DHowell.ES@Xerox.ARPA (09/14/85)
Here's a new subject to think about, start aguments, etc. Let's say I have a device that emits an electromagnetic wave. I put a certain amount of energy into it, and get most of that energy out as an electromagnetic wave (the rest being lost to heat in the wires). Now I set up another one of these, and place it exactly one wavelength away from the other. I have them emit in the same direction so that the waves overlap and are in phase. Now the output of this system is an elctromagnetic wave with the same frequency and twice the amplitude. I'm putting twice as much energy in and getting twice as much energy out. Here's the question... If I place them half a wavelength apart so that they are 180 degrees out of phase, the waves will cancel. Now I appear to be getting no energy out of this system, at least not in the form of EM waves. I am still putting as much energy into the system. All I did was move one of the devices. What is happening to the energy? Is there an output in another form of energy? Is it building up in one of the devices somewhere? Dan <DHowell@Xerox.ARPA>
rpw3@redwood.UUCP (Rob Warnock) (09/16/85)
+--------------- | Let's say I have a device that emits an electromagnetic wave. I put a | certain amount of energy into it, and get most of that energy out ... | Here's the question... If I place [two of] them half a wavelength apart so | that they are 180 degrees out of phase, the waves will cancel. Now I appear | to be getting no energy out of this system, at least not in the form of | EM waves... What is happening to the energy?... | Dan <DHowell@Xerox.ARPA> +--------------- Sorry. The waves only "cancel" on a line directly through the two devices. They add to a lesser or greater degree at other, off-axis, angles. All you have done is change the radiation pattern of the system, NOT the total energy emitted. Radio stations do this all the time to control their directional pattern so they don't (for example) waste kilowatts on open ocean... Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404
mwg@petrus.UUCP (Mark Garrett) (09/16/85)
++ > Here's the question... If I place [two identicle electromagnetic sources > operating at some frequency] half a wavelength apart so that > they are 180 degrees out of phase, the waves will cancel. Now I appear > to be getting no energy out of this system, at least not in the form of > EM waves. I am still putting as much energy into the system. All I did > was move one of the devices. What is happening to the energy? Is there > an output in another form of energy? Is it building up in one of the > devices somewhere? > Dan <DHowell@Xerox.ARPA> The key is to think of the problem in three (or even two) dimensions. True, along the line passing through the two points, you will have exactly canceled the transmission; and also at certain points in the plane or space. But generally, there will be regions of destructive interferance and of constructive interferance. That is, you can find a point where you are an even multiple of wavelengths from both transmitters, and will observe twice the amplitude. Just as much energy is being radiated, but the pattern will be more complicated. This is used quite often for AM (FM?) broadcasts. There will be an array of six or eight transmitter towers placed such that the resulting waves cancel in one direction and add in the other (more or less). This way, a station in New York can transmit over an area of which New York is not the center, to avoid interference with a station in Philly while covering Connecticut. -Mark Garrett
student@nmtvax.UUCP (09/16/85)
In article <> DHowell.ES@Xerox.ARPA writes: >Let's say I have a device that emits an electromagnetic wave. I put a >certain amount of energy into it, and get most of that energy out as an >electromagnetic wave (the rest being lost to heat in the wires). > >Now I set up another one of these, and place it exactly one wavelength >away from the other. I have them emit in the same direction so that the >waves overlap and are in phase. Now the output of this system is an >elctromagnetic wave with the same frequency and twice the amplitude. >I'm putting twice as much energy in and getting twice as much energy >out. > >Here's the question... If I place them half a wavelength apart so that >they are 180 degrees out of phase, the waves will cancel. Now I appear >to be getting no energy out of this system, at least not in the form of >EM waves. Nope. When you put two of these one wavelength the amplitude does NOT double. Instead there are twice as many photons emitted, each photon with energy E = h f where h is Plank's constant and f is the frequency in Hertz. The waves (photons) will destructively interfere only along the line of the antennas. At any angle there will not be complete interference so EM waves will propagate out with energy happily flowing out of the antennas. Sincerely; Greg Hennessy ..ucbvax!unmvax!nmtvax!student
gjk@talcott.UUCP (John) (09/17/85)
In article <543@sri-arpa.ARPA>, DHowell.ES@Xerox.ARPA writes: > Let's say I have a device that emits an electromagnetic wave... > Now I set up another one of these, and place it exactly one wavelength > away from the other. I have them emit in the same direction so that the > waves overlap and are in phase. Now the output of this system is an > electromagnetic wave with the same frequency and twice the amplitude. > I'm putting twice as much energy in and getting twice as much energy > out. > > Here's the question... If I place them half a wavelength apart so that > they are 180 degrees out of phase, the waves will cancel... First of all, the amount of energy in an electromagnetic wave is proportional to the square of the amplitude, so if the devices are completely in phase, the energy output will be four times as large. So something is definitely amiss. The answer is that the amount of work it takes to create an electromagnetic wave (or any electromagnetic field, for that matter) depends on the pre-existing electromagnetic field. This is analogous to mechanical force and energy: If you apply a constant force to an object, the amount of energy transferred to the object depends on its velocity. We can also analyze your specific example. Any device that emits light does so because electrically-charged particles in it are vibrating. It is not only the case that a vibrating charged particle creates an electromagnetic wave, but also an electromagnetic wave causes a charged particle to vibrate. Anyway, the electromagnetic wave from the first device would cause those very particles in the second device to vibrate, and would therefore change the behaviour (either amount of output or the energy consumption) of the second device in such a fashion that energy is conserved. -- abcdefghijklmnopqrstuvwxyz ^ ^^
dsi@unccvax.UUCP (Dataspan Inc) (09/17/85)
True, directionalisation of the antenna system is common in AM
stations. However, the use of directional antennas in FM is not
very common. The problem is obtaining sufficient directionalisation
in the XY-plane (say the Z-axis is normal to the point on the earth
where the tower is) without introducing severe "multipath like" distortion.
It would, for example, be very very easy for an FM station to just
stick up an ordinary Yagi-Uda directional antenna, but as you were driving
on a road in the "minor lobes" or nulls, you would get severe interference
due to constantly changing phase and amplitude from the various elements.
However, directionalisation in the family of Z planes is common,
to obtain antenna gain. Here, you are simply "squashing" the doughnut
of a single dipole as you add more bays (not exactly true) so that
you don't radiate all that RF into free space. The rate of change of
signals from the various elements would be very small (you'd have to climb
Pike's Peak in 2 minutes or so to get the "picket fencing" in thsi
plane!)
In addition, most (if not all) modern FM stations use circular
polarisation, which is very tough to directionalise in the XY plane.
(I have yet to see "Cavity Backed Radiators" (tm) used in FM, which
have approximately a 120 degree coverage range. They also have no
gain over a dipole) FM stations are solely assigned on distance / power
criteria, and short spacing is rarely a problem (except in North Carolina,
and South Carolina).
By the way: just what is it that is circularly polarized in a CP
antenna? Is it just that the vertical and horizontal E-fields are lagging
in phase quadrature (90 degrees) or something? A good (and rational)
explaination of this would be very helpful.
David Anthony
DataSpan, Inc.
.gjk@talcott.UUCP (John) (09/18/85)
In article <542@petrus.UUCP>, mwg@petrus.UUCP (Mark Garrett) writes: > > Here's the question... If I place [two identicle electromagnetic sources > > operating at some frequency] half a wavelength apart so that > > they are 180 degrees out of phase, the waves will cancel. ... > > Dan <DHowell@Xerox.ARPA> > > The key is to think of the problem in three (or even two) dimensions. > True, along the line passing through the two points, you will have > exactly canceled the transmission; and also at certain points in the > plane or space. But generally, there will be regions of destructive > interference and of constructive interference... Yes, this might happen with an antenna, but it certainly wouldn't happen with a laser. For an explanation of what happens when you do this experiment with lasers or other planar (as opposed to spherical) light sources, see my other posting. -- abcdefghijklmnopqrstuvwxyz ^ ^^
cjh@petsd.UUCP (Chris Henrich) (09/18/85)
[] In article <543@sri-arpa.ARPA> DHowell.ES@Xerox.ARPA writes: >Here's a new subject to think about, start aguments, etc. > >Let's say I have a device that emits an electromagnetic wave. >Now I set up another one of these, and place ... > them half a wavelength apart so that >they are 180 degrees out of phase, [so that] the waves will cancel. >Now I appear to be getting no energy out of this system, at least not >in the form of EM waves. I am still putting as much energy into the >system. All I did was move one of the devices. What is happening to >the energy? The two waves do not exactly cancel; at some points they do, but at other points they re-inforce each other. So, if the amplitude of one wave is 1, the amplitude of the combined wave varies from 0 to 2. What does this imply about the total energy? Well, the energy is a *quadratic* function of the field strength, so it varies from 0 to 4. Take an average, for a rough guess, and you get 2. Two sources, twice as much energy. This can all be made very much more difficult, by trying to decide exact patter of radiation is from each source, then setting up and solving a messy integration problem... Regards, Chris -- Full-Name: Christopher J. Henrich UUCP: ..!(cornell | ariel | ukc | houxz)!vax135!petsd!cjh US Mail: MS 313; Perkin-Elmer; 106 Apple St; Tinton Falls, NJ 07724 Phone: (201) 758-7288
floyd@brl-tgr.ARPA (Floyd C. Wofford ) (09/18/85)
In article <542@petrus.UUCP> mwg@petrus.UUCP (Mark Garrett) writes: >++ >a point where you are an even multiple of wavelengths from both transmitters, >and will observe twice the amplitude. Just as much energy is being radiated, >but the pattern will be more complicated. This is used quite often for AM >(FM?) broadcasts. There will be an array of six or eight transmitter towers >placed such that the resulting waves cancel in one direction and add in the >other (more or less)........ >-Mark Garrett Pick up an antenna theory book. This will describe the large scale effects of such a system. A line of radiating dipoles is a linear array. The same thing is done in two dimensions, hence a planar array. Vary the phase of the currents in the dipoles and space the dipoles some integer multiple of half a wavelength (arbitrary). You will be able to direct the main beam (or beams, depending on the array size) to any direction. This type of antenna forms the front-end of some of the nicest radars built.
usenet@ucbvax.ARPA (USENET News Administration) (09/21/85)
This discussion of EM waves triggered a thought. (I have
studied a lot of math but little physics ...)
Is it possible to direct radio waves towards a particular
location some distance away, in order to affect the behavior
of a person at that location? Is any research done on
questions like this? I would expect the Soviets would
have experimented with this kind of thing ...
Thanks very much,
-Tom
tedrick@berkeleymikes@AMES-NAS.ARPA (09/24/85)
From: mikes@AMES-NAS.ARPA (Peter Mikes) In response top Message-Id: <8509230749.AA19555@sri-unix.ARPA> Subject: Cancelling EM waves Is it possible to direct radio waves towards a particular location some distance away, in order to affect the behavior of a person at that location? Is any research done on To get a good focus you should adjust your wavelength to the size of your target - also as you move to higher frequencies you get beter directionality - so I would suggest you try microwaves - if you direct enough energy on target it will have similar effect on him/her is micro- vawe owen has on frozen chicken. For more information, look under "Star Wars".