II yPRR- FREQUENCY WAVE GUIDES 



295 



Figure 6 is based on calculations covering the case of a hollow 

 conductor (air dielectric) propagating the Ih type of wave. The 

 experimental data for plotting points on Fig. 6 were obtained at 

 frequencies extending from 1500 mc. (X = 20 cm.) to 2000 mc. (X = 15 

 cm.) on hollow cylinders ranging in diameter from four inches to six 

 inches. Relative velocity was determined from the length of standing 

 waves set up in short sections of these wave guides. The measure- 

 ments represented were made with much more refined apparatus than 

 utilized in obtaining the data for Figs. 4 and 5. 



z 



>- 04 



0.4 0.6 0.8 1.0 1.2 



RATIO OF WAVELENGTH TO DIAMETER 



(I) 



Fig. 6 — \'elocity ratio for the Hi type of wave in a hollow metal pipe. 



A ttenuation 



Figure 7 shows in graphical form the calculated attenuations suffered 

 by each of the four more common types of waves when traveling 

 through a hollow copper pipe 5 inches in diameter. It is immediately 

 obvious that the attenuation is infinite for all waves at their respective 

 cut-off frequencies. How^ever, at frequencies above the cut-off this 

 attenuation becomes finite, generally descending to values comparable 

 with attenuations experienced on ordinary conductors at considerably 

 lower frequencies. For the £o and £i types of waves the attenuation 

 falls from infinity at cut-off to a minimum at a frequency ^3 times 

 the cut-off frequency after which it again begins to increase and 

 ultimately varies in a linear fashion much as does attenuation over 

 ordinary conductors. For the Hi type of wave this minimum comes 

 at a frequency 3.15 V3 times the cut-off frequency. Thus we see that 



