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The term Mutual Coupling is very generic...
1. it applys to Transformers in which it couples with inductors to form K...
2.it applys to transmission lines (traces) on PCB/MMIC/RFIC which provides
mutual inductance or mutual coupling factor...
3. It applys to anennas mutual coupling effet when multiple antennas spaced closely..
I preassume that who want for antenna mutual coupling parameter...
Mutual coupling happen in any high frequency structures when they are exposed to each other. The current on a structure is creating EM field and the EM field induces current on another strucutre exposed to the field. There are EM energy transferred from the 1st structure to the 2nd structure. Also, the EM energy on the 2nd structure is also coupled to the 1st one. That is why people call it mutual coupling. In some sense, you can think all the wireless devices in modern days are operating based upon the mutual coupling. However, people use other terminology such as "radiation" for it due to the differences in applications.
For some applications, designers want to make use of mutual coupling. For example, designers use mutual coupling to design transformers and directional couplers etc. However, for many applications, people want to avoid it because mutual coupling can make their designs much more difficult or different from what they want. For example, a designer wants to build 2 inductors on an IC or PCB. The designer wants to achieve a speicifed L values. Asssume the designer can achieve the L values exactly. However, when the two inductors are put together on the same PCB and they are adjacent to each other. There will be significant coupling between them. The coupling between them will make each of them behaves differently from what it is supposed to be when they are not coupled. Then, performance of the circuit may not meet the specifications. There are so many similar examples.
Please simulate 2 patch antennas with 1 port on each as:
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| |__p1
| |
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---------
| |__p2
| |
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Then, the S(2,1) or S(1,2) is the mutual coupling between them. Please display the current distribution on the 2 patch antenna array:
You can set Voltage Source = 1 on port 1, and Voltage Source = 0 on port 2 while port 2 has a source impedance Rs2 not equal to 0. You will see strong current on patch 1 while relatively weaker current on patch 2. You will also see V2 and I2 are not 0. There will be power delivered to the Rs2. It means that we only excite the port 1 while the port 2 receive coupled power.
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