Antenna Mutual Coupling
Antenna to antenna mutual coupling describes energy absorbed by one antenna's receiver when another nearby antenna is operating. That is, mutual coupling is typically undesirable because energy that should be radiated away is absorbed by a nearby antenna. Similarly, energy that could have been captured by one antenna is instead absorbed by a nearby antenna. Hence, mutual coupling reduces the antenna efficiency and performance of antennas in both the transmit and receive mode. Recall that by reciprocity, antennas have the same properties in the transmit and receive mode, so by understanding coupling loss in the transmit mode we can understand coupling loss in the receive mode.
This is illustrated in Figure 1:
Figure 1. Illustration of Mutual Coupling. A nearby receive antenna interferes with a transmiting antenna and reduces some of the radiated energy.
For an antenna to radiate well, we want all of the resistance seen by the antenna to be radiation resistance. This means that all resistance (lossy dielectrics, resistors, etc) should be removed. This will maximize the power radiated away. When you have a closely spaced antenna, as in Figure 1, the receiver of the second antenna is a 50 Ohm load. The second antenna is collecting as much energy as it can, and is hence making a virutal "highway" from the first (transmitting) antenna to the 50 Ohm load. This will zap the power of the first antenna. This is undesirable, and this loss is known as antenna coupling, or mutual coupling. The more energy absorbed by the nearby antenna, the less will be transmitted for communication to the far field antennas we want to communicate.
Mutual coupling can be quantified by measuring the antenna isolation. In addition, the antenna efficiency can be measured with and without the second antenna present; this will determine the magnitude of the efficiency loss due to the mutual coupling.
Mutual coupling degrades not just the antenna efficiency, but it can alter the antenna's radiation pattern as well. This is one of the limitations involved in antenna arrays. If the energy received by all the antennas in your array are highly correlated, then you don't really have an antenna array; rather it will function more like one large antenna, which makes beam steering much less effective.
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