Clarification of understanding of modulation concept needed (1 Viewer)

[EpaX]

My understanding is that one of the reasons modulation is needed is because: for EM waves to travel further distances they need more energy, so they need to have higher frequency (E=hf?).

Is this correct? And is the following also correct?:

Waves are obstructed by objects whose dimensions are of similar or bigger size than the wavelength of the wave; e.g. this is why microwave links need a line-of-sight for transmission. This would also mean that gamma rays, which have extremely small wavelengths, wouldn't be able to travel through air due to obstruction by air molecules.

These two concepts seem to contradict each other... the first is saying that higher frequency waves travel further distances, but the second is saying that higher frequency waves travel less distance due to obstructions.

Another way to look at it is: if both concepts are correct, does this mean that carrier waves with frequencies in the radio spectrum are used in modulation because of the need for the information signal to travel further distances AND because of the need to travel with minimal obstructions by buildings, walls etc. (since radio waves have quite large wavelengths). In other words, radio waves are just right - enough energy to travel considerably far distances and not too short in wavelength to be obstructed by objects.


Please help, very confused :S

 

[EpaX]

bump

 

[QZP]

Modulation is the storing of information in EM waves; I don't think it affects propagation. In fact, EM waves should propagate forever so long as they are not absorbed. The reason why a signal (i.e. modulated wave) loses strength is because of attenuation (the signal spreading out and being absorbed).

If you talk about obstruction, you are not really looking at "energy vs. propagation distance" are you? (and I wouldn't even compare those two. Am I missing something? What topic is this?) If you want to compare radio waves to gamma rays, then understand that objects best absorb EM wavelengths that match their own dimensions (this is also the basis of the antenna design). Hence, gamma rays are quickly absorbed by the atmosphere's particles whereas radio waves can travel much further before being absorbed by anything as simply there is less things for radio waves to be absorbed by.

 

[EpaX]

In fact, EM waves should propagate forever so long as they are not absorbed.

But don't EM waves obey the Inverse Square Law? So EM waves attenuate with increased distance b/c of the Inverse Square Law + objects absorbing them.This would mean that all EM waves attenuate at the same rate (disregarding the object absorption bit)?

Anyway, although EM waves attenuate over distance, it is much less than sound waves (which also attenuate in the same way?), which is why we communicate with them over long distances?

 

[QZP]

But don't EM waves obey the Inverse Square Law? So EM waves attenuate with increased distance b/c of the Inverse Square Law + objects absorbing them.This would mean that all EM waves attenuate at the same rate (disregarding the object absorption bit)?

Anyway, although EM waves attenuate over distance, it is much less than sound waves (which also attenuate in the same way?), which is why we communicate with them over long distances?

The intensity of the EM wave follows the inverse square law. This is simply because the measured intensity at a point away from the EM wave source is weaker due to the EM waves spreading out. However, the individual EM waves that have spread out will continue to propagate forever.

The reason why we use radio waves/microwaves as opposed to gamma rays for communication is because of their ability to penetrate the atmosphere which relates back to wavelength and absorption by particles.

 

[EpaX]

The intensity of the EM wave follows the inverse square law. This is simply because the measured intensity at a point away from the EM wave source is weaker due to the EM waves spreading out. However, the individual EM waves that have spread out will continue to propagate forever.

ahh ok, that makes sense; so EM waves can only attenuate by being absorbed by objects. But how do sound waves attenuate then (besides being absorbed by objects)? [I know that sound waves are due to the oscillation of molecules parallel to the direction of propagation].

 

[QZP]

ahh ok, that makes sense; so EM waves can only attenuate by being absorbed by objects. But how do sound waves attenuate then (besides being absorbed by objects)? [I know that sound waves are due to the oscillation of molecules parallel to the direction of propagation].

Noo attenuation is mainly caused by spreading out of waves and then secondarily absorption of the waves. So for sound waves, they attenuate because they spread out in space.

 

[EpaX]

Noo attenuation is mainly caused by spreading out of waves and then secondarily absorption of the waves.

But didn't you say earlier that the intensity of, say, a light bulb, measured from a distance decreases with distance due to the spreading of the ''individual'' EM waves; and that each ''individual'' EM wave (which cannot spread out anymore b/c it is just itself) propagates forever unless absorbed by an object?

 

[anomalousdecay]

But didn't you say earlier that the intensity of, say, a light bulb, measured from a distance decreases with distance due to the spreading of the ''individual'' EM waves; and that each ''individual'' EM wave (which cannot spread out anymore b/c it is just itself) propagates forever unless absorbed by an object?

That's what QZP meant by spreading out of waves I think........

But yeah think of it this way.

Light attenuates due to absorption. It also depends on the source.

For example, if the source is just a single point, then the ability to attenuate over a long distance is higher as it spreads out more.

However, if the source is a straight stream of light (like a car's headlights by a parabolic reflectors), then the waves won't spread out hence the attenuation due to spreading is minor.

Of course, headlights for a car do have a different shape so that some light spreads out to the sides, because you wouldn't only want to see 2 spots of light in front of you, but rather a wide view of light in front to see what is within the surroundings of the car when turning, or if a car is merging, etc.

So in terms of applications (if you think of it that way), having purely one spot of sourced light in a 360 degree view is useful for a room, but for car headlights it would be inefficient, etc.

What I said above is just an example and a way for you to think of "spreading out" if you ever find yourself confused.

 

[EpaX]

The reason why we use radio waves/microwaves as opposed to gamma rays for communication is because of their ability to penetrate the atmosphere which relates back to wavelength and absorption by particles.

Then why is it that gamma rays' penetrating power makes it the most dangerous radiation to humans? If EMR is absorbed by objects of similar size to their wavelengths, then theoretically gamma rays would be blocked by, well, everything, seeing as they're as small as atomic nuclei.

 

[QZP]

Then why is it that gamma rays' penetrating power makes it the most dangerous radiation to humans? If EMR is absorbed by objects of similar size to their wavelengths, then theoretically gamma rays would be blocked by, well, everything, seeing as they're as small as atomic nuclei.

More importantly, gamma rays have high energy which is why they are dangerous. Gamma rays have sufficient penetrating power to go through the skin. Now compare this to radio waves. Higher penetrating power but less energy. Not dangerous.

 

[EpaX]

More importantly, gamma rays have high energy which is why they are dangerous. Gamma rays have sufficient penetrating power to go through the skin. Now compare this to radio waves. Higher penetrating power but less energy. Not dangerous.

Ahh i see; so the penetrating power of an EM wave is independent of the EM wave's energy - it only depends on the wavelength of the wave?
Anyway, I think I understand why radio waves are used in modulation now - very high penetrating power to go through buildings and hence travel long distances + not dangerous.

 

[anomalousdecay]

Then why is it that gamma rays' penetrating power makes it the most dangerous radiation to humans? If EMR is absorbed by objects of similar size to their wavelengths, then theoretically gamma rays would be blocked by, well, everything, seeing as they're as small as atomic nuclei.

Well gamma rays have a high enough energy to knock electrons out of orbits in their atoms or compounds and hey presto we have the problems of breaking down compounds or even atoms. As a result you have breakdown of DNA --> mutations.

Ahh i see; so the penetrating power of an EM wave is independent of the EM wave's energy - it only depends on the wavelength of the wave?

Yes.

Anyway, I think I understand why radio waves are used in modulation now - very high penetrating power to go through buildings and hence travel long distances + not dangerous.

Yes.

 

[EpaX]

ok thanks for the help QZP and Anomalousdecay