GB603119A - Improvements in or relating to electrically resonant cavities - Google Patents

Abstract

603,119. Electric resonators. PHILCO RADIO & TELEVISION CORPORATION. Oct. 24, 1945, No. 28121. Convention date, April 28, 1944. [Class 40 (viii)] [Also in Group XXXVI] In resonant cavities, undesired modes or wave patterns are eliminated by absorption in lossy material (such as semi-conductive plastic material, porcelain containing carbon, paint containing graphite, steel wool, plaster of Paris mixed with lamp black, or powdered iron with a cement binder) disposed in regions which are nodal for electric currents in the desired patterns. All wave patterns except those of type TEon where n is an integer, may be suppressed in a cylindrical resonator, Figs. 1, 2 (not shown), by locating lossy material in two annular gaps between the end plates and the cylindrical wall. In a cylindrical wave meter or echo box, Fig. 4, the effective length is determined by plates 12, 13 for which coarse and fine screw adjustments are respectively provided, and small gaps are left at 16, 17 so that energy of undesired wave patterns, viz. all except those of a TEon type, can penetrate the end cavities 18, 19 where they are absorbed by lossy material such as steel wool enclosed in a wooden box 20 or by imperfectly conductive paint 23. Wave patterns of all type except TEo2 are damped out in a resonator having cylindrical walls 9, Fig. 5, if a ring of lossy material 26 is embedded in each of the end plates 10, the ring having such a diameter that it corresponds with a nodal region of current in TEo2 patterns. For selecting patterns of the TEo3 type, two rings of appropriate diameters may be located in each end plate, the diameters being calculated in terms of Bessel functions. Larger numbers of rings may be used for selecting wave patterns belonging to higher modes. In an alternative form of resonator for selecting TEo2 patterns, Fig. 8, a circumferential gap 29 admits all TM waves to an annulus 30 of steel wool or the like, and also all TE waves whose halfwave-length is an even submultiple of the height of the cylinder. All patterns other than TEo2 are further suppressed by lossy rings 34, 35 located at a nodal region for current. On the other hand, TM waves are favoured and TE waves suppressed in a cylinder having longitudinal slots- 39, Fig. 10, leading to a jacket of lossy material 41. Alternatively, Figs. 11, 12 (not shown), strips of lossy material may be embedded in longitudinal grooves.