Noise Spectrum of Electron Beam in Longitudal Magnetic Field: Part II -The UHF Noise Spectrum

The reader is referred to Part I 1 for a description of the experimental apparatus and its operation. In this paper, measurements of noise power in the same electron beam are described, with frequencies chiefly in the 10- to 500-mc range, and relatively weak magnetic fields. For the U H F measurements, a calibrated coaxial step attenuator and a super-regenerative receiver (the Hewlett-Packard 417-A V H F Detector) are used. Relative noise-power amplitudes at fixed frequencies are measured as before, in terms of changes in attenuation between probe and receiver required to restore constant receiver output. To obtain qualitative information, however, such as the location of noise maxima along the beam, the series attenuation is fixed. The receiver output is amplified, rectified, and per855 86() T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , J U L Y 1957 mitted to register itself directly on the chart recorder, whose motion is synchronized with that of the probe. Very roughly, the detector output varies as the log of input power. Measurements are described (a) of the U H F noise spectrum in the beam, just outside the gun anode; (b) of this spectrum at the end of the drift region, in a longitudinal magnetic field; (c) of the noise-power distribution along the axis; and (d) transverse to the axis of the rippled beam in the drift region. Two calculations are then outlined, one of wave propagation along the rippled beam (to explain the observed distribution patterns), and the other to account for some spectacular peaks in the beam spectrum (b).