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Consider that we have a 50-ohm source generator that is connected to a 300 ohm transmission line. This line has a loss of 10 dB for every 100 ft and is connected to a 50 ohm receiver.

The source indicates an output of -30 dBm and we need to find the input voltage to the receiver in dB(uV).

My steps are as follows :

  • First I have converted the source output power (-30 dBm) to the corresponding voltage in absolute value to be equal to 7.07 mV.
  • Then I have converted this value to dB (uv) to be equal to 77 dB(uV).
  • Finally I have calculated the input voltage to the receiver as V = V source (dB (uv)) - loss (dB) = 77 -10 =67 dB (uV) .

However, the answer is 71.7 dB(uV). So what is wrong with my steps?

Edit : This problem is taken from Introduction to electromagnetic compatibility by C.paul.

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  • \$\begingroup\$ You're omitting what you did to come up with your loss number. \$\endgroup\$ Mar 1, 2019 at 21:24
  • \$\begingroup\$ It looks like you aren't accounting for mismatch between the transmission line and the source and load. \$\endgroup\$
    – The Photon
    Mar 1, 2019 at 22:58

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