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Measurements have been made of the atmospheric gaseous emission spectrum in the region from 85–118 GHz (wavelength range 3·5–2·5 mm), and the results have been used to estimate the attenuation in the zenith direction over this range, which includes the low frequency side of the oxygen absorption line centered at 118·75 GHz. The results are compared with theoretical predictions based on line shape models in current use, and the discrepancies between experiment and theory are discussed.

Following the suggestion of B arrett and C hung (1962) that the pressure-dependence of the strength and width of the 22·235 Gc/s (1·35 cm) water-vapour rotational emission line might make it possible to determine whether or not there is any appreciable increase with height in the water-vapour to air mixing ratio in the stratosphere and mesosphere, a series of measurements of sky radiation noise has been carried out at Slough, England from June 1964 to April 1965. The measurements of the noise-temperature spectrum in the vicinity of the centre frequency of the line have failed to produce any evidence of the enhancement which would exist close to the centre frequency if there was any appreciable increase in the stratospheric mixing ratio. If such an enhancement does exist at Slough, then it is less than 2°K This is consistent With a mixing ratio in the stratosphere and mesosphere which is nearly independent of height.

The paper describes the essential features of design and construction of field-strength measuring equipment for use at frequencies up to about 600 Mc./s., i.e. wavelengths down to about 50 cm. The equipment is of relatively narrow band-width (about 20 kc./s.) and is primarily intended for the measurement of continuous-wave fields of the order of tens or hundreds of microvolts per metre. The paper is supplementary to a previous paper on the same subject and is intended to bring the information in the previous paper up to date in respect of recent practical and theoretical developments. An appendix to the paper briefly outlines the theory of the transference of received radio-frequency energy from an aerial to a diode detector or other load, by way of a closed tuned circuit.

The paper contains a discussion of the principal features of the design of apparatus for the measurement of field strength at very short wavelengths by means of a loop aerial. The apparatus consists of a high-gain intermediate-frequency amplifier (about 1 Mc./sec.) associated with a signal-frequency and frequency-change unit of constant conversion efficiency. The e.m.f. of thermal agitation in the input tuned circuit of the intermediate-frequency amplifier is used as a standard signal for setting the amplifier to a known gain. The combination of intermediate-frequency amplifier and signalfrequency unit is calibrated over its signal-frequency range by means of a radiator which gives a horizontally-polarized radiation of calculable intensity. It is shown that a single-phase, i.e. unsymmetrical, frequency-change system has certain practical advantages over biphase systems, and that this can be used without loss of the necessary balance of the aerial system, if the latter is coupled inductively through a Faraday screen to the frequency-change circuit. The detailed design of the various couplings and of the diode frequency-conversion circuit are analysed in Appendices. The intermediate-frequency amplifier embodies continuously-variable gain-controls, calibrated in decibels, these components being a particular type of commercial carbontrack "volume control" which has been found satisfactory for this quantitative use. The set can be used for the measurement of field strengths down to the order of microvolts per metre at wavelengths from 7-11 metres (27-43 Mc./sec.) with the frequency-change and aerial units described.

Measurements are described in which the effective noise temperature of the type CV1881 argon discharge tube is determined by comparison with both a thermal noise source and a continuous-wave signal, at a wavelength of 3.2cm (frequency, 9.4Gc/s). The results obtained were 10590 ± 500°K and 11050 ± 1350°K, respectively. The results are compared with data previously given for argon noise sources.

It is shown that a field-strength measuring set having a symmetrical closed-loop aerial coupled inductively through an electric screen to an unsymmetrical amplifier, can conveniently be calibrated in terms of a measured current passed through the coupling coil. The calibration so determined was found to be in satisfactory agreement with standard-field calibrations at wavelengths of 8 m. and 20 m.

The instrument is in the form of a differential air thermometer, and consists of two similar glass cells connected by a glass capillary tube. A carbon-coated strip is placed in each of the two cells; one of these strips absorbs the radio-frequency power to be measured and the other is heated by direct current. A liquid pellet in the capillary tube is displaced as a result of the differential expansion of the air in the two cells arising from the heat dissipated in the carbon-coated strips. After balancing the system by adjustment of the d.c. cell the radio-frequency power is finally determined by the measurement of the d.c. power which must be substituted for the radio-frequency power in the same cell in order to maintain the state of balance. The instrument has been used to measure powers of 10 to 100 mW at a wavelength of 3cm with a discrimination of about 0.2 mW.

Measurements of the X-band magnetic properties of a series of nickel ferrite-aluminates, as a function of applied magnetic field, are described, together with static magnetization measurements. The results show considerable departures from simple theoretical (Landau?Lifshitz) behaviour, especially in the compensation region of small magnetization, which are probably due to porosity and anisotropy eftects. Resonant-cavity measurements of permeability and permittivity have been made on various nickel ferrite-aluminates and nickel-zinc ferrite aluminates, and compared with transmission measurements for thin slabs of the same materials placed in rectangular waveguide. A general correlation was obtained in all cases, but this was insufficiently precise for exact prediction of waveguide propagation data from intrinsic ferrite parameters. Possible reasons for this are discussed.

Measurements on a path 150m long, with narrow-beam transmitting and receiving antennas 2m above the ground at one end of the path, and a reflecting plate at the other end, at the same height, have shown attenuation in rain to be about 17dB/km and 8dB/km for rainfall rates of 48mm/h and 7mm/h, respectively. Amplitude variations of up to ±0.4dB can occur in a time interval of the order of 1s during sunny, humid conditions in summer.

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