INDUCTIVE LOADING FOR TELEPHONE FACILITIES 199 



loading above described, it is of interest to note that enough program coils 

 were manufactured during the period 1928-1948 to provide loading for more 

 than 100,000 pair-miles of B22 facilities. The demand for the new B22 

 loading is rapidly declining however, in consequence of the post-war de- 

 velopment of program transmission facilities using combinations of channels 

 in non-loaded cable carrier systems. From now on, it seems probable that 

 new B22 loading will largely be limited to short extensions of existing loaded 

 facilities, and to short links between cable carrier-system terminals and the 

 points where broadcasting programs are picked up or delivered. 



13.3 15-kc Program Transmission Circuits 



The development of new loading for use on cable facilities transmitting 

 FM broadcasting material was completed early in 1948. It provides a trans- 

 mission band extending up to 15 kc and is suitable for use on circuits that 

 connect FM broadcasting transmitters with their studios, in situations where 

 its use will reduce costs as compared with the use of intermediate amplifiers. 



Two weights of 15-kc loading are available, one having a nominal im- 

 pedance equal to that of B22 loading (800 ohms), and a lighter-weight 

 loading having a nominal impedance of about 480 ohms. 



The 800-ohm loading generally uses 1 1 mh coils at spacings which give a 

 theoretical cut-off frequency of about 23 kc, and a nominal velocity of 

 about 20,000 miles per second. In 0.062 mf/mi cables the coil spacing is 

 1500 ft. and in "high-capacitance" exchange cables, it is 1100 ft., in order 

 to obtain closely the same total loading section capacitance and similar 

 values of nominal impedance and cut-off frequency. Since studio-transmitter 

 circuits usually include tandem combinations of component cables which 

 have appreciably different unit-length mutual capacitances, this loading plan 

 minimizes reflection effects which would otherwise result from impedance 

 differences at the junctions of the component cables. 



800-ohm loading is sometimes provided on program pairs in coaxial cables 

 by using 7.5 mh coils at 1000 ft. spacing. When the coaxial cables are 

 installed in 1000 ft. lengths, this loading arrangement avoids the need for 

 making the (expensive) extra cable splices that would be required if 1500 ft. 

 spacing should be used. Although 50% more program loading coils are 

 required, the small additional cost of the loading is negligible in relation to 

 the savings in cable splicing costs. This 7.5 mh, 800-ohm loading has a 

 nominal velocity of about 20,000 mi/sec, and a theoretical cut-off frequency 

 of about 34 kc. 



The 480-ohm loading uses 7.5 mh coils at spacings twice as long as those 

 for the 800-ohm loading using 11 mh coils. The theoretical cut-off is a little 

 over 19 kc and the nominal transmission velocity is about 36,000 miles per 

 second. 



The 800-ohm loading is superior in all transmission features to the 480- 



