Bandwidth Enhancement of Antenna Arrays Utilizing Mutual Coupling between Antenna Elements

The mutual coupling effect between antenna elements on an array's bandwidth is investigated using scattering parameters instead of the mutual impedance. First, an approximate expression is derived for matched voltage standing wave ratio (VSWR) bandwidth of a tuned antenna, which reveals that the bandwidth is inversely proportional to the magnitude |Γ′0(𝜔0)| of the frequency derivative of the reflection coefficient. Next, considering linear antenna arrays with corporate feed as an example, closed-form expressions of the reflection coefficient are derived at the input port of the feeding network, which shows that the active reflection coefficient of an array is the linear superposition of elements' passive reflection coefficient 𝑆11 and the mutual coupling coefficient 𝑆12 from adjacent elements. The VSWR bandwidth expressions for an array imply that bandwidth enhancement of the overall array can be achieved when the element passive reflection coefficient 𝑆11 and mutual coupling 𝑆12 are cancelled, as well as the frequency derivatives 𝑆′11 and 𝑆′12 also cancel each other. Slot arrays and a two-element Vivaldi array are investigated to verify the validity of our theoretical analysis. Numerical and experimental results are presented to successfully demonstrate the bandwidth enhancement of antenna arrays utilizing mutual coupling effect.

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