Why don't radio stations interfere with each others broadcasts?

  1. I've learned the basics of waves but now I have a burning question. No amount of googling appears to help.
    I was wondering how radio station broadcasts don't interfere with one another causing loss of information on the signals. After all they are waves and interference occurs when waves overlap. Obviously the radio signals overlap because they can all be received at the same place. So why doesn't constructive or destructive interference occur?
  2. jcsd
  3. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    Radio stations are spread out across different frequencies in an area. For you to successfully cancel out one station with another, you would need them to be the same frequency so that the crests and troughs of one wave always coincides with another. In addition, the carrier frequency is modulated to encode the station's information, either via amplitude (AM) or frequency (FM) modulation. This adds constant changing in the signal from any given station making it even much less likely that one signal will encounter another signal that will destructively interfere enough to destroy the signal.

    When you receive radio signals on a receiver, you filter out the signals except for a small bandwidth centered around the desired carrier signal. So it doesn't matter that you may be receiving signals from twenty different stations since you can filter out all but a desired frequency. The only real danger that can corrupt the signal are multiple signals on the same frequency (which will usually create noise on the stronger signal which I'm sure we all have heard when driving long distance) or you may get a reflection of the station's signal. If the signal hits your antenna, moves on, reflects off a surface, and then hits the antenna again, it can cause ghosting in the signal. This can be seen visually in analog TV by a second "ghost" image offset in the picture or it can be an "echo" in a radio signal. The reflected signal, could cancel out the incoming signal, but the variations in the carrier due to the modulation may allow some information to still come through (like in the case when you watch a noisy ghosting TV program).
  4. But even if complete destructive interference doesn't occur there must be SOME degree of overlap between waves? What happens when the waves overlap like this?
  5. They do interfer with each other. By interfer, I assume you mean wave interference. It's the job of the reciever to limit the sidebands, and decode the audio information from the station center frequency.

    As far as stations in different locations, sharing the same wavelength, this is a second matter. A station is licenced to operate over a region, where everywhere else the signal strength is below some minimum required level. If it's not, they are likely to be infringing on someone elses licenced region. For AM radio, this mean that at dusk, they may, and probably do, have to turn down their transmitter power to maintain transmission strength to levels that don't cause infringment.

    Stations may use various arrayed antennas to cover as much of their advertising turf as they can, calculating and plotting all the various lobes, and changing the array to up their Nielson ratings if the cost advantages warrant.
    Last edited: Apr 14, 2009
  6. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    It doesn't matter because these overlaps are not constant in time unless you have two signals of the same frequency. If they are different frequencies, then you can remove the effects of the other signal using a frequency dependent filter. This is what you do when you tune to a particular station.
  7. jtbell

    Staff: Mentor

    The waves simply superpose (add together). If they're different frequencies / wavelengths, you can "take them apart" again. Mathematically, you use Fourier analysis. In an electrical circuit, you use a circuit that blocks all the frequencies except the one that you want. This is what a radio or TV tuner does. For FM radio and TV, you actually want to select a range of frequencies, but the basic idea is similar.
  8. There is another potential interference problem relating to the receiver design. In AM radios (550 kHz to 1600 kHz RF frequency), there is (usually) a local oscillator (LO) offset from the RF frequency by 455 kHz that beats against the received signal to create a 455 kHz fixed intermediate frequency (IF) signal that is amplified before demodulation. If the LO frequency were say 1100kHz, then either 1555 or 645 kHz RF signals could get into the 455 kHz (fixed frequency) IF amplifier. But the bandwidth of the AM radio RF amplifier is narrow enough to reject the unwanted sideband. But if the same IF frequency were used for FM (88 to 108 MHz) signals, both upper and lower sidebands (separated by only 910 kHz) would get into the IF amplifier. For this reason, the FM radio LO is (usually) 10.7 MHz above the RF frequency and the IF amplifier is set to 10.7 MHz.
  9. rcgldr

    rcgldr 7,395
    Homework Helper

    AM radio stations have to limit the rate of change of volume (amplitude) in order to limit the frequency range bandwidth used around their main frequency. Even ham AM morse code transmitters use a 5 ms ramp up and down rate to avoid excessive bandwidth usage. (An ramp up of amplitude appears as an increase in frequency, a ramp down as a decrease).

    For FM, the amplitude is encoded via frequency, so here the range of amplitude as opposed to the ramp rate is limited, before being encoded as a frequency deviation.

    The main point is that the transmitters in all devices (cell phones, radio control transmitters, garage door openers, radio stations, ...) are made to limit the bandwidth to their proper frequency range to avoid interference.
  10. Andy Resnick

    Andy Resnick 5,744
    Science Advisor

    Because the different radio stations are not mutually coherent- there is no coherence between the different broadcasts. It's the same reason light from one star does not interfere with light from another star.
  11. jtbell

    Staff: Mentor

    I don't think that's a good comparison. The light from a star is a superposition of wavetrains emitted from bazilions of individual sources (atoms), at random times and random phases. That's what makes it incoherent. The radio waves from a transmitter come from a single source (an oscillating current in the antenna).
  12. Here is a very simple way to visualize it. The analogy is not perfect, but I think it'll help you get the picture. Imagine a float in a pool of water. Now imagine making some waves on both sides of the float. The waves produced on the left side have a higher frequency than the waves produced on the right side. So the distance between each wave front from the left is shorter than the distance between each wave front from the right. What happens when the waves cross the float? Well, the float bobs up and down of course. But at what frequency? As it turns out, the float will bob up and down at both frequencies, one superimposed on the other. Only when the waves produced from the left and right are of the same frequency does interference occur. A very similar thing happens when radio waves cross a length of wire. It causes the electrons to move back and forth in the wire. There could be many radio stations transmitting on different frequencies and the electrons will be moving back and forth at all of those frequencies. When this radio frequency (RF) current is fed into a receiver it passes through circuits designed to filter out the unwanted stations.
  13. Andy Resnick

    Andy Resnick 5,744
    Science Advisor

    That's not exactly correct- stellar interferometry is used to null out the light from a single star. I mean light from one star cannot be used to null out light from another star.
  14. So basically there is no constructive of destructive interference between waves of different frequencies?
  15. Does anyone know where i might find an animation of this phenomenon?
  16. There's no CONSISTENT DESTRUCTIVE INTERFERENCE. You're going to have some random interference at any given moment which will degrade the quality of the signal, this is one of the sources of the random noise you hear, however, unless something is broadcasting at the same frequency (or at a very close frequency in which case you could get a beating effect) you're not going to cancel out another radio signal.
  17. That being said, interference can happen. For example, if you have wireless internet you may have noticed (if you're unlucky) that your internet dies when someone turns on the microwave. Now this isn't supposed to happen since microwave bands and wifi bands aren't supposed to coincide but I suppose if they are close enough in frequency and the microwave is outputting at such a high intensity it becomes impossible for your router to determine the wifi signal from the background white noise (the signal-to-noise ratio, as its called, is small). Of course, I'm not actually an EE but I always assumed that's what happens
  18. Thanks for your replies... but according to the applet TurtleMeister linked to (thanks for the link) if the frequency of the second wave is much higher than the first it appears that the resultant wave will be completely garbled. So how can there be very little interference? (I know.. I've been asking this since the beginning of the thread and I must seem pretty dumb but I just can't understand it. After all.. the applet does show a weird resultant wave.)
    Last edited: Apr 19, 2009
  19. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    Just because it's a mess in the temporal spectrum doesn't mean it is that way in the frequency spectrum. Again, the two signals are at different frequencies, and barring the noise concerns that were mentioned by maverick, you can easily separate the two by using the appropriate filter. EM waves, in the classical sense, follow linear superposition. They simply add on top of eachother. You can only destroy the wave by having two waves of the same frequency and amplitude but with 180 degree phase shifts. Anything else is going to be background noise (although with two signals of the same frequency you can no longer acccurately distinguish between the two sources unless you do some intelligent encoding rules).
  20. Try adjusting the amplitude of one and then the other (in the applet). It's a little difficult to see it when both amplitudes are the same. Even though it may look distorted, both waves are still present and can be recovered in their original form. You can see the same thing in a real world experiment if you have access to an oscilloscope and two audio tone generators. Connect the o'scope to a microphone. Turn on the two tone generators and adjust their frequencies. You will get the same effect on the o'scope display as you get in the applet.

    I think the confusion my be in the use of the terms constructive and destructive interference. It makes it sound like something is being destroyed or altered in an unrecoverable way. And that is not necessarily the case. If what you are thinking (that waves from two different sources interfere destructively) were true then you would not be able to view (normally) a red light and a blue light simultaneously because their waves would distorted each other. You would not be able to hear two audio tones without distortion. But I'm sure you realize this, you're just having trouble visualizing it. Try adjusting the amplitudes in the applet.
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