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AM power

Discussion in 'Amplitude Modulation - AM Fans' started by N2RDQ, May 15, 2018.

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  1. KA4KOE

    KA4KOE Ham Member QRZ Page

    I don't know what is the basis of the personal animus. But I'm getting tired of folks bashing Don.
    N6YW likes this.
  2. K4KYV

    K4KYV Subscriber QRZ Page

    Look back to Paragraph 18 of the original FCC Docket 82-624 notice of proposed rulemaking (attached in the initial post at ). You will see that the FCC admitted that "accurate" pee-e-pee measurement would be beyond the capability of many licensed amateurs, so they deleted the previous rule, §97.67(a), that had required hams to possess instruments capable of accurately measuring transmitter power whenever the nominal level exceeded 90% legal limit.

    It could be argued that "pee-e-pee" measurements depend on the type of instrument used, whether it measures in the time domain, or frequency domain. The so-called "wattmeters" used by hams are time-domain. These are not true watt-meters at all, but r.f. voltmeters equipped with dial scales to indicate watts of power when working into a perfectly matched, purely resistive, non-reactive ( 50Ω + j0) load. This works fine for broadcast and other commercial stations assigned to a single licensed frequency with one specific antenna whose impedance has been precisely measured, but amateurs transmit on many frequencies within multiple bands. What is the likelihood that the load presented to an amateur transmitter will always be exactly 50Ω + j0?

    And now, let's consider a frequency domain instrument, for example, a spectrum analyser. Here, the display clearly shows the AM carrier and sidebands off to each side. The "peak" amplitude of the carrier extends greatly above any of the multitude of peaks displayed in the sidebands. We know mathematically that the total peak power of the sidebands of an AM signal is 1/2 the carrier power. If we add up the peak power levels displayed on a spectrum analyser, we see a 1000-watt carrier in the middle, with 500 watts of peak sideband power spread out over both sides adjacent to the carrier. 1000 watts of carrier + 500 watts p.e.p. of sideband power = 1500 watts pee-e-pee.

    The definition of pee-e-pee in time-domain, ASSumes a principle discarded more than a century ago: that an AM signal consists of a single carrier frequency, varying in amplitude in step with the modulation. The spectrum analyser clearly shows a steady carrier of unvarying amplitude, dominating over two sidebands, generated on separate frequencies each side of the carrier, by the modulation of the signal.
    N2RDQ, WD4IGX, N5DMC and 3 others like this.
  3. N5RFX

    N5RFX Ham Member QRZ Page

    This thread is hilarious.
  4. N1BCG

    N1BCG Ham Member QRZ Page

    Now you've got me curious if a modulating tone of <10Hz will show a visibly fluctuating carrier amplitude. I guess I can cross off the BC1-G for this experiment...
  5. G0GSR

    G0GSR Ham Member QRZ Page

    If you could achieve the resolution bandwidth required on your spectrum analyser, you would indeed see a fixed carrier plus the two sidebands +/- 10Hz.
    The fact that maybe you can't is just a deficiency in the measurement accuracy, it doesn't change the theory.
    AC0OB likes this.
  6. AC0OB

    AC0OB Subscriber QRZ Page

    Frank and Don are right. Spectrum Analyzers with sufficient resolution will show you the theory is correct.

    See post #80:

    With a 1khz modulating tone and a higher resolution setting on the SA, I can indeed see a steady carrier with varying amplitude sidebands.

  7. N1BCG

    N1BCG Ham Member QRZ Page

    Perhaps it's the phosphor holding the image, much like raster scanning on analog televisions, and the inability of the eye to see rapid changes. A modulating signal is varying the amplitude of that carrier, thus "AM", so in theory, it should be visible but only at single digit modulating frequencies.
  8. AC0OB

    AC0OB Subscriber QRZ Page

    It has nothing to do with the phosphor or the display.

    Here is a spectrum display that I use in one of my classes, compliments of and by permission of Richard Fry and clearly shows the varying amplitude sidebands with a constant center frequency carrier. The HF audio content is due to NRSC pre-emphasis:


    Attached Files:

    N6YW likes this.
  9. W7TFO

    W7TFO Subscriber QRZ Page

    Now I'm seeing things....

  10. WA3VJB

    WA3VJB Ham Member QRZ Page

    On the other hand, trying to get 10hz through a BC-1G might cause a little carrier shift as the power supply groans ...
    N1BCG and N6YW like this.

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