1244 THE BELL SYSTEM TECHXICAL JOURXAL, NOVEMBER 1954 



being (in an extreme case) 25 db greater for TE91 than for TE13 in the 

 5'' pipe. For the majority of modes, howe\'er, the latter variation is ±3 

 db, and the maximum probe response as a function of curcumferential 

 and (to a hmited extent) longitudinal position can be determined with- 

 out excessive labor. 



The probe technique of mode conversion evaluation Avas first applied 

 by M. Aronoff to the individual sections of the h" diameter experimental 

 line. He found that the average indication of conversion for the (ap- 

 proximately) 20 ft. lengths of pipe was 29.5 db below the signal wave 

 power; since the power loss due to dissipation in the walls for a 20 ft. 

 section is about 27 db below the signal wave power, the individual pipe 

 measurements gave an order-of -magnitude estimate of 0.55 for the ratio 

 of conversion loss to heat loss (aixA^ift)- Four mechanically distorted 

 sections of line, previously considered satisfactory, were identified and 

 discarded on the basis of this approach. 



A. C. Beck and M. Aronoff next applied the probe technique to the 

 5" diameter line assembled into lengths of 145 feet, 270 feet, and 500 

 feet. The indications of converted power were —17 db, —10.5 db, and 

 — 13 db respectively (at wavelengths near 3.2 cm) which is compatible 

 with the hj^pothesis of random addition of a number of conversion com- 

 ponents. Since the heat-loss power for the 500-foot line is about — 13 db 

 compared to the incident signal power, the 500-foot line radial probe 

 measurement gave an order of magnitude estimate of 1.0 for Oix/ai/, , in 

 fair agreement with the value of 0.55 from single pipe measurements. 

 The probe indication of conversion as a function of frequencj^ for the 

 500-foot hne is plotted in Fig. 23, which shows that quite a number of 



3.04 3.08 3.12 3.16 3.20 3.24 3.28 3.32 3.36 3.40 3.44 3.48 3.52 3.55 

 WAVELENGTH, Aqj '^ CENTIMETERS 



Fig. 23 — Probe recording of converted power in the 500-foot line. 



