Abstract :
The properties of the lower portion of the ionosphere have been studied at selected low and very low frequencies by observations on commercial continuous-wave transmitters located at distances of about 100 km from the receiving point. Attention has, in general, been confined to the horizontally-polarized electric component of the ionospheric wave received by means of a vertical loop set normal to the plane of propagation. Using mainly twin-channel recording equipment, an outline description of which is given, the pick-up on this loop has been compared in phase and magnitude with that on another vertical loop arranged in the plane of propagation. Changes in the apparent height and conversion coefficient of the reflecting layer have been studied, mainly at 16 and 70.8 kc/s, in terms of a simple model under both normal and disturbed conditions. The data serve generally to confirm the measurements of other observers, but some discrepancies are discussed. It is considered that pulse-sounding technique has advantages over the continuous-wave method for ionospheric measurements in the frequency band under review, particularly at the higher frequencies.
