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Thread: AM Question

  1. #1

    Default AM Question

    I am very embarrassed to ask this question, especially since I have an extra class license, but here it goes... I just can't completely get my head around how AM is transmitted. I know about sidebands, and it's the audio signal varying the amplitude of the carrier, but just what exactly does that mean? What exactly is the amplitude? I've seen it in diagrams, and on scopes, but I just don't fully get it. On a scope, the varying amplitude is shown as a variation in the voltage of the carrier, but does that mean that the strength of the carrier is being varied? You can hear an unmodulated carrier on the radio of course, and see it has a certain s-meter level. I've been inactive for a long time, but I seem to remember the signal strength not changing with modulation... at least not plate modulation (BTW, what would plate modulation be called in a solid state transmitter?). I think the s-meter did vary with screen modulation. Is the modulation increasing the strength of the carrier above the unmodulated level? I think I remember something about at 100% modulation the carrier would increase 50%, but maybe I'm just remembering that it take a 50 watt modulater to plate modulate a 100 watt transmitter. Can anyone clarify this for me?

  2. #2


    In a AM signal there is the carrier and two sidebands the carrier + the
    modulation and - the modulation.
    Example: WECZ AM 1540 in Punxsy has a carrier frequency of 1540 kHz.
    To make things easy, Let's assume that they were sending a single 1KHz audio tone
    for a minute. If you looked at the output of the transmitter on a spectrum
    analyzer you would see the main carrier at 1540 kHz and a signal at 1539Khz
    and one at 1541KHz. The two sidebands would be 1/2 the level of the main
    carrier. The receiver only uses one of the sidebands and discards the
    other. The main signal, the carrier, is there just to carry the
    information modulation through the air.
    Last edited by R2D2; Tue 12th May 2020 at 16:52.

  3. #3

  4. #4


    One of the key reasons for the use of amplitude modulation was its ease of use. The system simply required the carrier amplitude to be modulated, but more usefully the detector required in the receiver could be a simple diode based circuit. This meant that AM radios did not need complicated demodulators and costs were reduced - a key requirement for widespread use of radio technology, especially in the early days of radio when ICs were not available.
    In order that a radio signal can carry audio or other information for broadcasting or for two way radio communication, it must be modulated or changed in some way. Although there are a number of ways in which a radio signal may be modulated, one of the easiest is to change its amplitude in line with variations of the sound.

    In this way the amplitude of the radio frequency signal varies in line with the instantaneous value of the intensity of the modulation. This means that the radio frequency signal has a representation of the sound wave superimposed in it.

    In view of the way the basic signal "carries" the sound or modulation, the radio frequency signal is often termed the "carrier".

    Amplitude modulation, AM, is one of the most straightforward ways of modulating a radio signal or carrier. It can be achieved in a number of ways, but the simplest uses a single diode rectifier circuit.

    Other methods of demodulating an AM signal use synchronous techniques and provide much lower levels of distortion and improved reception where selective fading is present.

    One of the main reasons for the popularity of amplitude modulation has been the simplicity of the demodulation. It enables costs to be kept low - a significant advantage in producing vast quantities of very low cost AM radios.

    Advantages & disadvantages of amplitude modulation, AM

    As with any technology there are advantages and disadvantages to be considered.
    The summary below gives a highlight of the basic pro's and con's.


    It is simple to implement
    it can be demodulated using a circuit consisting of very few components
    AM receivers are very cheap as no specialized components are needed.


    It is not efficient in terms of its power usage
    It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency
    It is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

    To visualize AM, hold your finger out in front of your face and wiggle it up and down, this is am, then wag it side to side, this is FM.
    FM the amplitude does not change, only the frequency.
    Last edited by R2D2; Wed 13th May 2020 at 13:33.

  5. #5


    Thank you all for taking the time to reply, however I've understood the content of your replies since my novice days, what I'm looking for is an explanation of what is meant by "amplitude". I know what changes in amplitude look like in a diagram, but what is it in the real world? Is it a change in the strength of the carrier (corresponding to the modulation of the carrier) or what?

  6. #6


    Assuming a 50ohm load that remains constant, a change in voltage across that load results in a change in current through it, therefore, that results in a change in RMS power in watts. On an oscilloscope, you will see the voltage vary as the load of 50ohms remains the same.

  7. #7

  8. #8


    Brandon - TNX for that YouTube link. That was the explanation I was looking for.

  9. #9


    Make sure to thank W2AEW by hitting his subscribe button! Every video he makes is educational gold!

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