88 TRANSMISSION LINE FORMULAS 



Therefore, 



DB* + DB p - DB s = DB . P - p 2 

 or DB 2 - DB s + p 2 = o. 



Solving this quadratic equation in DB, we have 



[The negative value of the radical must not be used, 

 since it would give the value of DA instead of DB.] 

 Therefore, 



p ' 4p* 

 which may be expressed as 



c- ^ 77 - v 



4 cosh" 1 (- L ) 



\2p/ 



or it may be expanded by the Binomial Theorem to give 

 the approximate value 



T 



per centimeter, 



or, very nearly, 



p 



* It is evident that the expression 

 C = 



which is sometimes published, is less accurate than the simpler expression 



C--L- 



4 log,- 



