# Question on amplitude of a wave

#### jaygatsby

Joined Nov 23, 2011
182
Amplitude modulation (AM) implies that the amplitude of a received wave must be known to recover the intelligence from the modulated signal. However at low frequencies, AM quarter/half wave antennas only pick up a part of the wave. This means that the antenna only gets a part of the height of the wave, and thus cannot know the true amplitude, only the frequency. So how can AM work under these conditions? What do I misunderstand?

Thanks,
J

#### MrChips

Joined Oct 2, 2009
21,634
Amplitude modulation (AM) implies that the amplitude of a received wave must be known to recover the intelligence from the modulated signal. However at low frequencies, AM quarter/half wave antennas only pick up a part of the wave. This means that the antenna only gets a part of the height of the wave, and thus cannot know the true amplitude, only the frequency. So how can AM work under these conditions? What do I misunderstand?

Thanks,
J
The signal received by the antenna is not affected by the length of the antenna except for the actual signal strength received.

• jaygatsby

#### jaygatsby

Joined Nov 23, 2011
182
The signal received by the antenna is not affected by the length of the antenna except for the actual signal strength received.
Then, is the amplitude of the signal not a physical wave height, but a level of energy? And isn't the energy in a wave proportional to its frequency, not amplitude?

Thanks

#### BillO

Joined Nov 24, 2008
990
I think where you are going wrong is in assuming that the amplitude of the received wave must be know. This is not true.

AM works by 'detecting' the modulating signal. This is usually done by rectifying and filtering out the carrier such that all that is left is a voltage that varies with the amplitude (whatever that is) of the received signal. This is then usually amplified to a usable level. No knowledge of the amplitude of the originating signal is required.

• jaygatsby

#### MrChips

Joined Oct 2, 2009
21,634
Then, is the amplitude of the signal not a physical wave height, but a level of energy? And isn't the energy in a wave proportional to its frequency, not amplitude?
Whoa! You're mixing a few things here, quantum mechanics and wave motion.
The energy of a wave quantum is proportional to its frequency determined by Planck's constant.

Here we are talking about the energy in a sine wave which is proportional to the square of the amplitude.

• jaygatsby

#### jaygatsby

Joined Nov 23, 2011
182
Whoa! You're mixing a few things here, quantum mechanics and wave motion.
The energy of a wave quantum is proportional to its frequency determined by Planck's constant.

Here we are talking about the energy in a sine wave which is proportional to the square of the amplitude.
Ok... Thanks. Is the amplitude of the wave the physical height, for instance in meters, so that a physical wave resembles what is on an oscilloscope, but scaled?

#### jaygatsby

Joined Nov 23, 2011
182
(I am speaking of the wave in space or air...)

#### Papabravo

Joined Feb 24, 2006
14,205
No. The amplitude of a wave is measured in volts. The wavelength is measured in meters (or any distance unit), and is related to frequency by the speed of light.

A wave which is a function of time is called a traveling wave. A wave that is a function of distance is called a standing wave. Both kinds of waves can be present in an antenna. The geometry of an antenna will favor signals of a particular frequency (wavelength) over all others.

• jaygatsby

#### jaygatsby

Joined Nov 23, 2011
182
No. The amplitude of a wave is measured in volts. The wavelength is measured in meters (or any distance unit), and is related to frequency by the speed of light.

A wave which is a function of time is called a traveling wave. A wave that is a function of distance is called a standing wave. Both kinds of waves can be present in an antenna. The geometry of an antenna will favor signals of a particular frequency (wavelength) over all others.
The wave amplitude in a circuit is measured in volts... What about in air? So, the amplitude does not affect the physical dimensions (in space) of an EM wave?

#### MrChips

Joined Oct 2, 2009
21,634
Whoa! You're getting off into a tangent again. An electromagnetic (EM) wave does not occupy space. A light bulb emits EM waves. The waves do not occupy space, they just travel outwards in all dimensions. An analogy is like when you throw a stone into a pond. The waves radiate outwards from the source of the energy.

• jaygatsby

#### jaygatsby

Joined Nov 23, 2011
182
Whoa! You're getting off into a tangent again. An electromagnetic (EM) wave does not occupy space. A light bulb emits EM waves. The waves do not occupy space, they just travel outwards in all dimensions. An analogy is like when you throw a stone into a pond. The waves radiate outwards from the source of the energy.
I guess I'm still confused. If they don't occupy space, how do we measure the wavelength in meters?

#### Papabravo

Joined Feb 24, 2006
14,205
In space or in a circuit it is the same thing. We measure wavelength by frequency using the relationship that:
c(the speed of light) = (frequency in Hz.)*(wavelength in meters)

c = 299,792,458 meters/second in a vacuum

If you have a wire with a standing wave in it you could measure the voltage at various points and infer the wavelength of the signal by measuring the distance from a null (0 VAC) to a peak value (e.g. 150 microvolts) and this would be 1/4 of a wavelength. You might be able to do the same thing with a field strength meter in the near field of an antenna, but I've never tried this.

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#### antonv

Joined Nov 27, 2012
148
Back to the original question: I think you can say that to demodulate AM you are looking at amplitude changes, not absolute amplitude.

• jaygatsby

#### WBahn

Joined Mar 31, 2012
26,047
I guess I'm still confused. If they don't occupy space, how do we measure the wavelength in meters?
The amplitude of an EM wave is not a distance (i.e., a displacement of something), but rather an intensity. Imagine a long bar of metal that is heated in such a way that as you move along the bar it alternates between hot and cold regions. Let's assume, for the sake of visualization, that the hot regions are hot enough to glow and that the cold regions are cold enough to gather frost (the colder, the more frost) and also that a given spot on the bar can go from white hot to frostly cold instantaneously. So now you look at this bar and what you see are regions that are pulsing between white hot and deep frost separated by regions that are steady at room temperature. The distance between the center of one white-hot region and the next (assuming they are evently spaced) is the wavelength and it can be measured in meters. But the amplitude is the difference in temperature between the steady point (which I'm simply stating is room temperature) and either the peak hot temperature or the peak cold temperature (which I'm implying is evenly up and down from room temp -- in otherwords, the average time happens to be room temp). See how the concept of amplitude need not have anything to do with distance and that, even if it doesn't, that doesn't affect the ability to have a wavelength that is a distance?

In an EM wave, the amplitude refers to the intensity of the electric field.

Switching gears to a different aspect of your question, the "intelligence" as you put it (or the "information signal") of an AM wave can't be encoded into the absolute amplitude of the wave because that amplitude is going to change with distance (generally die out as the square of the distance from the transmitter) and many, many other factors will affect it. So, as others of pointed out, the information is encoded into changes in the amplitude of the signal.

• jaygatsby
Thanks! 