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Homework Help: Microwave optics

  1. Apr 28, 2013 #1
    1. The problem statement, all variables and given/known data

    We're working through the first Microwave Optics lab described in the following PASCO Scientific manual:

    We're using a slightly modified version of this setup, with a voltmeter hooked up to the receiver.
    As the receiver is moved away from the transmitter in 2 cm increments, the meter reading is recorded.

    We want to demonstrate that the intensity is proportional to the inverse square of the distance between the transmitter and receiver.

    What is the relationship between intensity and our voltage reading at each position?
    Can we reasonable assume that the intensity is proportional to the square of the voltage?

    2. Relevant equations

    3. The attempt at a solution

    E field is proportional to Voltage.
    Intensity is proportional to square of E field.
    Therefore, Intensity is proportional to square of Voltage reading.

    Have we gone off in a wrong direction?
  2. jcsd
  3. Apr 28, 2013 #2


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    Your question is answered in the PASCO manual in the section describing the microwave receiver (p. 2 of manual, or p. 6 of pdf file). See the first sentence there.
  4. Apr 28, 2013 #3
    Right, I did read that. Unfortunately we seem to be using an older version of the apparatus. Our receiver didn't actually have a meter on it. We hooked it up to a voltmeter to take measurements.
  5. Apr 28, 2013 #4
    The university-compiled manual I'm working with states the following:
    At small amplitudes the diode voltage is approximately proportional to microwave intensity; this, in turn, is proportional to the square of the E-field amplitude.

    But this follows:
    At higher levels the diode voltage becomes more nearly proportional to the E-field amplitude. This voltage is measured with an ordinary voltmeter.

    Going back to the Pasco manual (p. 9) it states that E = 1/R and I = 1/R^2. So... if E is proportional to V, then I should be proportional to V^2.

    My question is: Is this a correct conclusion?
  6. Apr 28, 2013 #5


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    Not necessarily. The E they refer to is that of the microwave field. The V you are measuring is across a diode somewhere in the receiver circuit. So the diode's V is not necessarily proportional to the microwave field's E.

    It would have been helpful if either of the manuals said what they consider to be a small amplitude (V proportional to I) or a higher level (where V is proportional to E).

    It might be worth a phone call to the tech support number given in the back of the manual. If you tell them what level or range your voltage readings are, they might know if it's in the low or high level regime.
  7. Apr 28, 2013 #6
    Thanks for your help! It's really appreciated!
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