Design considerations for a group of broad-band calorimetric power meters, capable of accurately measuring low (zero to one milliwatt) and medium (zero to 100 milliwatts) power levels over a frequency range from zero to 75,000 mc, are presented. The power meters are of the nonadiabatic, twin, dry-load type and utilize the substitution of dc power. The conflicting requirements imposed upon the design by the need to realize broad-band performance, adequate sensitivity, reasonably short response time, negligibly small rf-dc equivalence error, freedom from "zero" drift and from other types of error are discussed. An analysis is given of the known sources of error which enables the accuracy of the individual instruments to be reliably estimated.

Broad-Band Calorimeters for the Measurement of Low and Medium Level Microwave Power I. Analysis and Design

Thermistors are components of comparatively recent development, although the phenomenon of conduction in metallic salts was recorded by Faraday over a century ago. The paper, in a broad survey of the subject, describes the theory of their operation, explains why they, in common with other semiconductor devices, possess a negative temperature coefficient of resistance and develops the expressions which govern their parameters. Methods of manufacture of the various forms of thermistor are outlined in general terms, and numerous applications of the device are described and discussed. Guidance is given on the approach to circuit problems, and the possibilities of exploiting some of the device's interesting and unusual properties are indicated. The point is made that the thermistor can now be used in many fields as an adequate alternative to components of more specialized application.

Thermistors,Their Theory,Manufacture and Application

The paper describes the instruments used in the microwave band for the measurement of power, frequency and impedance; the majority of them use waveguide methods of transmission, but coaxial systems at the longer wavelengths and free-space semi-optical systems at the shorter wavelengths are also described. Emphasis is laid upon more recent instruments and on those for the millimetre-wavelength region. Methods of manufacture and their influence on behaviour and design are also discussed.

Instruments for use in the microwave band

Microwaves are one form of electromagnetic radiation — that is, a wave motion associated with electric and magnetic fields — the known spectrum of which is shown in Fig. 1. Here, frequency is plotted as abscissa and free-space wavelength as ordinates, while, for future reference, the energy and equivalent temperature of corresponding photons are also included. Microwaves occupy a unique place in this spectrum because they are, as a rule, susceptibie to electrical, as distinct from purely optical, treatment, whilst requiring techniques [1] that differ completely from those used at lower, conventional radio, frequencies. This radiation is coherent, having a clearly defined frequency and spatial distribution of phase and polarization. In this respect, the field of microwave techniques has recently been extended into the infrared and visible regions and some consideration will also be given to this work.

G R Nicoll

Papers

Discrepancies in the Measurement of Microwave Power at Wavelengths below 3 cm.