GB1392452A - Waveguides - Google Patents

Abstract

1392452 Coaxial waveguides NATIONAL RESEARCH DEVELOPMENT CORP 19 July 1972 [2 Aug 1971 20 Dec 1971] 36203/71 and 58951/71 Heading H1W Electromagnetic waves are transmitted in the dipole mode in the dielectric between co-axial cylindrical surfaces of two guiding members. These modes are designated EH n and HE n and the field pattern of the EH 1 mode is shown (Fig. 1). In this mode, the electric field follows a single diameter (n = 1) and has the same direction on both sides of the inner member 10, in contrast to the symmetrical E 0 , H 0 modes where the fields are opposite. Where n > 1, there are n diameters each containing an electric field vector. There is no clear distinction between the EH n and HE n modes which both have E and H fields in the direction of propagation. It is shown in the Specification that these modes may be propagated between co-axial members 10, 11 with low attenuation, low dispersion and without frequency cut-off. The Specification derives the conditions to be satisfied by a co-axial structure required to propagate waves of a given frequency. In particular, these conditions can be satisfied if (a) the longitudinal impedances of the inner and outer surfaces are resistive and inductive, and the transverse impedances are resistive and capacitive, or if (b) the longitudinal impedances are resistive and capacitive and the transverse impedances resistive and inductive. These conditions are summarized by where Z<SP>2</SP> 0 = Á/# is the characteristic impedance of the waveguide. Case (a) is satisfied if the inner 10 is a dielectric rod, e.g. of polythene, polystyrene, polyethylene or P.T.F.E., a dielectric tube, or a conductive rod with a thick layer of dielectric material. Transverse capacitance can be obtained by using a number of spaced longitudinal conductive members 14, Fig. 2d, (at least three per wavelength) which may be mounted on a dielectic core 15, or by longitudinal grooves 16 in a conductive inner 10, or by radial fins 18 extending from a conductive core 17 into a thick dielectric layer 19. The required impedance conditions for the outer member 11 are achieved by similar methods, e.g. Fig. 3a shows the method of Fig. 2d applied to the outer member 11. In a further method, applicable to both inner and outer members, a number of longitudinal wires are embedddd just below the surface of a dielectric tube. The wires may be skewed into a helix of long pitch. The outer surface of the outer member may be covered by a dielectric braid. The ratio of the radii of the inner and outer surfaces r 1 /r 2 may have any values satisfying the equation of general conditions given in the Specification, but a convenient practical limit is 20 : 1. The Specification gives some detailed examples of waveguides including dimensions and materials. The dipole wave may be lauched into the co-axial guide from a waveguide or cavity supporting the H 01 or H 11 mode, the co-axial structure being coupled to an annular aperture in the centre of a transverse wall of the waveguide or cavity (Fig. 4a, to 4d not shown). Alternatively, the end face of the co-axial guide may penetrate a waveguide or cavity having an end face which is left open. In the arrangement of Fig. 4e the wave is launched from a two-conductor transmission line 35 supporting a TEM mode. The co-axial waveguide has a tapered inner 10 and an outer 11 composed of conductive strips (see Fig. 3a). It penetrates the end of the TEM line 35 and is connected thereto by adjustable short-circuiting members 36 which may be used it provide impedance matching. The dimensions of the co-axial waveguide are such that it is below cut-off for unwanted waveguide modes dependent upon transverse resonance.