Measuring RF transmitter output watts

Discussion in 'Wireless & RF Design' started by doug08, May 24, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    How do you do measure a transmitters output without having a Power/SWR meter? Also, if you have a small transmitter, can you measure the voltage of the antenna output to arrive at the power rating? I tested a small FM transmitter and got 3.5mv for the antenna output. How about connecting an ammeter and measuring the current draw?

    Thanks.
     
  2. curry87

    Member

    May 30, 2010
    101
    0
    Been wondering this myself, anyone know the answer ?
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    I always use an oscilloscope with a 50 Ohms input.
    You measure the peak to peak voltage.
    You can calculate the power with the formula (Vpp X Vpp) / ( 8 X RL)
    Where RL is the input impedance aka 50 Ohms.

    Bertus
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,039
    287
    Measuring voltage at the antenna is fine for RELATIVE power measurement....if you just want to tune for maximum output. For actual power measurement, you need to know the load impedance precisely, then you can use E^2/R. If you use an oscilloscope, you have to be sure the scope's bandwidth is comfortably higher than your frequency of operation.

    Eric
     
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