Patent GB-687203-A (GB)
Title: Travelling wave tube
IPC: H01J23/24
Filed: 1949-10-28
Abstract: 687,203. Travelling-wave tubes. STANDARD TELEPHONES & CABLES, Ltd. Oct. 28, 1949 [Nov. 4, 1948], No. 27722/48. Class 39(i) [Also in Group XL(b)] In a travelling wave amplifier the wave transmission path is a coaxial line 6, 7, Fig. 1, loaded with baffles attached to the inner and/or outer conductor, and an annular electron beam passes through the annular space between the conductors. The outer conductor 6 is formed with a plurality of longitudinal slots 32 to suppress angle dependent' modes which require circumferential currents in the outer conductor. The operating frequency band is preferably in the region of anti-resonance of the slots but this is not necessary provided that the gap between the baffle edges and the outer conductor is small compared with Ag at the antiresonant frequency. The input and output coupling to coaxial lines 20, 22 is made by impedance matching slugs 23 and spider rings 19 with apertures for the annular beam from indirectly heated cathode. 2. A magnetic coil 17 focuses the beam to' travel preferably as close to the baffle edges as possible where the axial field is strongest or the beam may fill the entire annular space. The spacing between the baffles 8 is approximately equal to the spacing between the outer edges of the baffles and the outer conductor 6, and both spacings are very small compared with #g at any desired frequency of operation. By using a small gap between the edges of the baffles and the outer conductor the band width of the device is increased i.e. there is less variation of phase velocity for a given change in frequency. The space occupied by one baffle and one gap may be equal to #g/4 where #g is the wave-length in the coaxial line. In a modification, Fig. 4, the coupling is by internally tapered ring 33 with projections 34 contacting the outer conductor 6 and spaced apart to allow the beam to pass through. The baffles 8 attached to the inner conductor 7 are tapered correspondingly. The tapering is such as to make the impedance change between successive baffles a constant. The longitudinal slots 35 are closed by wall parts 36 and are about #g/4 in depth so as to be anti-resonant. They suppress angle dependent' modes as in Fig. 1. In another modification, Fig. 6 (not shown), the arrangement is similar to that of Fig. 4 except that the baffles are attached to the outer conductor and the input coaxial line extends normal to the axis of the tube and is coupled to the coaxial line inside the tube #/4 from its adjacent end. The latter end is provided with a grid which forms a short-circuit and also allows passage of the beam. In another modification, Fig. 8 (not shown), baffles are provided on both the inner and the outer conductors. The spacing between the two sets of baffles is small compared to the diameter of the baffles on the inner conductor. In another modification, Fig. 10 (not shown), the two sets of baffles are in staggered relation. The devices described may be made frequency selective by adjusting the D.C. accelerating voltage on the coaxial line to give a beam velocity equal to a phase velocity in a dispersive region.