IlYPEli-riiliQUENCY WAVE CUIDES 285 



set by the properties of available materials. These have for con- 

 venience been called hyper-frequencies. When these electromagnetic 

 waves are propagated through either of the two forms of guide men- 

 tioned above that incorporates a metal sheath, there is little or no 

 external field and consequently little or no interference from static 

 or other extraneous noises. 



As already mentioned there is no return conductor, at least of the 

 kind with which we are generally familiar in ordinary wire or coaxial 

 cable transmission. Corresponding to this difference in physical struc- 

 ture there are striking differences in the character of the waves prop- 

 agated. On the other hand, when we compare this transmission with 

 radio, where there might at first sight appear to be great similarity, 

 we find little or no correspondence, for it turns out that as regards 

 both the velocity of propagation and attenuation per unit length, 

 radio and wave guides follow quite different laws. 



In answer to the natural question as to what practical use there 

 may be for transmission methods of this type the following considera- 

 tions may be of interest: The size of structure that may be used as 

 a guide is directly proportional to the wave-length. It happens that 

 in structures that are at all convenient in size, the necessary frequencies 

 correspond approximately to the highest range now being tried out 

 in radio. If the size of structure is further reduced to make it more 

 economical for use for long distance transmission, it is then neces- 

 sary to use frequencies above this range. Thus far these can be 

 produced and handled only with serious difficulty. Although it is 

 possible to reduce the size of the guiding structure for a given fre- 

 quency by the use of a suitable dielectric we are met with a conflicting 

 difficulty of producing at reasonable cost the necessary medium that 

 will incorporate high dielectric constant with sufficiently low losses. 

 The situation then is that the art at these extreme frequencies is not 

 yet at a point which permits a satisfactory evaluation of practical use. 

 However, for short distance transmission or for use as antennas or 

 projectors of radio waves or for selective elements analogous in nature 

 to the tuning elements so commonly used in radio, there are not the 

 same economic conditions limiting the size of structure. For such 

 uses, then, structures of this type deserve serious consideration. 



Theory indicates that one of the four types of waves (designated 

 below as //o) has progressively less attenuation as its frequency is 

 increased. It happens, however, that this type requires for a given 

 guide a higher range of frequencies than any others. This puts it, 

 therefore, in a frequency range where the art is even less developed 

 than for the other types of transmission and where it is even more 



