WAVEGUIDK AS A COMMrXlC ATIOX MKOITM 



-I X 



q-Iq. 



0.1 0.2 0.4 Q6 0.8 1.0 2 4 6 8 10 



TEoi HEAT LOSS IN DECIBELS 



P'ig. 20 — Ratio of TEoi signal power (Pi) to X-mode power (P^) versus line 

 lougtli for conversion coefficients (ai^ and Oxi) equal to one-half the licat loss co- 

 efficient (fli;,). 



filters is very unlikely to be satisfactory for long distances as a communi- 

 cation medium. However, the addition of mode filters will rai.se the heat 

 loss to the inidesired waves, and the latter improves the signal to re- 

 con^'erted-power ratio directly as the ratio of heat loss in the unused 

 wave to heat loss in the signal wave. Thus, as shown on Figure 21, a 

 heat loss ratio on the order of 500 would produce a ratio of signal ])ow(>r 

 to reconverted wave power on the order of 20 db at a transmission line 

 length of 60 db. It may be shown that the magnitude of the signal to 

 reconverted wave power varies as the square of the conversion to heat 

 loss coefficient ratio aix/fli/j . 



The sharp break downward on the right hand end of the curves for 

 CLjh/a^h — 2 in Figs. 20 and 21 represents the condition wherein the 

 power in the reconverted wave becomes comparaljle to the power re- 

 maining in the signal wave. 



We next consider the improvement in signal fidelity which results 

 from the introduction of ideal mode filters. We shall assume the ideal 

 mode filters have a matched impedance for all modes, very high trans- 

 mission loss to the unused modes, and no transmission loss for the signal 

 mode. The impro\em(Mit in tlu^ ratio of signal power to reconverted wave 

 power due to the addition of such filters is shown in Fig. 22. This plot 

 has been calculated for a total line length of 20 db heat loss, but the 

 conclusions are valid for any line length wherein the signal wave power 

 remains appreciably larger than the reconverted waxc ))()\vcr. We ob- 

 served that mode filters improve the signal-to-recon\-ertt'd-\va\'e powers 

 very slowly when placed far apart, typical improvements ranging be- 



