[Paper Review] Passive Decoupling of Two Closely Located Dipole Antennas

Masoud Sharifian Mazraeh Mollaei, Anna Hurshkainen+3 more|arXiv (Cornell University)|Feb 21, 2018

Energy Harvesting in Wireless Networks14 references3 citations

TL;DR

This paper proposes a passive decoupling technique for two closely spaced dipole antennas using a single passive dipole placed symmetrically between them. By leveraging electromagnetic interference cancellation via induced electromotive forces, the method achieves complete decoupling—suppressing mutual coupling for any excitation amplitudes and phases—verified analytically, numerically, and experimentally at distances as small as d = 3 cm (≈λ/10), with S12 < -17 dB at resonance when h = 0.

ABSTRACT

In this paper, we prove that two parallel dipole antennas can be decoupled by a similar but passive dipole located in the middle between them. The decoupling is proved for whatever excitation of these antennas and for ultimately small distances between them. Our theoretical model based on the method of induced electromotive forces is validated by numerical simulations and measurements. A good agreement between theory, simulation and measurement proves the veracity of our approach.

Motivation & Objective

• To address the challenge of strong mutual coupling in closely spaced dipole antennas, especially when d ≪ λ/10.
• To develop a passive, low-cost, and broadband solution for decoupling that avoids active circuitry or EBG structures.
• To achieve complete decoupling—valid for arbitrary excitation magnitudes and phases—without requiring adaptive or tunable components.
• To validate the theoretical framework through numerical simulations and experimental measurements across varying heights of the passive scatterer.
• To demonstrate feasibility in practical applications such as MIMO systems and MRI array coils where low crosstalk and high SNR are critical.

Proposed method

• A theoretical model based on the method of induced electromotive forces is developed to analyze mutual coupling between two active dipoles and a passive dipole in between.
• The system is modeled using Kirchhoff’s voltage law, with self-impedance Z and mutual impedances Z12, Z21, and additional shared impedances Z1s, Z2s introduced by the passive scatterer.
• The passive dipole is assumed to be resonant and symmetrically positioned at the midpoint between the two active dipoles, with its length and radius chosen to match the resonance of the active elements.
• Numerical simulations are performed using full-wave electromagnetic solvers to validate the theoretical predictions across varying heights h of the passive scatterer.
• An experimental setup is constructed using two dipole antennas and a passive copper wire scatterer on a foam board, with S-parameters measured via a VNA to assess isolation.
• The measured S12 is normalized using a MATLAB-based method to simulate ideal dual-side matching, enabling comparison with simulations under matched and mismatched conditions.

Experimental results

Research questions

• RQ1 Can a single passive dipole placed symmetrically between two active dipole antennas achieve complete decoupling for any arbitrary excitation amplitudes and phases?
• RQ2 Is passive decoupling feasible at extremely small inter-antenna distances (d ≪ λ/10), where conventional EBG structures become impractical?
• RQ3 How does the height h of the passive scatterer above the plane of the active dipoles affect the degree of decoupling and isolation?
• RQ4 To what extent does the theoretical model based on induced electromotive forces accurately predict the observed mutual coupling suppression?
• RQ5 Can experimental measurements confirm the theoretical and simulated results, particularly in the matched and mismatched regimes?

Key findings

• Complete passive decoupling is achieved when the passive dipole is placed at h = 0 cm (in the same plane as the active dipoles), confirmed by identical minimum frequencies of S12 in both matched and mismatched conditions.
• The deepest isolation of S12 = -17 dB is achieved at 294.2 MHz in the matched case when h = 10 mm, but this is not complete decoupling due to frequency shift between matched and mismatched regimes.
• For h = 0, the frequency of minimum S12 remains consistent across matched and mismatched cases (293 MHz), confirming complete decoupling as per theoretical criteria.
• Theoretical predictions, numerical simulations, and experimental measurements show excellent agreement, validating the proposed method across all tested configurations.
• The method achieves significant mutual coupling suppression even at d = 3 cm (≈λ/10), demonstrating feasibility for applications requiring low crosstalk at small inter-element spacing.
• The operational bandwidth for complete decoupling is narrow (relative bandwidth ~0.1%), which may limit efficiency in wideband applications, though it remains viable for narrowband systems like MRI.