Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Light Interference
Name: N/A
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A


Question:
How is that given the myriad of electromagnetic waves generated by radio, TV, mobile phone, remote control devices, etc. etc. these wave forms appear to retain their integrity and do not interfere with one another ?



Replies:
Well, they DO interfere with each other, but that doesn't make them go away. You can think of it like waves in the ocean. Many waves with vary different periods and amplitudes combine, without any of them disappearing. The large swells roll onto the beach, and atop the swells are myriad smaller waves going every which direction. If you swim out beyond the breakers and throw a rock into the water, ripples will radiate away from the impact just as they would in a glassy pond. They do this on top of all the other waves, and their effect is added to the effects of all the other waves, but they still are there.

The circuitry that decodes the signals from radio waves is sensitive to only a small range of wavelengths, and anything outside that range isn't even detected. So, if your circuit is tuned to the "ripple" frequency, it won't even know that the "sea swells" are there. Of course, that's only part of the problem. If a hundred people are throwing pebbles into a glassy pond, it is very difficult to discern the ripples from any given rock. That's the challenge of maintaining a cellular phone network. I don't know exactly how that is done, though I know that it involves a number of clever tricks, such as assigning slightly different frequencies to different devices depending on what other frequencies are in use at the time and special identifier signals so that the network knows which signal comes from which phone.

Richard Barrans Jr., Ph.D.


Hello,

They actually do. That is why when you are on your cordless phone, you can may hear your neighbors! To avoid that, you sometimes can switch channels. Anyway, in the US, FCC, the Federal Communication Commission, regulates the frequencies and strengths of the broadcasting centers as well as other sources of electromagnetic emission form devices such as motors, microwave ovens, etc. (Look on the label on the back of these and you see FCC compliance).

If broadcasts are made at different frequencies (channels) OR are adequately distant from one another, then your receiver (radio or cordless phone) which is tuned to a particular frequency will pick only the one you are tuning to.

Do these waves collide in the air, sure they do. Do they get distorted a little, yes. Can we tell just by listening in? Not usually, and, in addition, all kinds of "filters" are used to take out these interferences. Interference effects also depend on the kind of broadcast, frequency modulated (FM) and amplitude modulated (AM). They are affected differently.

Finally, if the interfering source is both broad and strong enough, you can get the wave you are tuning to distorted enough to overcome your filtering.

This This is the limit of my knowledge. May I suggest that you look at FCC site (www.fcc.org) and links therein for more information.

Dr. Ali Khounsary


The quick answer is they actually due interfere with each other. A prime example is solar flares interacting with radio and satellite signals on the earth.

Dr. Myron


Gracious, of course they interfere with each other. Perhaps the conceptual block here is that you assume "interfere" means "destroy the information content of", that is, you are wondering why the presence of the local Top 40 FM radio station antenna in your neighborhood does not hopelessly muddle your reception of TV signals, or mobile phone signals, etc.

When two radio signals are in the same region of space, they "interfere" in the same way as two ocean waves that occupy the same area of ocean interfere: the net amplitude of the electric field (or ocean height) is the sum of the amplitudes of the two waves. Granted, this produces a much more complicated-looking electric field distribution (or sea surface distribution), but no information has been lost. You can readily disentangle one wave from the other with your receiver. The first thing your receiver does is represent the complicated looking electric field by the sum of a large number of pure AC sinusoids of various frequencies. Fourier's Theorem says this can always be done. It then throws away all of these sinusoids that are at uninteresting frequencies, for example those below 88 MHz and above 105 MHz if you are listening to FM radio. Actually, you select, by means of your tuning dial, an even sharper filter that throws away all waves at frequencies more than a Mhz or so away from a particular target frequency, KROQ 102.6 FM, or whatever. It is at this point, you see, that your receiver has thrown away all the "interference" form TV, mobile phone, etc. transmitters nearby.

Dr. C Grayce



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory