This paper describes instrumentation for automatically measuring the phase and amplitude of randomly fluctuating pulsed radio signals reflected by the ionosphere. The phase detecting system is based on the null principle of servo-mechanics, wherein the phase difference between transmitted and received signals is considered as an error to be annulled. The equipment is designed for pulsed operation at a carrier frequency of 60 kc, pulse width of 100 μsec, and a repetition rate of 12 pulses/sec. Amplitude and phase data are presented on a dual channel pen recorder.

The problem of electromagnetic scattering by homogeneous spheres with relative impedance equal to unity is considered. From the Mie series expression for the induced internal fields, the details of the condition responsible for producing a null in the backscattering cross section are deduced. The existence of this null is attributable solely to destructive interference, regardless of frequency, and is valid for either lossy or lossless media. The returned signal in the vicinity of the null is determined from numerical computation of the bistatic cross sections.