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When I think of waves traveling through a medium, I tend to think of the double slit experiment or waves in a pond. In those cases, waves are canceled out by destructive or constructive interference.

I'm curious as to how the EM wave can be sent out through an antenna without being canceled out by other EM waves from nearby antennas (or attenuated when it travels through some attenuating medium)? How is it possible that I am able to hear the sounds from the radio knowing that it was transmitted by a radio tower miles and miles away?

Or rather, if the magnitude of the EM wave is not of concern, then what exactly is the information which is sent and what information is received and decoded by the receiving antenna?

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  • $\begingroup$ In fact in the days of 405-line television, occasionally a passing aeroplane would cause the received signal to fluctuate as the reflection off the plane alternately cancelled and reinforced the direct signal. $\endgroup$ – peterG Nov 19 '14 at 23:35
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So, in the case of waves traveling through a pond, if you imagine the transmitter as the rock hitting the pond and the receiver as, say, a frog sitting in the water, then you can see that the frog feels the waves that bounce back or interfere with each other, but the first wave that hits him will be the strongest.

While a wave is traveling, a significant amount is attenuated/lost, and only a small amount (10^-6 or less) of the original signal makes it through to the receiver. The receiver then filters the signal received from the antenna to only "look at" a particular wavelength of the RF spectrum and then demodulates it.

The exact details of the signal sent by the transmitter depend on the transmission mode. There are many kinds or modes of transmission. Some common modes are FM (Frequency Modulation, AM (Amplitude Modulation), SSB (Single Side Band) for transmitting voice, or one of the many modes for data transmission.

If you are talking about the kind of radio that they play music over for you to listen to in your car, then you are most likely talking about FM. Frequency Modulation (FM) works by changing the frequency of the radio wave that is sent. If you tuned your radio to 95.5 MHz, then what you are receiving is a radio wave that has an unmodulated frequency of 95.5 MHz. What the transmitter does is take this 95.5 MHz signal, and modulate it (i.e. they change the frequency slightly to transmit voices or music). Then, at the receiving end, your radio takes this modulated signal and extracts the sound from it.

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  • $\begingroup$ Would it be possible to receive several radio-waves at a given frequency when you are listening to radio in your car? What would be some reason that this frequency becomes distorted (i.e. becomes faster so that it interferes with another signal)? Also when I think of EM wave it is always a wave that is in 3D (x,y,z) where say the E component may point to $\hat x$ and the M component may point to $\hat y$, whereas FM, AM is always drawn as 2D figures. Is modulation done for both the electric and magnetic field components separately? Sorry if this is too many questions at the same time $\endgroup$ – Carlos - the Mongoose - Danger Nov 19 '14 at 21:48
  • $\begingroup$ @IllegalImmigrant, you frequently do pick up waves on multiple frequencies when listening to the radio. The difference is that (due to governments regulating the usage of the radio spectrum) you are usually only able to hear one channel at once. Sometimes there are frequencies that sound sort of strange, like two stations were broadcasting on the same channel. This is because they are. As far as the frequency becoming distorted, unless you are moving away at a significant frequency of the speed of light, it probably would not be significant, although atmospheric effects may have an effect. $\endgroup$ – Stack Tracer Nov 20 '14 at 4:24
  • $\begingroup$ In addition, FM and AM are only drawn as 2D figures for the sake of simplicity. Because electricity and magnetism are the same thing, we can simplify the pictures of waves by only showing one of the two. This is because the electrical and magnetic fields are modulated at the same time. In fact, we just modulate the electric current, and this drives the antenna, which produces an electromagnetic field. $\endgroup$ – Stack Tracer Nov 20 '14 at 4:29
  • $\begingroup$ I am a "ham" amateur radio operator and my primary mode of operation is CW (Morse Code). Many times I am listening to multiple stations that are not only within the small window of spectrum that I listen (~500 Hz) but also sometimes the multiple and different stations are right smack dab on the same frequency yet sending different information. With CW mode, this is very apparent and there are different audio affects that result from the interference such as two close signals beating together creating a new beat tone. Or, warbles, and so on. Skill is required to pick the one signal you want $\endgroup$ – K7PEH Nov 20 '14 at 6:08
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Good question.

If you look at a list of radio stations in the UK you will see that most of them (and all of them that cover the whole country) use several different frequencies to broadcast precisely because of this problem. (look at the AM list which is clearest on this point - AM = amplitude modulation).

Radio stations that cover a large area use several frequencies so that in places between transmitters they do not have the interference problems that you talk about in your question. So neighbouring transmitters broadcast on different frequencies.

The electronics in radios can pick up individual frequencies very nicely so that you only here the station you are tuned into, but if two stations were broadcasting on the same frequency there would be real problems because of intereference.

So to answer your question inteference is a concern and stations that broadcast from several antennae use different frequencies.

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Or rather, if the magnitude of the EM wave is not of concern, then what exactly is the information which is sent and what information is received and decoded by the receiving antenna?

Let me give you some perspective on this, as it's a point that is often overlooked.

The whole reason you can receive a (weak, as you say) signal in the distance, is due to the massive redundancy of using an oscillating EM field, as opposed to, say, a single EM spike.

To give the idea in a super-simplified form, imagine a 1 MHz transmission as hammering on a telegraph key (EM "clicks") 1 million times a second, and then your buddy is listening to this and keeps the rythm with his left foot (he is very fast!). He knows the speed the beats should be coming with, and just tries to find the corresponding rythm.

Even if the signal is very weak and he misses say 80% of the beats, he can fill in the rest since he is keeping the rythm and knows where to listen.

So by deciding beforehand what rythm you and your buddy should communicate using, you can counteract the lossy medium and send a resilient signal (you could encode the message by slightly perturbing individual groups of beats backwards or forwards in time for example, this would correspond to FM radio then).

If someone else transmits on another frequency it doesn't affect your transmission as it would just drown in the rest of the noise the receiver hears.

Fortunately, you don't need to click on the telegraph key millions of times a second and find a buddy with a fast ear. In amazing convenience, nature provided us with the electrons, photons and metals. Depending on the actual frequency, you and your buddy are electrons spinning around atoms or flowing through a conductor, and the beats' best analogues are virtual photons, building up the transmitted photon.

This turned out a bit long-winded, but it's such a basic feature of physics and information that is often not appreciated.

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What makes you think they don't?

When I was a wee lad, my physics teacher showed a video to me of exactly that. A man was driving down a road listening to a radio station (BBC if memory serves). At regular intervals, the music would get quieter and cut out.

The issue was that the road in question was nearly along the line between two radio transmitters, which setup a standing wave between them.

This is typically a rare occurrence, as governments have setup agencies to govern usage of radio frequencies, in hopes that this wouldn't happen.

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protected by ACuriousMind Feb 5 '17 at 18:41

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