WAVEGUIDE AS A COMMUNICATION' MKDIUM 1259 



Another conclusion the writer has reached is that sonic form of signal 

 rt'^cncration is hkely to be reciuirod at each repeater, and that tlie 

 nio(hilation process should be selected in such a way as to permit this 

 regeneration with limited comiilexity. 



CONCLUSION 



Single-mode waveguide is unattractive as a long-distance communi- 

 cation medium due to limited bandwidth and either unreasonable size 

 or excessive loss. 



The circular-electric mode in a '1" diameter round pip(^ has a theoreti- 

 cal attenuation coefficient of 2 db per mile for carrier frequencies near 

 50,000 mc. Delay distortion due to waveguide cut-off will require ecjual- 

 ization if baseband widths on the order of 500 mc are to be provided on 

 repeater spacings of 25 miles. Using fre(]uency-division multiplex, such 

 a waveguide might be exploited over the 40,000-mc band from 35,000 to 

 75,000 mc, for which the theoretical attenuation coefficients are 3 db/ 

 mile and 1 db/mile respectively. 



Transmission experiments were conducted at 9,000 mc in a 500-foot 

 copper pipe 4.73" I.D. for \vhich the theoretical circular-electric wave 

 loss was 1.9 db/mile. Under favorable conditions the observed losses 

 were within 25 per cent of the theoretical value; under unfavorable con- 

 ditions (which are not likely to occur in practice) the observed losses 

 were as high as 75 per cent greater than the theoretical value. Surface 

 roughness accounted for losses about 20 per cent above the theoretical 

 \'alue, and the remaining excess losses were due to conversion of energy 

 to other modes of propagation. Direct observation of the power trans- 

 ferred between modes in the 500-foot line confirmed the latter conclusion. 



Other observations in the 500-foot line showed that the mode con- 

 version process produced a signal-distortion or signal-crosstalk effect 

 through reconversion of energy from the unused modes of pro])agation 

 back to the signal (circular electric) mode. This type of interference seri- 

 ously limits the capabilities of bare copper pipe for use as a long-distance 

 communication medium. However, it has been shown experimentally 

 and theoretically that the effects of the conversion-recon\-ersion process 

 are greatly reduced through the addition of mode filters, which absorb 

 energy present in the unused modes of i)ropagation. 



The combination of solid pipe plus mode filters remains attractive as 

 a communication medium. Average losses for the unused modes of 

 propagation should be on the order of 500 times the lo.ss to the signal 

 wave, in order that the conversion-reconversion effects be tolerable in 



