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In a high-quality coaxial cable, the power drops by a factor of 10 approximately every 5 km. If the original signal power is 0.25 W (=2.5 x 10-1), how far will a signal be transmitted before the power is attenuated to 25 μW? As part of your answer, include a Table showing the signal power vs. distance in 5 km intervals. If optical fibre is used instead of the coaxial cable, briefly explain how you would expect the above calculated distance value to change. You are not required to include another Table.

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sqdancefan

Answer:

  • 20 km for 40 dB loss
  • 80 km for 40 dB loss, or 10 dB loss for 20 km

Explanation:

Here's your table of (distance, power level):

  (0 km, 250 mW), (5 km, 25 mW), (10 km, 2.5 mW),

  (15 km, 250 μW), (20 km, 25 μW)

The signal can be transmitted 20 km before being attenuated to 25 μW.

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Reportedly, the loss in fiber optic cable is about 0.5 dB/km. This compares to 10 dB/5 km = 2 dB/km for the coaxial cable. Loss in dB/km is a factor of 4 less for fiber optic cable, so the distance for the same loss would be multiplied by 4.

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