electromagnetic waves

[umar karim]

hi people! is there any method for attracting electromagnetic waves which can be practically implemented? this might seem absurd cause filters are there to respond to a certain frequency. what i want is to completely nullify a certain frequency EM wave within a region. discussing it in an electronics room means that i need to know the specific electronic equipment. i will be thankful.

 

[RadioRon]

It would be possible to do such a thing, but it would be very dependent on the size of the region, the frequency of the EM waves, the shape of the EM waves (that is, if they are arriving from a distant source or come from a local source). The method would probably involve emitting a complementary wave, that is, one that is the same amplitude but exactly 180 degrees out of phase, to cause cancellation. So, the kind of equipment involved would include an emitting transducer (in other words, an antenna), a signal source with sufficient amplitude and frequency capability, and I think a monitor receiver to implement a feedback arrangement wherein a controller manipulates the signal source amplitude and frequency to cause a minimum signal to be heard in the monitor receiver. In this case the monitor receiver would have its own sense antenna somewhere in the region.

It would not be possible to recommend specific equipment without understanding the conditions mentioned above. In any case, detailed design of such a system is beyond the reasonable capability of a bulletin board such as this.

 

[umar karim]

thanx mate. i am thinking of taking it up is as my final year project.

 

[jpanhalt]

There has been a fair bit of research trying to do the same thing with sound (see: Bose). In brief, you can do sound cancellation over a very small area, like inside an earphone, but it can't be done over larger areas because of reflections.

I suspect there would be similar problems with EM waves. In some areas you would get cancellation, but in other areas you would not or might even get augmentation. In other words, interference patterns as seen with coherent sources. The other problem to contend with is that the waves from most emission sources are not coherent. John

 

[RadioRon]

It would probably be easier to implement a demonstration system at lower RF frequencies, like perhaps 2 to 30 MHz. At the lower frequencies reflections are not as complex and in some cases may not occur very strongly at all. The down-side to this suggestion is that antennas are often very large at these frequencies, unless you use loops or non-resonant types. At frequencies above 50 MHz, the reflection problem becomes more difficult. In addition, the definition of what constitutes a "small" region is dependent on the wavelength, so a "small" region at 10MHz will be a lot bigger than a "small" region at, say, 500 MHz.

 

[justDIY]

you could demonstrate it using visible light - I remember reading about constructive and destructive color mixing somewhere -there was a simple experiment using prisms..

i hate it when electromagnetic waves interrupt my sleep too early in the morning. the problem is remedied easily with simple filters made of cotton installed near the aperture, attenuating the amplitude quite nicely.

 

[jpanhalt]

Unfortunately, light waves are so short that the "small" described above become less than a micron.

As for the EM in the morning or night, it's more likely sound waves of low frequency, which is why I got interested in noise cancellation in the first place. Laws seem to have no effect, so I am seriously considering experimenting with a sound "laser" as a counter measure.
John

Edit: "Small" is about one wavelength. I assume Radio Ron was using the same assumption.

 

[audioguru]

"Nullify a certain frequency".
It sounds like you want to jam a radio station that plays rap.
It is illegal. Some morons actually likerap and buy the products that are advertised on the station. If the station has a licence then there is nothing you can do about it.

 

[justDIY]

some R&D company was bragging on slashdot today about a disabler that would completely disable a car with a few nanosecond burst of microwave radiation

what I dont understand is how the radiation gets to the computer? the ECM/PCM for my '94 caprice are sealed inside an aluminum box. even my head unit and amps are all sealed in aluminum. the head unit is ventilated, but the holes are much too small for microwaves to fit through.

even the main fuse block, under the hood, is protected by metalized plastic.

 

[RadioRon]

some R&D company was bragging on slashdot today about a disabler that would completely disable a car with a few nanosecond burst of microwave radiation

what I dont understand is how the radiation gets to the computer? the ECM/PCM for my '94 caprice are sealed inside an aluminum box. even my head unit and amps are all sealed in aluminum. the head unit is ventilated, but the holes are much too small for microwaves to fit through.

even the main fuse block, under the hood, is protected by metalized plastic.

It is a common misconception that a metal box is a good shield at RF. In fact, a solid box with no seams, no holes and no wires is indeed a very good shield. But the moment you pass a wire through a hole in that box you destroy the effectiveness of the shield. The wire that extends outside the box is an antenna and when some RF is picked up on the antenna, it just flows down the wire and right through the hole in the metal box. The longer the exterior wire, the better it works as an antenna and the lower the frequencies it will pick up.

There are ways to fix this problem. The most effective way is to provide a lot of capacitance between that wire and the wall of the box so that RF that comes down the wire conducts onto the outside wall of the box and doesn't go through the hole. The best kind of capacitor for this function is a coaxial capacitor, also called a feed-through cap, that has capacitance in a circle around the wire, the most effective geometry for very high frequencies.

 

[umar karim]

jamming reminds me of something else. if jamming is all that easy as detecting the probe frequency, and transmitting a complementary wave,why are'nt sensitive locations safeguarded against probe waves(sattelite waves that is)? isnt it worth implementing?

 

[RadioRon]

jamming reminds me of something else. if jamming is all that easy as detecting the probe frequency, and transmitting a complementary wave,why are'nt sensitive locations safeguarded against probe waves(sattelite waves that is)? isnt it worth implementing?

Because it is not easy to jam in this way. There are many difficulties in the way of its use in practical situations.

As far as I know (which is only learned from Dale Brown novels) most jammers use either brute force noise power to overwhelm receivers, or trick receivers by mimicking valid signals but with false information.

 

[jpanhalt]

if jamming is all that easy as detecting the probe frequency, and transmitting a complementary wave,why are'nt sensitive locations safeguarded against probe waves(sattelite waves that is)? isnt it worth implementing?

I am not sure jamming by producing a complementary wave is all that easy. In fact, I am not sure it has been done with non-coherent EM waves, except maybe as a coincidence.

The problem with sound cancellation is in detecting and producing the counter sound with proper characteristcs in the time available. That is why Bose sells headphones, but not a room deadener. I would imagine that with EM, timing would be a serious if not insurmountable limitation.

For example with sound, one can have a pick-up microphone, process the information, produce an anti-sound wave signal, transmit to a emitter on the other side, and have the two sound waves meet and cancel somewhere in between. The processing speeds and transmission speeds greatly exceed the speed of sound. With EM, that is not the case. The wave would have already passed before the anti-signal (if travelling in a wire) could get to the emitter. An EM wave simply can't "catch up" with another EM wave in the same medium. None of this is to imply that you cannot demonstrate and use interference (i.e., adding and substracting) with EM waves, as in laser measurements. But those uses do not include the detection, characterization, and cancellation of EM that is non-coherent and arriving randomly. John

 

[Hero999]

A resonant circuit will do just what you want.
http://amasci.com/tesla/tesceive.html

 

[umar karim]

thanx people for your help.