Meaning
The electromagnetic coupling between closely spaced radiating elements affects the input impedance and radiation pattern of each individual antenna. Calculations of spatial mutual impedance help designers predict how much energy from one antenna is absorbed or reflected by its neighbor. This parameter dictates the layout of multi-antenna arrays in compact wireless devices.
Coupling Mechanism
Electric and magnetic fields from a transmitting antenna induce currents on the surfaces of adjacent conductive structures. These induced currents alter the effective impedance seen at the feed point of the neighboring antenna. When spatial mutual impedance is high, the power transfer efficiency to the antenna declines, leading to signal degradation.
System Impact
MIMO systems rely on independent signal paths to increase data rates and improve network capacity. High coupling between the antenna elements ruins this independence, reducing the spatial multiplexing gain of the system. In severe cases, the transmitter power amplifiers can be damaged by the high levels of reflected power caused by this impedance mismatch.
Mitigation Strategy
Increasing the physical separation between the radiating elements represents the most effective way to reduce the coupling. When space is limited within a compact enclosure, engineers use decoupling networks or parasitic isolation elements to cancel the induced currents. Testing the finished array on a network analyzer allows developers to measure the scattering parameters and adjust the matching networks to compensate for the remaining coupling.
This step ensures that the wireless module maintains its target throughput in high-density device configurations.
