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How do two EM fields interact?

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GeekWizard

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Hello,

This may be simple thing for most of you but

theoretically speaking how do two or more EM fields interact with each other if they are at a different frequencies and are at close proximity?
The fields are originating from different antennas.

Many thanks.
 

Free space is linear and so superposition results. The field intensity (magnetic and electric) at any point is the sum of the values at that point in time of all of the fields taken one at a time from all of the sources.

There is a second order effect of the antennas picking up energy from the other antennas and it affecting the power amplifier driving that antenna. This is why it is common to use circulators. These divert power picked up by a transmitting antenna into a terminating load and therefor it does not go to the transmitting amplifier.
 

hi flatulent,

I got a little of your idea about circulators? Can't understand exactly what they are? Are they some filters tuned at a particular frequency, so they are able to reject these frequencies comming from different antennas nearby...or something else.

Thanks
 

Circulators and isolators are nonreciprocal devices. They contain ferrite materials with a constant magnetic bias. They have three ports. Power into one port emerges at the next port clockwise or counterclockwise depending on the polarity of the magnetic bias.

They have been around for decades and you can buy them from many suppliers. The port isolation is only large at one frequency, but they are usually useful over a few percent bandwidth.
 

How far should the transmitting antennas be if I want minimum interaction between them?
I had the idea that the inter-distance should be more than the antenna aperture ( Dλ²\4Π ) so that the radiation patterns do not interfere with each other. Is this true? What about antennas like monopoles , dipoles - what should be the inter-distance?

Many Thanks[/i]

Added after 3 minutes:

You can say I'm more interested in isolation at the transmission side. Any links explaining this ud be helpful.
 

The key parameter for interaction between the two sources is the mutual impedance between the antennas. If your frequencies are far apart, you could get along with band pass filters at the transmitter outputs.


What sort of antennas are you using? What is the power level? and frequencies?
 

they penetrate each other and keep their original shape,

that's wave's characteristic.

best regards




GeekWizard said:
Hello,

This may be simple thing for most of you but

theoretically speaking how do two or more EM fields interact with each other if they are at a different frequencies and are at close proximity?
The fields are originating from different antennas.

Many thanks.
Click to expand...
 

they penetrate each other and keep their original shape,

that's wave's characteristic.
Click to expand...

Isn't this contradicting to what Flautulent posted? ??

What sort of antennas are you using? What is the power level? and frequencies?
Click to expand...

I'm using monopoles and dipoles. The frequencies are VHF,UHF and S-band.

Actually I was interested in investigating the effects of radiation patterns on a small surface. I'm using Win-NEC for this purpose. Since I was using it for the first time, later I found out that all NEC based algorithms only simulate only for one single frequency, and so I'm trying to understand theoretically what happens before I can go on to any practical construction.
 

theoretically, in free space any two single freqency signal (of course in this case it carry zero information) with different frequency wont interact with each other.

and, theoretically antenna is a linear element, so there should be no interaction in antenna too. but with nonlinear element, such as mixer, two different frequency signal may produce another frequency which may be harmful to the system.
 

GeekWizard said:
they penetrate each other and keep their original shape,

that's wave's characteristic.
Click to expand...

Isn't this contradicting to what Flautulent posted? ??
Click to expand...

The two waves won't interact, (think about the wave sent out by the thousands of cell phones).

However the two antennas (source of the wave) would interact (coupled to each other) because the antenna can receive waves at different frequency.
 

However the two antennas (source of the wave) would interact (coupled to each other) because the antenna can receive waves at different frequency
Click to expand...

How far apart should the antennas be in order to avoid coupling?

Is there any precise calculation involved or just based on trial...

What other factors come into play in antenna coupling ?

Many Thanks
 

I think there is no such general rule as "if A and B are xxx apart, then there is no coupling". I suppose this is one of the reasons that people use full wave electromagnetic simulator to study the coupling. (Isn't this one type of EMC problem? )

I suppose you can play with the circuit (to filter out the unwanted frequency), the polarization of the radiated signal, etc. to reduce and even nullify the coupling..
 

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    GeekWizard

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