What is pep?

Discussion in 'General Technical Questions and Answers' started by G4ALA, Jul 24, 2008.

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  1. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I guess no one read the article you refered early on:)
    The use of "RMS" in reference to power is a total misnomer but pretty much everyone in this discussion continues to use it any way:( Even the ARRL handbook warns us that RMS is not correctly used in regard to power.
    The extra class license guide covers it as well and might be a reprint of "watts" in one of the HB's:)
    You simply measure the peak voltage, mulitiply that times 0.707, square the result and divide by the load resistance (Esquared over R). Because the harmonics are usually and should be low (even -50 dBc for my old 718), the RF cycle is pretty close to a sinewave for all intents and purposes so you can assume the RMS VOLTAGE is pretty close to 0.707 times the peak. It doesn't matter what the waveform is (fixed envelope or SSB)...grab the peak voltage with a calibrated scope and you can have the PEP as fast as you can punch in the numbers or move the cursor on your slide rule.
    73, Pete
  2. AG3Y

    AG3Y Guest

    Thanks, Pete, for backing me up ! You will note that the URL is from EZNec, the developers of the antenna modeling software, so either they are right, or all the antennas that they model aren't worth a hoot ! I would prefer to think the 1st choice !

    73, Jim
  3. N5RFX

    N5RFX Ham Member QRZ Page

    Its all the same.

    Watts rms is just a term and it has a definition which you have described in your quote above. Just because you and others don't like a term doesn't negate its meaning. I don't particularly like the term either; but, I recognize that a definition does exist. I think that folks mistakenly look at the term too literally and mistakenly think that the powers are squared, then the mean of the squares are found, then the square root is taken. This is simply not true. What has been described in the quote above is the definition of Watts rms or rms power, or average power. The accepted definition of rms power gives a value that is equal average power.

    Mark N5RFX
  4. M0DSZ

    M0DSZ Ham Member QRZ Page

    The multipliers such as 0.707, 1.414 and 1.732 are only useful for sine waves, a thermal method of measurement sounds good but must be subject to some inaccuracies due to heat loss.

    With some of the strange power waveforms encountered in drive and motor engineering I have resorted to things like measuring oscilloscope trace area, not trusting digital power meters at all. Different meter makes give different readings.

    RF power meters as supplied to amateurs can have capacitors in the diode rectifier/detector circuits which give a higher reading than meters without, and some meters have them switchable, MFJ I believe provide this. Is the implication that all meters should have capacitors and therefore show a peak voltage reading, therefore can be calibrated as "peak power" assuming the load is actually 50 ohms?
    Last edited: Jul 25, 2008
  5. N5RFX

    N5RFX Ham Member QRZ Page

    That is a good question. If the capacitor is holding the peak RMS voltage of the waveform then the meter can be calibrated in PEP. Circuits in better Watt meters that are peak reading do some massaging of the detected voltage so that what is presented to the meter movement is analogous to PEP.

    Other circuits like the AD8363 provide a DC voltage that is analogous to some dB level. Notice that this device does not really measure power, but detects a voltage that is used to produce a voltage that represents power.

    Mark N5RFX
  6. AG3Y

    AG3Y Guest

    "The multipliers such as 0.707, 1.414 and 1.732 are only useful for sine waves, a thermal method of measurement sounds good but must be subject to some inaccuracies due to heat loss."

    This is quite true, but the latency on the cooling liquid ( water/glycol mixture ) required the transmitter to stabilize for a significant period of time ( like a half hour IIRC ) and the thermometers in the test I mentioned, were located immediately on both sides of the dummy load that was generating the heat . The "charts" I referred to were accurately calibrated for the specific dummy load used with that specific transmitter. There was very little margin for error.

    As they used to say, "if it's good enough for the F.C.C. . . . "

    And BTW, a "Black" signal with no setup, combined with sync pulses, and averaged over time, is hardly what I would consider a sine wave! The time averaging factor was also figured into the equation.

    All this talk about "peak" "average" "RMS" power is an academic exercise , and this thread is starting to sound a bit like the "SWR" thread that at this point is well over 100 individual posts, of only a few actually gave a meaningful reason to the original poster as to why his meter would have read differently at different power levels. Are we going in the same direction with this thread ? I think so .

    73, Jim
  7. AC0FP

    AC0FP Ham Member QRZ Page

    I'm with Jim, some of the conversation is making me think I'm back in an academic environment. Most voltmeters read the peak value of a sine wave and calculate the voltage in average (peak * 0.636) or RMS (peak * 0.707). True RMS reading meters cost more bucks!


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