TRANSMISSION LINES FOR RADIO SYSTEMS 



441 



to the heat insulation of the air space between the conductors. Small 

 lines laid in a sinuous manner have been found remarkably free from 

 mechanical breakdowns brought about by temperature variations of 

 length. 



Sliding joints may be employed to accommodate variations in line 

 length brought about by temperature changes. It is very difificult to 

 make such joints water-tight without recourse to expensive fittings. 

 There is also the possibility of microphonic contacts which are particu- 

 larly objectionable in receiving work. 



The expansion joints shown to the right of V'lg. 19 have been em- 



Fig. 19 — Experimental expansion and lock joints for large sizes of concentric tube line. 



ployed with some success. Dimensions and shapes should be chosen 

 to minimize the irregularities in line impedance caused by expansion 

 joints. It is a step in the right direction to maintain constant the ratio 

 of conductor diameters at the joint. Even then, it has been found that 

 the irregularities caused by 10 such joints in a 600-foot line are observ- 

 able (approximately 10 per cent standing waves). 



It is necessary that expansion joints be employed in conjunction 

 with lock joints so arranged that no one joint is required to take more 

 than a predetermined portion of the line expansion. One lock joint 

 with an expansion joint 25 feet in either direction has been found to be 



