- http://www.amazon.com/gp/product/0521646456/
- at Google Book Search
- Page 82:
- As an historical example, we can analyze the first transatlantic telegraph cable, laid in 1865. The cable was 3,600 km long and weighed 5,000 tons. The insulator was a vegetable gum called gutta-percha . For this cable L = 460 nH/m, C = 75 pF/m, and R = 7 mQ/m. At a frequency of 2.4 kHz, wL = R, and so the high-resistance assumption is well satisfied for frequencies below 100 Hz. At 12 Hz, we can write a and v as
- a = v/wRC/2 = 4.4 x 10...
- v = ...
- The loss for the entire line is al = 140 dB and the delay is l/v = 210 ms. For comparison, at 3 Hz, the loss in dB and the delay change by a factor of 2, to 70 dB and 420 ms. Thus the 12-Hz component attenuates 70 dB more than the 3-Hz component. In addition, the 12-Hz component arrives 210 ms ahead of the 3-Hz component. In order to improve these characteristics, the signalling speed had to be drastically reduced, to about one word per minute, which was twenty times slower ...
Saturday, July 12, 2008
Telegraphist's Equations
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