Short period fluctuations in the characteristics of wireless echoes from the ionosphere

A new experimental technique is described for studying the directional and polarization characteristics of wireless echoes. The method makes use of an ordinary spaced frame direction-finding system, combined with a twin channel amplifier and cathode-ray tube, so that by observations of an ellipse on the screen the phases and amplitudes of the e. m. f. s in the aerial system may be determined. By appropriate connexions of the aerials either of the two characteristics mentioned may be studied. The equipment is made applicable to pulse transmissions by means of an 'echo selecting’ device, which has the function of desensitizing the apparatus for all but a small element of the time-base cycle. The duration of the active period is of the order of 15μsec., and by means of suitable phasing circuits it may be made to coincide with the arrival of any echo of the pulse pattern, causing the ellipses appropriate to that element to appear alone on the cathode-ray tube screen. In polarization measurements a single ellipse is produced on the screen corresponding to the e. m. f. s in two loops at right angles. In directional measurements, the aerials are switched at 25 cyc./sec. by relays so that two ellipses are produced, one representing the phase difference in one pair of spaced frames, and the other that in the other pair. These are separated laterally on the tube by a synchronized magnetic deflecting circuit. The figures are photographed on a moving film, and the ellipses due to each pulse from the transmitte rare thus recorded separately. In this way rapid changes in polarization and direction can be studied. The method has been applied to the various types of ionospheric echo, and records are shown to illustrate their behaviour. In particular it has been used to study the short-lived scatter echoes obtained on high power transmissions, whose characteristics change too rapidly to be followed by manual methods. The results of both polarization and directional measurements are considered in relation to the general problem of ionospheric irregularities.