Antenna Fundamentals and Integration Advanced Antenna Topics Informational

What is the scan impedance of an antenna element in an array and how does it differ from isolated impedance?

Q: Can I simulate scan impedance?

Yes. Use the infinite array approximation: simulate a single unit cell with periodic boundary conditions (Floquet port excitation in HFSS or CST). The periodic boundary conditions enforce the correct mutual coupling for the specified scan angle. Sweep the scan angle from broadside to the maximum scan angle. The result is the exact scan impedance for an infinite periodic array, which is a good approximation for large finite arrays (> 10x10 elements). For small arrays: simulate the full finite array with all elements excited.

The scan impedance (also called active impedance) of an antenna element in an array is the input impedance seen at the terminals of one element when all other elements in the array are simultaneously excited with the appropriate amplitude and phase for beam steering to a specific scan angle. It differs from the isolated impedance because mutual coupling between elements causes each element's impedance to change depending on the excitation of its neighbors. The scan impedance is: Z_scan = Z_self + sum over all other elements of (Z_mutual_0n x (I_n / I_0) x exp(j psi_n)), where Z_self is the element's self-impedance, Z_mutual_0n is the mutual impedance between the reference element (0) and element n, I_n/I_0 is the amplitude ratio of the excitation, and psi_n is the phase difference (determined by the scan angle). At broadside (all elements in phase): the mutual impedances add constructively, and the scan impedance may be significantly higher or lower than the isolated impedance (for closely spaced dipoles: the scan impedance at broadside is approximately Z_self + 2 x Z_mutual for nearest neighbors). At large scan angles: the mutual coupling terms add with different phases, and the scan impedance can have large reactive excursions. At certain scan angles, the scan impedance can become very large (scan blindness) or very small (poor match), making the array unable to transmit or receive at that angle.

Category: Antenna Fundamentals and Integration

Updated: April 2026

Product Tie-In: Antennas, Arrays, Feeds

Array Scan Impedance Analysis

Understanding scan impedance is essential for phased array design because the array elements must be matched to the transmit/receive modules across all scan angles and frequencies. A 50-ohm match at broadside does not guarantee a good match at 45 degrees scan.

Scan Impedance Behavior

Broadside (theta = 0): All elements are in phase. Mutual coupling contributions add coherently. For closely spaced elements (d < 0.5 lambda): the real part of Z_scan decreases (more current sharing reduces the radiation resistance per element). For widely spaced elements (d > 0.5 lambda): Z_scan approaches the isolated impedance
E-plane scan: As the beam scans in the E-plane, the scan impedance decreases (approaching zero at 90 degrees, grazing). The impedance changes smoothly
H-plane scan: As the beam scans in the H-plane, the scan impedance increases. At specific angles, surface wave coupling can cause resonances (scan blindness)
Diagonal plane scan: The impedance behavior is a combination of E-plane and H-plane effects

Scan Impedance Calculations

Scan impedance: Z_scan(theta, phi) = Z_self + sum Z_mutual_0n × exp(j k d_n sin(theta))
Infinite array: Z_scan = Z_Floquet(theta, phi) [exact for periodic arrays]
Broadside scan impedance: R_scan ~ R_isolated × (lambda/(2d))^2 for dense arrays
For dipoles at d = 0.5 lambda: Z_scan(broadside) ~ 70-100 ohms
At d = 0.3 lambda: Z_scan(broadside) ~ 120-200 ohms

Common Questions

Frequently Asked Questions

How do I design a matching network for varying scan impedance?

Design the matching network for the average or most-used scan angle (often broadside). Accept higher VSWR at extreme scan angles. Use a wideband matching approach (wideband balun or tapered transition) that provides a reasonable match across the scan impedance locus. Alternatively: design for the scan impedance at the worst-case scan angle to ensure the VSWR never exceeds the specification. Advanced approach: use a tunable matching network or reconfigurable feed that adjusts with scan angle.

What is scan blindness?

Scan blindness occurs at specific scan angles where the scan impedance becomes very large (or small), causing near-total reflection of power at the element terminals. It is caused by a resonance between the array and a surface wave or Floquet mode: at the blind angle, a surface wave is strongly excited and carries power along the array surface instead of radiating it. Scan blindness is most common in arrays with substrate-backed elements (microstrip patches) where surface waves propagate in the substrate. Prevention: use thin substrates (h < 0.05 lambda), low-Er materials, or substrate perforation to suppress surface waves.

Can I simulate scan impedance?

Yes. Use the infinite array approximation: simulate a single unit cell with periodic boundary conditions (Floquet port excitation in HFSS or CST). The periodic boundary conditions enforce the correct mutual coupling for the specified scan angle. Sweep the scan angle from broadside to the maximum scan angle. The result is the exact scan impedance for an infinite periodic array, which is a good approximation for large finite arrays (> 10x10 elements). For small arrays: simulate the full finite array with all elements excited.

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