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What is mutual coupling between antennas?

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Mutual coupling is the interaction between antennas in proximity: current on one element induces current on another, changing each element’s impedance, pattern and efficiency. It is described by mutual impedance Zmn or the S-parameter matrix, and in arrays leads to scan-dependent active impedance and, in severe cases, scan blindness.[1][2]

How is mutual coupling quantified?

For an N-port array, V = ZI or b = Sa. Element m sees an active reflection coefficient Γmact = Σn Smnan/am, which depends on the excitations of all elements and therefore on scan angle.[2] Two parallel half-wave dipoles side by side at 0.1λ spacing have mutual resistance near 70 Ω, falling and oscillating with distance.[1]

What is an embedded element pattern?

The pattern of one element excited with all others terminated in matched loads. It includes coupling effects and, in large arrays, its gain relates directly to the active reflection coefficient: Ge(θ) ≈ (4πAcell/λ²)(1−|Γact(θ)|²)cosθ, a result connected to Hannan’s element-gain paradox.[3][2]

Why does coupling matter for MIMO and compact devices?

Coupling between closely spaced terminals lowers total efficiency (power is absorbed in the other port) and increases pattern correlation. Decoupling techniques include spacing, orthogonal polarization, neutralization lines, defected ground structures, parasitic decouplers and decoupling/matching networks.[4][5] See MIMO antennas.

Frequently asked questions

Is mutual coupling always bad?

No; Yagis and connected or tightly coupled wideband arrays depend on it.

What isolation is typical between phone antennas?

Designers often target 10–15 dB or better, harder at low bands.

Can coupling be simulated?

Yes; full-wave simulation of the whole array gives the S-matrix and embedded patterns.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  2. “Phased Array Antenna Handbook, 3rd ed.”. R. J. Mailloux, Artech House. 2017.
  3. “The Element-Gain Paradox for a Phased-Array Antenna”. P. W. Hannan, IEEE Transactions on Antennas and Propagation, vol. 12, no. 4, pp. 423-433. July 1964.
  4. “Foundations of MIMO Communication”. R. W. Heath Jr. and A. Lozano, Cambridge University Press. 2018.
  5. “Modern Small Antennas”. K. Fujimoto and H. Morishita, Cambridge University Press. 2013.
Categories: Arrays and beamforming