between the broadwall to the nearest channel 30 and is also equal to the spacing between the periphery of the face 28 and the outer channel each of these spacings being equal to one quarter wavelength of the midband radiation the depth of each channel 30 is also equal to one quarter wavelengths at the midband radiation the width of a channel 30 is less than approximately one tenth of the midband wavelength the spacing between the two flange assemblies 24 and 26 at their closest point of approach is 0 01 inches in the preferred embodiment of the invention this being less than the aforementioned one twentieth of the midband wavelength this spacing is maintained by a gimbal mounting 36 which will now be described with reference to fig3 5 as shown in fig3 5 the gimbal mounting 36 extends radially outward in a central transverse plane of the joint 16 and encircles each of the flange assemblies 24 and 26 the mounting 36 permits pivoting in two orthogonal planes corresponding to first and second pivot axes of pivots 38 and 40 the mounting 36 comprises a circular ring 42 which supports the pivots 38 and 40 a pair of struts 44 extend radially outward from the flange assembly 26 and then curve towards the central transverse plane of the joint 16 to engage with the pivots 38 all of the.