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Noise comparators and associated thermal standards have been developed to provide accurate noise source calibration facilities at S and X Bands. The comparators are based on a null-balance form of the Dicke radiometer, but eliminate problems of RF switching by using an IF switch. Matching facilities are incorporated. The thermal standards include both waveguide and coaxial forms and operate at 400°C. The most successful type is described in detail. Gas discharge noise sources have been developed as highly reproducible transfer standards, with outputs stable over many thousands of hours to within a few hundredths of a decibel.

This paper outlines the work currently being carried out at the Royal Radar Establishment (RRE) in the RF and Microwave National Standards Division of the British Calibration Service, on both hot and cold noise standards and comparator systems. The design of three cryogenic noise sources for use in WG15 are described in detail along with the theoretical calculation of their output noise power spectral density. A noise-adding radiometer is analyzed, and finally the sources of uncertainty involved in a recent calibration are discussed.

An easily constructed differential radiometer is described which can be calibrated analytically from its geometric and physical parameters. An estimated 3% accuracy is achieved using common handbook values for the physical parameters, and the accuracy of the calibration is confirmed by experiment. The instrument measures net thermal radiation exchanged between surfaces near normal room temperature. It responds quickly to changes in incident flux and reaches equilibrium within about 1 min. It is sufficiently stable and rugged to be considered for student experiments and for demonstration lecture work. A number of elementary experiments are described which use the radiometer to demonstrate the behavior of long wavelength infrared radiation and to determine its interaction with materials.

Of modern British work seeking to establish a scale of radiation intensity the most important is that of the late Professor H. L. Callendar (1910), who developed a very accurate instrument, the Callendar Radio-balance, for measuring radiation. Subsequently Callendar used the radio-balance to calibrate various types of radiometer produced by British instrument makers. After his death, arrangements were made for this work to be continued at the National Physical Laboratory. The instrument used to establish a scale of radiation intensity at this Laboratory was a radio-balance made to an improved design of Callendar’s shortly before his death. It was made in the Physics workshops of the Imperial College of Science and calibrated at the National Physical Laboratory as described in the second of these papers. Other scales, notably those known as the Smithsonian Scale of 1913 and the Ångström Scale, have also been employed in this and other countries, particularly for the calibration of meteorological instruments; and it has been recognized for some time that these scales are not in agreement. The discrepancies indicated the importance of obtaining independent evidence of the accuracy of the N. P. L. scale rather than continuing to rely on the properties of a single instrument. Accordingly, the first steps taken in a long investigation, which ultimately led to complete justification of the faith reposed in the radio-balance, concerned the development of absolute radio-meters of an entirely different type. Descriptions of these new instruments, and of the method of using them, form the subject of this first paper.

The response of a net radiometer to a sudden irradiation is shown to give generally a marked overshoot which during calibration is mostly overlooked. This overshoot also simulates a high speed of response. An improved design of net radiometer minimizes this effect and also gives a high sensitivity.

Long-wave calibration procedures which expose a shielded net radiometer to different air temperature on the two sides or which expose an effective-pyranometer to an air temperature different from the instrument temperature cause a major increase in apparent sensitivity due to (1) an unwanted sensible heat flux through the instrument and (2) differential radiation coming from the shielding if the latter is not very transparent. Calibration procedures using cavities directly placed over the shielding should therefore be avoided.

Laboratory calibration of Beckman-Whitley's long wave radiometer was carried out by using two radiators of the Dines type, as shown in Fig, 1. Of these radiators, the surface temperature of one is near the room temperature, about 10°C, the other about 20-45°C. Let the temperature of the surface be T1 and T2, the electric motiv force V and V′, then have(1-e)⋅κ⋅a⋅σ(T14-T24)=k⋅(V-V′), where e is an absorptivity of water vapour and CO2 in the wooden box, a is blackness of radiator, κ is a constant depending upon the solid angle of the radiator seen from the element. k is the calibration constant, and obtained from Fig. 2.These value of k is significant for the column radiation from the upward. However, a value for the total radiation from all parts on the hemisphere is desired in the measurement of atmospheric radiation. Assuming the relative value of absorptivity of the receiver surface depended on the direction of incident radiation and intensity of the atmospheric radiation from each direction, we can obtain a value required, that is larger than the value obtained from Fig. 2.

To allow the absolute calibration of field and laboratory radiometers in the range 0.1 to 1.5 cal/cm(2) sec, a continuous flow radiation calorimeter has been constructed, based on a design originally developed by the British Joint Fire Research Organization. The temperature difference established between a water outlet and an inlet is sensed by a thermistor bridge and the radiation intensity measurement is made in terms of the level of the electrical power, applied to a built-in immersion heater, which is necessary to reproduce the same temperature difference., Reprinted from: Canadian Journal of Physics, v. 39, no. 11, November 1961, p. 1583-1594. ISSN: 0008-4204

The accuracies of conventional microwave radiometers are influenced by instabilities mainly in their RF components A radiometer is described that uses two reference temperatnres. The signal processing eliminates the influences of all RF parameter changes on the radiometer indication. Thus the overall stability depends only on the reference temperatures and the low-frequency and dc components. These can be made extremely stable by careful design. The paper presents a detailed analysis of the radiometer performance. The minimum detectable signal is greater than that of a comparable Dicke radiometer, but approaches this, if the reference temperatures are appropriately chosen according to the particular measurement problem. The great advantage of the radiometer is that no measurement time is lost for frequent recalibration. Practical results of an X-band radiometer for airborne use are given at the end of the discussion. They are in good agreement with the theory and demonstrate the feasibility and superiority of the concept.

Impulse generators are not conventional instruments and present their own calibration problems. A new method for calibrating the output spectral density level of impulse generators is explained. A Dicke-type radiometer is used for measuring the equivalent noise temperature of the output power spectral density which can easily be converted to the spectral density (amplitude). The switching sequence of the radiometer should be synchronized with the impulse generator in order to have all the impulses delivered to the receiver. This method eliminates the need for computing impulse bandwidth and the need for measuring bandpass characteristics of the receiver. It also does not require a sophisticated radiometer but a simple Dicke radiometer. An impulse generator with 50 pulses per second (pps) rate is calibrated with a Dicke radiometer with a square wave switching sequence of 400 Hz. There is reasonable agreement between the calculated and measured values.

The quarts gauge apparatus for measuring the vapor pressure of postum was completed. The first sample of postum oxidized in the course of preparation, while in a presumably sealed and evacuated quartz tube. an error of 0.4 per cent appears to be present in the vacuum weighing of postum. a diffusion pump is being installed to reduce the pressure, in the hope of minimizing the radiometer effect. Repetition of the effusion experiment with a rebuilt crucible yielded four points below those reported last time by a factor of ten in pressure. A leak appeared in the crucible, requiring construction of a new one. X-ray patterns were again obtained for the Q metal sample, {number_sign}33, and oxide sample, {number_sign}42. Work on the calibration of the two new powder cameras is proceeding satisfactorily. Two samples of cadmium were made for the purpose of developing a technique for producing single crystals of metals. A sample of tellurium nitrate yielded the same pattern as that reported in the literature. Data on the characteristics of a newly designed tub type moderator as compared with the Hanson cylinder are presented. Some notes on the calculation of probable error show the minimum error to depend upon the ratio of source strength to background.

THERMAL methods for the measurement of power at centimetre wave-lengths normally involve a substitution process in which the high-frequency power is assumed to be identical in magnitude with the d.c. power causing the same rise in temperature or change in resistance. While in some techniques this assumption results in no appreciable error, it is a general limitation of thermal methods that the high-frequency and d.c. powers are not dissipated in precisely the same manner. This feature of existing methods is particularly evident at low power-levels, of the order of 1 milliwatt, at wave-lengths of 3 cm. and less. Although a calibration of centimetre-wave milliwattmeters can be made against standard equipment operating at higher power-levels, this operation requires much auxiliary apparatus and attenuation measurements of high precision. There is a need, therefore, for further convenient power-measuring techniques at the milliwatt-level in which the limitations inherent in existing methods are avoided. It seems probable that a form of film radiometer or bolometer, in which the ideal of accurate substitution of d.c. power for high-frequency power is approached very closely, will fulfil these requirements; the results of some preliminary experiments in the development of such an instrument are summarized in this communication.

Klumb-Schwarz type radiometer gauge (Knudsen gauge) is an absolute gauge for high vacuum measurements, but since its theoretical sensitivity is so far unknown, in practice it needs calibration against another absolute gauge (e. g. McLeod). In this paper a simple theory to determine the sensitivity of a Klumb-Schwarz gauge is described. The optimum angle of the vane is decided as 24° by theoretical considerations. It is further shown that in this gauge the force due to electromagnetic radiations limits the lower range and is something in analogue with the soft X-ray effect in conventional hot-cathode ionization gauges.

A long-wave radiometer is described in considerable detail. Physical and transmission characteristics of the hemispherical “windows” are presented, and calibration and use of the instrument are discussed. The results of a series of nighttime and daytime measurements of the infrared emissions of the atmosphere are compared with theoretical values obtained from the Elsasser radiation diagram, and satisfactory agreement is found to exist.

The letter describes a calibrator for millimetre-wave radio-meters, using the radiation from a load immersed in liquid nitrogen.