WAVEGUIDE AS A COMMUNICATION MEDIUM 1227 



pulse sliape was preserved for as manj' as 40 miles of travel (Fig. 13), to 

 a condition wherein the shape of the pulse was l)adly distoi-ted after only 

 3 or 1 miles of tra\'el. This general beluiN'ior is illustiatcd liy (he ))hoto- 

 gra])hs shown in Fig. 15. We will concentrate for the moment on tiic top 

 two rows of photographs which record tiie pulse transmission in the l)are 

 waveguide as a function of distance of pulse travel for both favorable and 

 unfavorable piston settings. However, all of the rows of the })hotographs 

 were taken under such conditions as to permit direct comparison. 



The photographs at the extreme left end represent the outgoing pulse 

 and the first echo pulses which travelled one round trip in the line, ap- 

 proximately 340 yards. All the other photographs show two principal 

 pulses which record two successive trips of the pulse as it passed the 

 transmitting end (Fig. 10). The second photograph from the l(^ft re])re- 

 sents the pulse as it passed the sending end after 10 and 1 1 round trips. 

 The third picture from the left records the 20th and 21st trip, the fourth 

 picture the 30th and 31st trip, etc. The numbers placed directly beneath 

 the individual photographs represent the relative sensitix'ity of the re- 

 ceiver for that particular photograph. Reference receiver sensitivity was 

 taken as the condition under which the 10th and 11th trip in the bare 

 waveguide were recorded with a favorable piston setting (0 db beneath 

 the photograph), and the designation — 6 db under the adjacent photo- 

 graph indicates that 6 db more receiver sensitivity was used in the lattei' 

 case. The relation between display amplitude and actual pulse ampli- 

 tude was approximately square law. The distance of pulse travel asso- 

 ciated with each of the pictures is given at the bottom of the figure. 



Comparing the top two rows representing favorable and unfavorable 

 piston positions, we note that the attenuation was appreciably differ- 

 ent ■ — the values being 2.6 db per mile and 3.1 db per mile respectively. 

 Serious distortion of the transmitted pulse also occurred for the un- 

 favorable piston setting. Since the receiver in the experiment was sensi- 

 tive to very many modes, one might suspect that the spurious i)ulses 

 which appeared at more than 7,000 yards with the unfa^'orabl(> piston 

 setting might represent energy present in some of the other modes. 

 Actually, all of the pulses and wiggles shown in the photographs at 

 ranges greater than 3,500 yards were in the circular electric mode. This 

 was deduced by first noting that every two successive pulses were not 

 appreciably different from each other (see Fig. 15). If some of the dis- 

 tortion effects shown by the received i)ul.se were due to energy l)eing 

 received in modes other than the signal mode, then successive pul.ses 

 would be different in shape because the \arious modes have different 

 phase constants. The very gradual change in pulse shape which did 



