What is the relationship between amplitude and strength in radio waves?

[jshuford]

This may be a very dumb question, but I was wondering, does amplitude function as a measurement for "strength" for a radio signal (by strength I mean the ability of one wave to dominate another on a receiver)?

I know that hertz defines the frequency, which I understand as the part of the spectrum a given signal occupies (which is, if I'm not totally off base, simply a measurement of where in the possible range of wavelengths, that particular wave's wavelength falls).

I know that amplitude is a measurement of the magnitude of change in a wave. So frequency is the distance between peaks, and amplitude is the distance of peak-to-trough. I also know that amplitude is a measurement of strength for sound waves. When I looked it up, amplitude on sound waves are measured in decibels. So far so good. The more amplitude, the louder it is. I can understand that easily from a lay-perspective.

But apparently the unit of measurement for amplitude isn't the same for all waves. I looked on Wikipedia and it says that displacement is the unit of measurement for waves moving through a medium or down a string, which seems to me is measuring a different quality. First, last time I checked sound moved through air, so why is it measured with a different unit than other waves traveling through a medium? I could see how displacement could be an effective measurement of wave strength (the more stuff it moves the stronger it is) but will that correlate to how far the wave will move through the medium? For instance, a sound can be very loud, can travel very far, and not displace a large amount of air. I'm thinking particularly of high frequency sounds, which can travel far, and be very loud, without producing any air compression (a function of air displacement) detectable by the human body. I'm sure the same could be said for any wave moving through a medium.

Clearly there is more than one possible measurement of "strength" for a wave. I think it's very likely I'm confusing myself, but the various definitions of amplitude aren't helping. :) Why are multiple units of measurement needed to define the same quality of various waves?

Now back to radio waves. I basically understood none of what Wikipedia said on radio wave amplitude. Does it still function as a unit of measurement to define strength? If so, how does it work? If two radio signals were being broadcast at the same frequency, say, the one that car radios use, but one is twice the amplitude, how would my car radio translate that? If amplitude isn't an effective measurement of strength, what serves this function for radio waves? I know that when I'm in my car often I can hear two signals on the same frequency but one is dominate and almost completely drowns the other out. I also know that my little iPod FM broadcaster can drown out some stations but not others, so obviously there is a function of radio wave strength independent of frequency, but what is it and how does it work? Is there more than one way to measure strength on radio waves (like there seems to be for waves in a medium)?

 

[Andy Resnick]

There's a lot of different concepts in your question, so let me take a first cut. "signal strength" can be measured in lots of different units- decibels, watts, etc, and to some degree one can interconvert from one to the other. Competing sources are difficult to understand without knowing if the signals can interfere or not- if there is mutual coherence between the sources.

There's different terminology for radio and optical detectors due to the ability to coherently detect radio waves (i.e. the instantaneous phase of the signal), but not optical frequencies.

The power of a traveling wave can be measured in Watts, and is proportional to the energy or intensity (amplitude^2) rather than the amplitude.

Sound waves are a different beast than electromagnetic waves (longitudinal vs. transverse, pressure vs. electromagnetic field) which may account for the difference between dB and W.

The propogation of a wave in a medium involves the interaction of the wave with the medium- scattering/absorption- and is highly dependent on the specifics.

Does any of that help?

 

[jshuford]

Ah, that actually helps a lot. I thought it might be some fundamental difference between waves that require a solid medium and waves that can travel in a vacuum. That gives me a lot of new stuff to look up though. :)

So why do waves that don't need a medium degrade over distance? Like a sound wave or a wave moving through water expels energy into the medium it's moving through, but a wave moving through a vacuum doesn't (or does it?). Does it still "use" part of it's energy to move? I can't imagine how it could. Is it simply accumulated interference that eventually degrades a radio or microwave so that, for instance, as I drive away from a radio tower the receiver in my car gets more static? And if that's the case, then how could increasing the strength of the radio wave make it coherent over longer distances (or maybe it doesn't)?

And I guess that radio waves or microwaves strength is described in Watts, which makes sense (based on the box my microwave oven came in :) ), but is that an intrinsic aspect of the wave like amplitude, or is it more like the "kenetic energy" of the wave, just a modifying factor that doesn't affect it's physical properties? Or, wait, I know radio waves and their ilk don't have physical properties, but their wave properties that have physical analogs (like frequency). Hmph, maybe part of the problem is my trying to imagine a non-physical wave in physical terms. . .

But anyway, what I'm getting at is what is the difference between a radio waves of different strengths? I looked up Watts and it seems to just be a generic energy term. I guess that the difference in a 1 Watt and a 100 Watt radio wave would be the 100 Watt one had 100 times the energy tied up in it, but how is that measured? What is being measured to get the wattage?

Oh, and thanks a ton! As you can see this is my first post here, but I'm sure it won't be the last.

 

[Andy Resnick]

Travelling waves disperse (diffract) as they propogate, this "spreads out" the energy, making your reciever, which then gets a smaller and smaller portion of the signal, pick up a weaker and weaker signal. This is different than scattering, which requires an interaction between medium and wave.

Now be aware that just because EM waves don't need a medium to propogate in, that's different from saying they do not propagate in a medium- EM waves scatter off air, for example.

As for the measurment of power, optically at least, the detector is a highly absorbing object. I'm not sure of the full details, but I think the absorption of the EM wave by the object is converted to thermal energy (increase in temperature), and if the specific heat of the detector is known, the incident energy can be calculated. 1W = 1 J/s, and energy (Joules) detectors are primarily calorimetric (heat) devices.

 

[skeptic2]

Radio waves in free space obey the inverse square law. Twice as far away the signal is 1/4 as strong. Why? The pattern of radiation from a dipole is donut shaped and twice as far away the surface of the donut has four times the area. Spreading the same signal over 4 times the area means it must be 1/4 as strong. This is why most fields that propagate through 3 space obey the inverse square law. It is not due to dissapation.

 

[jshuford]

Radio waves in free space obey the inverse square law. Twice as far away the signal is 1/4 as strong. Why? The pattern of radiation from a dipole is donut shaped and twice as far away the surface of the donut has four times the area. Spreading the same signal over 4 times the area means it must be 1/4 as strong. This is why most fields that propagate through 3 space obey the inverse square law. It is not due to dissapation.

Twice as far as what? That seems to imply to me that signal strength decreases quickly over distance, but don't we detect signals over interstellar distances?

 

[jtbell]

Twice as far as what?

When the distance from the source to the receiver doubles, the power received falls by 1/4 (assuming the same source and receiver). For example, if a receiver picks up 0.4 watt from a source that is 10 km away, an identical receiver 20 km from the source picks up 0.1 watt.

That seems to imply to me that signal strength decreases quickly over distance, but don't we detect signals over interstellar distances?

The sources tend to be rather powerful in those cases.