Antenna Arrays: A Computational Approach

6 Mutual Coupling

An antenna never exists in isolation. Radiation from a transmitting source illuminates the receive antenna as well as everything in its environment. The time-varying fields strike various objects in the environment that absorb and reradiate the fields. As a result, the antenna performs differently in its environment than in free space. Mutual coupling is the interactions between an antenna and its environment is called mutual coupling. It has three components [1]:

1. Radiation coupling between two nearby antennas.
2. Interactions between an antenna and nearby objects, particularly conducting objects.
3. Coupling inside the feed network of an antenna array.

This chapter examines the radiation coupling between elements in an antenna array. This is the final step toward the design of realistic phased array antennas.

Figure 6.1a is a diagram of an isolated antenna that is matched to its transmission line: The antenna impedance (Z_a) equals the transmission line impedance (Z₀). A wave travels from the transmitter to the antenna (V⁺) with no reflection (V⁻ = 0), so the reflection coefficient is zero, Γ = 0. In Figure 6.1b an identical second antenna is placed near the first one. The second antenna radiates a wave that is received by the first antenna. This received wave travels from the antenna back to the transmitter (|V⁻| > 0). Thus, the signal received from the second antenna looks like a reflected wave in the first antenna. A reflected wave is associated with an impedance ...