EMC Question of the Week: February 26, 2024

Three large spools of coaxial cable

At 1 GHz, a 1-volt input to a 25-meter cable produces a 100-mV output due to cable loss. What would the output voltage be if the cable length were 50 meters?

  1. 200 mV
  2. 100 mV
  3. 50 mV
  4. 10 mV


The correct answer is “d.” Cable attenuation is exponential with length. Decibel notation is ideally suited for expressing electrical quantities that vary exponentially, so cable attenuation is usually specified in dB-per-unit-length. In this example, the voltage is reduced by a factor of 10, which is 20 dB/(25-meters) or 0.8 dB/meter. Doubling the cable length would result in an attenuation that was twice the number of decibels. In this case, 40 dB (or a factor of 100 in voltage).   

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