Standards

Field-Strength Calibration.—A calibration service for field-strength and other precision receivers has been developed for the 1 to 10 GHz range. This new service includes calibration of the receiver as a twoterminal rf voltmeter, calibration of the signal attenuators, and calibration of the overall linearity of the receiver. The voltage measurements are made in terms of a coaxial power standard and are valid for rms CW measurements only. The attenuation and linearity measurements are made in terms of calibrated rotary vane attenuators. The calibration uncertainties vary from 2 to 10 percent, depending on frequency.

Antenna Field Measurements.-Significant progress has been made in the development of techniques for computing antenna characteristics (absolute gain and patterns) from near-zone data. In one NBS method, the radiation pattern of an antenna is expressed in terms of an angular spectrum of vector plane waves. The spectrum is obtained

from a two-dimensional, spatial Fourier-transform of the transverse components of the complex electric field in an "aperture plane." The antenna pattern can then be calculated for any distance beyond the aperture plane.

Dielectric and Magnetic Standards

High-Temperature Dielectrometer.—An accurate high-temperature (to 815 °C) microwave dielectric measurement service was activated and compared with previous results of five other laboratories. This service utilizes a measuring system developed in response to needs of industry for refined experimental techniques for evaluating high temperature dielectrics for radio-electronic applications. The technique uses a resonant cavity and a rod specimen.

Millimeter-Wave Dielectrometer.-The mm-wave interferometric dielectric measuring system was further evaluated and improved. Accuracy, defined in terms of "goodness" of fit of the theoretical to the experimental interference curve, is now of the order of 1/2 percent for the real part and 1 percent for the imaginary component.

Survey of Dielectric Measurements.-A critical survey and literature review of dielectric and magnetic measurements was undertaken from the viewpoint of radio-electronic and communications applications. Of special interest was the way in which these measurements depend upon the state of development of basic radio frequency-attenuation, impedance, etc.—and on how the measurements relate to the geometrical structure of the specimen and the electromagnetic environment. Dimensional gaging and small departures of the system from the form assumed in the ideal boundary value problem are often the limiting factors, rather than the electrical observations.

Narrow Linewidth Measurements.-A measuring service for the linewidth of single-crystal ferrimagnetic materials was activated. A nonresonant shorted waveguide method for the ferrimagnetic resonance linewidth is employed. Measurement of the linewidth was examined in detail by orienting the rf magnetic field relative to the static magnetic field on a given spherical specimen. It was concluded that the unloaded (intrinsic) linewidth can be correctly obtained from the measured linewidth and the magnetic resonator model of the specimen.

Tensor Permeability Measurement. It was demonstrated that a cylindrical TM110 mode cavity is very satisfactory for measuring the tensor permeability of rod samples to as low as 1 GHz. It is possible to obtain good linewidth measurements on cylindrical samples with standard rectangular waveguide techniques normally used in studies of

ferrimagnetic resonance. A previously unreported ferrimagnetic absorption was also observed in this study.

Vibrating-sample Magnetometer.-Saturation magnetization is an important index or "figure of merit" of a ferrite for use in designing nonreciprocal microwave devices. The vibrating-sample magnetometer is a practical and accurate means for determining this quantity. Two of the most widely accepted methods for calibrating such magnetometers were examined in detail and the sources of error delineated. The study included nickel and pure iron as standard reference specimens.

THERMAL QUANTITIES

High-Temperature Heat Content Standard.-Standard materials for high-temperature heat content (enthalpy) measurements are essential for comparing different methods and apparatus and ensuring consistency of tables of standard reference data. The present standard material is aluminum oxide-synthetic sapphire-which melts around 2300 °K and is therefore an unsatisfactory standard material for higher temperatures. As part of a program to develop a standard material for higher temperatures, the enthalpy of tungsten has been measured up to 2700 °K with new apparatus developed at NBS. The precision of these measurements is higher than any previously reported.

Thermal Expansion Measurements at High Temperatures.-To meet the increased demands of high-temperature technology in both industry and defense, apparatus for measuring thermal expansion of solids at temperatures between 800 and 1600 °C has been developed. Length measurements made with this apparatus have an accuracy of 30-50 ppm, and are the most accurate available in this temperature range. Platinum, single crystal aluminum oxide, magnesium oxide, tantalum carbide, and pure tungsten are being investigated as possible reference standards for thermal expansion at high temperatures.

PHOTOMETRIC AND RADIOMETRIC QUANTITIES

Freezing Point Blackbodies as Primary Standards of Special Radiance. Freezing-point blackbodies having efficiency of better than 0.999, and whose operating temperatures are known to better than one part in 50,000, were constructed and delivered to the Department of Defense Calibration Laboratories for use as primary standards of spectral radiance. Each furnace comprised a cavity, machined from high-purity graphite, immersed in a bath of liquid metal, contained in a crucible of high-purity graphite. As the metal freezes, the walls of the cavity are maintained at a very uniform temperature. High

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