Longitudinal Corrugations Research Articles

A method of changing the dominant mode in a hollow metal waveguide and its application to bends

It is the usual practice to use, where possible, the dominant mode in a waveguide, because, by making the cross-section of the tube just sufficient to support that mode, all others can be effectively excluded. For some applications the normal dominant mode is unsuitable, and the paper describes arrangements by which another desired mode can be brought to the dominant role. Thus the H01 mode in a circular-section guide can probably be made effectively dominant by employing an anisotropic surface impedance, yielding an inductive reactance to the H modes and a capacitive reactance to the E modes. Advantage is also taken of the conversion of all H modes with the exception of the H0n family to corresponding E modes as a result of longitudinal current in the wall of the guide. The net result of this technique is to make the guide effectively larger for the H modes and smaller for the E modes, while virtually suppressing all H modes requiring for their support longitudinal current. The necessary anisotropic surface impedance is provided by transverse and longitudinal corrugations or by coating the metal with a thin layer of dielectric.

A Stag-horn Head from Crete

The curious head which is illustrated, in actual size, on Plate VI., was bought by my colleague, Captain F. N. Pryce, and me from a well-known Greek dealer at Cairo in December 1918, and is now in the British Museum. It is carved in the beam of a stag's antler, the natural burr or coronet of the horn representing either a crown or curled, upstanding hair, while the longitudinal corrugations imitate hanging tresses. The smooth, round base of the shed antler very aptly resembles the top of a man's head (Fig. 1). All these features are unworked. The rest of the horn is carved in the shape of a human face wearing a full beard and turned-up moustaches. Across the forehead is a heavy ridged moulding, which runs into the edge of the beard on each side of the face. Whether this moulding represents the band of a headdress, or a ceremonial fillet, or the rim of a crown, or is simply a decorative device to help the transition from the projecting hair to the receding face, it is not possible to decide, for its details will not bear strict interpretation. The hair of eyebrows, moustache and beard is marked with close striations.