Backward Waves in Longitudinally Magnetized Ferrite Rods

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.