Low-Noise Microwave Oven Magnetrons With Fast Start-Oscillation by Azimuthally Varying Axial Magnetic Fields

Abstract

A technique has been proven to significantly reduce the noise and to hasten the startup in microwave signals from dc operating oven magnetrons. The technique is based on imposition of an azimuthally varying axial magnetic field in magnetrons. For an -cavity magnetron operating in pi-mode, the optimal number of periods of azimuthal variations in the axial magnetic field is 2. This technique demonstrates the fastest startup and lowest start-oscillation current by prebunching the electrons. Measurements show dramatic reduction in noise of 600-800 W oven magnetrons operating dc near 2.45 GHz. The sidebands are eliminated ( 30 dB) and the noise within 50 MHz from the carrier is reduced by 30 dB. Noise reduction is observed at low (up to 60 mA), medium (60-200 mA) and high currents (200-300 mA), for both aged and fresh oven magnetrons. The reduction in power and in electronic efficiency is of order of 10%, in this low-noise, fast startup operation.Index Terms-High power microwaves, magnetron, microwave noise.

Noise Performance of Frequency- and Phase-Locked CW Magnetrons Operated as Current-Controlled Oscillators

“…It is of interest to consider now how different levels of ripple and changing load condition affect the locking performance. Recent work by Neculaes et al [11] has indicated that there is scope for improving the intrinsic noise performance of CW magnetrons. The techniques suggested may well allow systems to be created with a substantially more robust locking performance at the to dB injection level.…”

Rapid kinematic bunching and parametric instability in a crossed-field gap with a periodic magnetic field

“…R ECENT experiments have shown that the sidebands and the close-in noise in a microwave oven magnetron may be eliminated, at the limit of spectrum sensitivity, by the addition of an azimuthally varying axial magnetic field [1], [2]. Noise reduction for 2.45-GHz microwave oven magnetrons is an important issue, as there exists an increasing concern about signal interference for cordless phones and wireless communications systems which also operate in this unlicensed frequency band [3], [4].…”

“…Noise reduction for 2.45-GHz microwave oven magnetrons is an important issue, as there exists an increasing concern about signal interference for cordless phones and wireless communications systems which also operate in this unlicensed frequency band [3], [4]. Not only the low noise behavior was observed, but also the startup was found to be substantially hastened when the number of the perturbing magnets equals the number of electron spokes in the operating mode of the magnetron (usually the pi-mode, in which the radio frequency (RF) electric field differs by in neighboring cavities [2], [5]). We call this technique "magnetic priming" and for an -cavity magnetron operating in the pi-mode it consists of imposition of /2 azimuthal magnetic field perturbations.…”

“…They have few counterparts in the much better understood electron beam-driven devices such as the klystron, traveling wave tube, gyrotron, and free electron laser [18]. Thus, the additional embodiments associated with the azimuthal perturbations in the axial magnetic field in the low-noise magnetron [1], [2], [5], [6] prompted an analysis beginning with single particle orbits. Here, we report the findings of such a study.…”

“…In these normalized variables, the familiar cycloidal orbital equations for the case of uniform magnetic field read (2) In one cycloid, the hopping time is , the hopping distance in is also , and the maximum excursion in is 2. This maximum excursion in is roughly (but slightly larger than) the Brillouin hub height, which is typically about one third of the anode-cathode separation in a magnetron.…”