How does broadside coupling affect phased array performance?

In a phased array, each element's input impedance is affected by the fields radiated by all neighboring elements (mutual coupling). As the beam is scanned away from broadside, the active impedance at each element changes, potentially causing significant mismatch. At certain scan angles, the active reflection coefficient can approach unity (scan blindness), where virtually no power is radiated. The broadside coupling coefficient between adjacent elements (typically minus 10 to minus 20 dB for half-wavelength spacing) determines the magnitude of these active impedance variations. Array designers must characterize the full mutual coupling matrix to predict scan performance and design element matching networks that minimize mismatch across the scan volume.

What is the difference between broadside and endfire coupling?

Broadside coupling occurs between elements whose broad radiating surfaces face each other, with radiation directed perpendicular to the array surface. The coupling mechanism is primarily through space waves and surface waves in the array substrate. Endfire coupling occurs between elements arranged along the direction of radiation, with energy coupling through traveling waves along the array axis. For a planar array, the H-plane coupling (between elements along the magnetic field direction) is typically 3 to 6 dB stronger than E-plane coupling at the same spacing, because the element pattern has wider beamwidth in the H-plane.

How is broadside coupling measured?

Broadside coupling between array elements is measured as the S-parameter S21 between two element ports while all other elements are terminated in matched loads (the embedded element pattern condition). For a two-element measurement, this is straightforward: connect a VNA to ports 1 and 2, terminate all other ports in 50 ohms, and measure S21 versus frequency. The coupling typically ranges from minus 10 dB at half-wavelength spacing to minus 25 dB at one-wavelength spacing, decreasing approximately as 1/distance for space-wave coupling and exponentially for surface-wave coupling.

Antennas & Arrays

Broadside Coupling

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The electromagnetic coupling between antenna elements or transmission line conductors whose broad faces are parallel and facing each other. In phased arrays, broadside coupling (mutual impedance) between adjacent elements affects the active impedance at each port as the beam is scanned, potentially causing scan blindness at certain angles. In transmission lines, it describes the stronger coupling achievable when conductors overlap vertically rather than sitting edge-to-edge.

Category: Antennas & Arrays

Typical: −10 to −20 dB (d = λ/2)

Risk: Scan blindness

Understanding Broadside Coupling

In antenna arrays, every element radiates electromagnetic fields that are received by neighboring elements. This mutual coupling modifies each element's input impedance from its isolated value: the active impedance at element n depends on the excitation amplitude and phase of every other element in the array. When the beam scans from broadside (perpendicular to the array) to endfire (along the array), the active impedance changes because the relative phase between elements changes the way their coupled fields add.

At broadside (all elements in phase), the mutual coupling tends to raise the active resistance above the isolated value, which can actually improve the bandwidth. As the beam scans toward endfire, certain scan angles cause the active reflection coefficient to spike (scan blindness), typically associated with surface wave excitation in the substrate. The coupling coefficient between adjacent elements at half-wavelength spacing is typically −10 to −20 dB, depending on element type, substrate, and polarization plane.

Mutual Coupling Equations

Active Impedance (element n):
Z_active,n = Σ_m Z_nm (I_m/I_n)

Mutual Coupling Decay (space wave):
|S₂₁| ∝ 1/d for large d (d = element spacing)

Scan Blindness Condition:
k₀ sinθ_blind = β_sw ± 2πn/d

Where β_sw is the surface wave propagation constant and d is the element period.

Coupling Mechanisms in Arrays

Mechanism	Decay	Plane	Magnitude	Mitigation
Space Wave	1/d	Both E and H	−10 to −15 dB	Element spacing, pattern shaping
Surface Wave	Exponential	E-plane dominant	−15 to −25 dB	Thin substrates, EBG, defected ground
Feed Network	Layout-dependent	Both	−20 to −30 dB	Isolation, good grounding

Common Questions

Frequently Asked Questions

How does it affect phased arrays?

Mutual coupling changes active impedance as the beam scans. At certain angles, the active reflection coefficient approaches unity (scan blindness). Coupling of −10 to −20 dB at λ/2 spacing determines impedance variation magnitude. Designers must characterize the full coupling matrix to predict scan performance.

Broadside vs endfire coupling?

Broadside: elements' broad faces parallel, radiation perpendicular to array. Coupling via space and surface waves. Endfire: elements along radiation direction, coupling via traveling waves. H-plane coupling is typically 3 to 6 dB stronger than E-plane at the same spacing due to wider element beamwidth.

How is it measured?

Measure S21 between two element ports with all others terminated in 50 Ω (embedded element condition). Ranges from −10 dB at λ/2 spacing to −25 dB at λ spacing. Decreases as 1/distance for space waves, exponentially for surface waves.

Related Terms

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