Mmm...I''ve been having an annoying time with this. I can''t find a decent explanation. Basically the problem is modelling a radio reciever. At different locations, the strength of the recieved signal depends on...something. I want to know what that something is. For example, if you walk under power lines, you lose the signal with a radio if have one with you. Why is this? Some building seem to absorb radio waves totally (you get virtually no signal inside) while other much larger, denser buildings seem to have decent reception all round. How would you accurately model the propagation of radio waves is what I guess I''m really asking. I would have -thought- you''d use a standard wave propagation model (like for light); but clearly there are things that I dont understand: I can''t fathom how power lines could affect an EM signal, and the complication of radio wave reflection/absorbtion makes this...quite tricky. Any suggestions? Suggested reading? I''ve tried a couple of physics textbooks, but although they talk about EM radiation, etc. etc. they basically gloss over how/if electric/magnet fields affect/are affected by EM radiation, especially with respect to wavelength.

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Original post by Shadow Mint
At different locations, the strength of the recieved signal depends on...something. I want to know what that something is.

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1/d² where d is the distance to the source. Essentially this is because the power transmitted from the source is spread out over a spherical area so
intensity = (Avg Power)/(4*PI*d²)

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For example, if you walk under power lines, you lose the signal with a radio if have one with you. Why is this?

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The signal in your radio is provided by currents in the antennae. These can be caused by changing magnetic fields as well as propagating radio waves. Power lines have (often strong) magnetic fields around them which change if the current in the lines changes. (or equivalently the magnetic field at the receiving antenna changes if you are moving near a power line)

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Some building seem to absorb radio waves totally (you get virtually no signal inside) while other much larger, denser buildings seem to have decent reception all round.

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I imagine that is due to wave interference effects which depend on the geometry of your surroundings. Just walking around a room with a tv or radio antennae you sometimes pass through regions of constructive or destructive interference and get bad or good signals.

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How would you accurately model the propagation of radio waves is what I guess I''m really asking.

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To a first approximation you could take a wavefront propagating outwards. To be more precise you''d use the fact that radio waves from a transmitter are caused by on oscillating electric dipole

like this. These fields are quite interesting.

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Original post by sQuid
The signal in your radio is provided by currents in the antennae. These can be caused by changing magnetic fields as well as propagating radio waves. Power lines have (often strong) magnetic fields around them which change if the current in the lines changes. (or equivalently the magnetic field at the receiving antenna changes if you are moving near a power line)

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From a real-world standpoint, the magnetic fields surrounding power lines are fluctuating at whatever frequency the current is alternating; therefore, the "current" is always "changing."

Peace,
ZE.



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Original post by Shadow Mint
Mmm...I''ve been having an annoying time with this. I can''t find a decent explanation. Basically the problem is modelling a radio reciever.

At different locations, the strength of the recieved signal depends on...something. I want to know what that something is.

For example, if you walk under power lines, you lose the signal with a radio if have one with you. Why is this?

Some building seem to absorb radio waves totally (you get virtually no signal inside) while other much larger, denser buildings seem to have decent reception all round.

How would you accurately model the propagation of radio waves is what I guess I''m really asking.

I would have -thought- you''d use a standard wave propagation model (like for light); but clearly there are things that I dont understand: I can''t fathom how power lines could affect an EM signal, and the complication of radio wave reflection/absorbtion makes this...quite tricky.

Any suggestions? Suggested reading? I''ve tried a couple of physics textbooks, but although they talk about EM radiation, etc. etc. they basically gloss over how/if electric/magnet fields affect/are affected by EM radiation, especially with respect to wavelength.

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Sounds like your in need of a crash course in phyisics. Specifically the phyisics of electromagnetic radiation. ARRL (Amatuer Radio Relay League) has stuff on RF progration and other things you might find interesting. You''ll have to do a search to find the website thought.

*humms to self* Let me refine this question, since although useful, what has been said isnt going to be much help.

Are the following correct:

1) EM radiation cannot be affected during its propogation through an eletric or magnetic field. (If it can, I''d love an example and a reference)

2) Radio receivers can be affected by local fields due to induced current (or electric field: different voltage difference at antenna).


In the second:

If that is true, why is the signal lost. Taking it frequency domain, and looking at the current in the antenna. You have the basic current C, and you have some additional component A, ok?

Unless the magnitude of A is >> C, the circuit should still be able to use the resonance to grab only the frequency compoent it needs. In the case of a fixed frequency addition (eg. from a nice contantly rotating power line) the A component should be completely irrelevant, since it is stuck in only a limited band. All the other frequency information should be preserved.

Now heres the thing right: If you stand on a road with a power line on it, and walk around a bit, you can find places where the reception is bad, and places where the reception is good. The positions are not proportional to distance from the power lines. therefore, there must be something else going on as well.

It could well be the combination of destructive interferance, from the surroundings, combined with the induced current/extra voltage from the power lines, but you get the same sort of thing from computers. You can put a walkman on the desk in front of your computer and move it left and right and back and forth and find spots of better and worse reception. Now, I admit the fields coming from a computer are fairly complicated compared to those from power lines, but the basic point remains:

How to model that. And I dont know the answer, or even where to look to find the answer. I''m -guessing- eletric field, magnetic field and geometry are the only factors to consider, but I dont know. Anyone prepared to tell me for sure they are? Or maybe point out anything else which could be causing the effect?

I''m certainly not an expert in this field, but I can recall my physics studies fairly well...

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Original post by Shadow Mint
1) EM radiation cannot be affected during its propogation through an eletric or magnetic field. (If it can, I''d love an example and a reference)

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True only in a vacuum. True to a good approximation in many gases (not plasmas). Not true for any environment that can propogate induced currents.

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Original post by Shadow Mint
2) Radio receivers can be affected by local fields due to induced current (or electric field: different voltage difference at antenna).

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Yes. The ambient EM field also wreaks havoc on the properties of solid state components in the radio unless it is well shielded (which cheap radios are not).


The one thing you might be forgetting in all of this is that the power lines (and your computer) are emitting a distorted form of the radio signal as well as creating their own ambient field. So, ignoring the ambient field (time varying noise source), you still have to deal with the superposition of the source radio signal and its perverted cousin re-emmitted by the power lines (or computer).

Cheers,

Timkin