Backward Waves in Longitudinally Magnetized Ferrite Rods

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Abstract
A quasi‐static approximation (▿×H=0) is used to investigate the backward waves that propagate along a longitudinally magnetized ferrite rod. These waves propagate in the frequency range where the perme‐ability of the ferrite is negative, γH0<ω<γH0[1 + (M0/μ0H0)]½, and can have a phase velocity much smaller than the velocity of light. The quasi‐static approximation is shown to be valid whenever the free‐space wavelength is greater than the circumference of the ferrite rod. Traveling‐wave interaction of a drifting electron stream with these backward waves is considered, and a start‐oscillation length of 1 in. at 4000 Mc is shown to be theoretically possible for a 600‐v, 0.4‐ma electron beam. Experimental verification of these waves is provided by measuring the phase velocity and signal strength along a longitudinally magnetized ferrite rod that completely fills a waveguide. Phase velocities as small as one one‐hundredth of the velocity of light are measured.