1260 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



the presence of conversion losses of the magnitude observed in the 500- 

 foot experimental line. 



Improved forms of circular-electric waveguide have been demon- 

 strated. These waveguides have the common property of providing very- 

 large attenuation coefficients for the unused modes of j^ropagation while 

 mamtaining essentially the same circular-electric wave attenuation as 

 in solid pipe. Thus, these guides are essentially continuous mode filters. 

 Structurally, such a medium may consist of a series of metallic rings 

 supported in a lossy housing (Fig. 26), or may consist of a helix supported 

 in similar manner. The helical circular-electric waveguide appears more 

 attractive from the standpoint of ease of fabrication. 



Where bends must be sharp, they may be negotiated in a number of 

 special ways. Gradual bends may be introduced in the improved forms 

 of circular-electric waveguide with modest increase in heat loss. For the 

 2" diameter guide at 48,000 mc, it is estimated that a bending radius of 

 about 2,000 feet would double the heat loss of a helical or spaced-ring 

 waveguide. For a 1" diameter helical or spaced-ring guide (theoretical 

 heat loss of 15 db/mile in solid pipe) the corresponding bending radius 

 is about 200 feet. The extra heat loss due to bending varies inversely as 

 the square of the bending radius. 



The type of modulation to be used in a waveguide system will prob- 

 alily be dominated by conversion-reconversion effects. An upper limit on 

 the time delay between the signal component and the associated recon- 

 verted-wave components lies in the range 1 to 100 millimicroseconds for 

 2" pipe at 48,000 mc without mode filters; the addition of mode filters or 

 use of the helical circular-electric guide should reduce these time delays 

 by factors of 100 or more. Thus, it appears likely that basebands on the 

 order of 500 mc or more should be usable. 



It is concluded that a waveguide signalling method must be capable 

 of withstanding large amounts of signal interference, and that some 

 form of regeneration is likely to be required at each repeater. 



ACKNOWLEDCiMENT 



The encouragement of R. Bown, H. T. Friis, and J. C. Schelleng is 

 gratefully acknowledged. The contributions of numerous colleagues (as 

 noted throughout the paper) form the building blocks without which the 

 present understanding of waveguide transmission could not have been 

 obtained. 



