TRANSMISSION LINES FOR RADIO SYSTEAfS 443 



nearly all traced to faulty construction at the braced support which 

 clamps the lock joint. 



("opper pipe lines may be too costly to permit the installation of 

 more than one line per transmitter. In such cases a selector switch is 

 required if several antennas are to be associated with one transmitter. 

 Some of the details of such a switch may be obtained from the experi- 

 mental arrangement shown in P'ig.20. 



The switch is an irregularity on the line and a source of undesired 

 reflections. This difficulty may be corrected by making the design such 

 that capacitive reactance of the switch predominates and then anti- 

 resonating this reactance with a suitable inductance. This scheme is 

 effective provided the irregularity is not too great. In the latter event 

 the corrective coil transforms the load impedance to a value different 

 from the surge impedance so that the reflections arising from the mis- 

 match are more serious than from the switch alone. 



VH. Other Applications of Transmission Lines 



In this section are described a number of transmission line applica- 

 tions to radio work some of which are feasible only at high frequencies 

 because the wave-length is short. 



Small concentric lines approximately 3/8-inch in diameter may be 

 employed as radio-frequency wiring in radio stations. Such lines 

 owing to the flexibility of the tubing may be snaked behind partitions 

 in very much the same manner that armored or leaded conductors are 

 installed. For this purpose refrigerator tubing has been found desir- 

 able because it is flexible and because it may be procured in long lengths. 

 The inner conductor is insulated from the sheath by means of small 

 porcelain beads spaced at intervals of approximately one inch. The 

 beads are held in position by small metal ears extruded from the inner 

 conductor. The beads fit loosely in the inner conductor so that the 

 line may be bent into arcs as small as six inches radius. Construction 

 details for small concentric lines may be obtained from Fig. 21. 



Lines constructed from refrigerator tubing may be buried in the 

 ground. Since only a few splices are necessary the possibility of faults 

 arising from water seeping into the line are correspondingly small. A 

 buried line constructed in this manner has been in service for more than 

 a year without developing faults. 



A number of the above-described lines may be terminated upon a 

 jack board and circuits set up with patch cords as in telephone prac- 

 tice. Of course, the beads in the patch cords may be more closely 

 spaced to assure flexibility and freedom from short circuits. The 



