OPEN-WIRE TELEPHONE LINES 



749 



total distance of about 2100 miles. The highest frequency employed 

 in this system is approximately 28,000 cycles. Using the attenuation 

 values for a 165-mil pair at 28,000 cycles, it appears that the dry 

 weather attenuation for the entire length of this system might be 

 approximately 220 db or less, and the wet weather attenuation about 

 330 db. This means that without amplification along the line the ratio 

 of the transmitted power to the received power might vary from 10^^ to 

 10^^. Since the attenuation of a repeater section is ordinarily limited to 

 from 25 to 40 db, ten repeaters are employed to span the total distance, 

 and in order to compensate for the attenuation variations a gain regu- 

 lating mechanism known as a pilot channel must be used. 



20 



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15 



10 



12 



Fig. 13 — Measured variations in the attenuation of an open-wire pair. 



12 



In the preceding illustration it was assumed that the range of varia- 

 tion in attenuation increases in direct proportion to the length of the 

 circuit. Although this may theoretically be possible, it has been found 

 in practise that the attenuation variations during any given period of 

 time increase less rapidly than the circuit length. The reason for this 

 is that augmenting the length of the circuit obviously reduces the likeli- 

 hood of experiencing extreme wet weather conditions simultaneously 

 over the entire line. A practical example of how the open-wire atten- 

 uation varies from time to time is afforded by Fig. 13, which shows the 

 measured attenuation changes on a line 110 miles long during the period 

 of two light rainstorms. 



