1242 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



tween 2 and 8 db for 1 db reparation between mode filters. The larger 

 improvement is obtained when the heat loss in the undesired mode is 

 more nearly comparable to the heat loss in the signal mode before the 

 addition of the mode filter. For very small spacing between the mode 

 filters, addition of the ideal mode filters improves the signal-to-recon- 

 verted wave power by very large factors. 



A somewhat different form of effect due to the addition of mode filters 

 is observed if the line before the introduction of the mode filter has a 

 reconverted wave power larger than the signal power. Under this con- 

 dition, relatively large mode-filter spacings bring about a large improve- 

 ment in signal-to-reconverted wave power, but this is due to the very 

 poor condition present before filtering. It is doubtful whether the trans- 

 mission line would become very useful without rather strong mode 

 filtering of the type represented in Figure 22. 



We may conclude that the mode filters should be placed very close 

 together, preferably at spacings of less than .1 db heat loss to the signal 

 wave. We may also conclude that the transmission of signals over dis- 

 tances corresponding to the order of (50 db heat loss will require either 



oTltf 



0.4 0.6 0.8 1.0 2 4 6 8 10 



TEo, HEAT LOSS IN DECIBELS 



40 60 100 



Fig. 21 — Ratio of TEoi signal power (Pi) to reconverted TEoi power (P„) 

 versus line length for aix = 0.5 au . 



