

1118 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



inside wall. To avoid an offset at the joint it is therefore important that 

 the thread be centered relative to the inside diameter. After a section 

 was threaded the ends were faced off to make the ends square and thus 

 avoid any tilt between sections when the ends are butted together. 



Of the two sizes tested the smaller diameter (Ke" I-D. X Ke" wall) 

 was chosen to provide a moderate line loss, while limiting the number 

 of propagating modes. In the band of interest (5.2-5.7 mm) the theoreti- 

 cal TEoi wave attenuation is about 4 db/100 ft. The number of modes 

 which can be supported at X = 5.2 mm is limited to 12 modes and to 

 only one of the circular electric modes. The higher order TEon modes are 

 beyond cut-off. These features limit the number of spurious modes and 

 simplify the mode filtering problem. Furthermore, in this smaller sized 

 waveguide, the associated components which may set up TEon waves, 

 for example conical tapers, need not be as long proportionately as in 

 larger waveguides. The %6" I-I^- guide has the advantage of smaller 

 size, lower cost and greater ease of transmitting TEoi through specially 

 constructed bends. The attenuation of this smaller diameter guide is 

 large enough that system requirements will usually restrict its usage to 

 lengths of line of a hundred feet or so. 



The larger size (J^'' I.D. X 3^" wall) is exactly twice the diameter of 

 the small size discussed in the preceding paragraph but has only one- 

 tenth the attenuation, or about 0.4 db/100 ft. The low loss of this larger 

 size becomes more attractive for runs as long as several hundred feet. 

 This diameter guide will, of course, support more modes, 50 at X = 5.2 

 mm; four of which are circular electric modes — • TEoi , TE02 , TE03 

 and TE04 . Some of the disadvantages which accompany the increased 

 diameter are: (1) greater care must be taken as to line straightness, (2) 

 longer conical tapers are required when converting from one guide diam- 

 eter to another, and (3) longer mode filters are required since the desired 

 mode-filtering attenuations vary inversely with the filter diameter at a 

 given frequency. Flexible spaced-disk lines employed as uniform bends 

 for TEoi transmission require much greater bending radii than bends in 

 the smaller diameter guide if the bend loss is to be kept proportionately 

 low. This problem is considered in some detail in another paper.^ With 

 reasonable care the accumulative effect of these foregoing factors can 

 be held to a reasonably low value. Expressed in terms of the ratio of 

 measured to theoretical attenuation the values are, on the average, 

 about 10 per cent higher in the %" I.D. waveguide than in the J4.6" 

 I.D. waveguide. 



A. P. King, forthcoming paper on bends. 



