Tightly-Coupled Dual-Polarized AVA

• TC-AVA is a wideband, dual-polarized antenna design using overlapping antipodal Vivaldi elements to achieve impedance matching from 3 to 20 GHz.
• It employs a novel interleaved-leaf topology on thin PCB substrates, enhancing low-frequency extension and ensuring high port isolation and polarization purity.
• The design supports compact dual-polarized arrays for applications like UWB positioning, DoA sensing, and frequency-agile communications with robust simulation-validated performance.

The tightly-coupled dual-polarized antipodal Vivaldi antenna (TC-AVA) is a wideband, dual-polarized antenna array architecture utilizing overlapping antipodal Vivaldi elements to achieve instantaneous impedance matching across 3–20 GHz. Employing a novel interleaved-leaf topology in thin PCB substrates, the TC-AVA demonstrates significant lower-band extension compared with isolated Vivaldi elements, alongside high port isolation and polarization purity. The approach facilitates compact, PCB-manufacturable, dual-polarized aperture arrays for applications in wideband positioning, sensing, spectrum monitoring, and modern frequency-agile communications (Lindvall et al., 20 Nov 2025).

1. Antenna Geometry, Materials, and Fabrication

Each AVA element is realized on a 0.254 mm Rogers RO4350B substrate (relative permittivity $\varepsilon_r\approx3.66$, loss tangent $\tan\delta\approx0.0037$), with 35 µm copper traces on both sides, forming the antipodal tapers. No additional ground plane is present; the copper cladding constitutes the entirety of the radiating structure. The geometric profile of each leaf is defined via exponential taper equations:

$x_i(y) = \pm c_i \cdot \exp(k_i y) \mp c_a$

$x_o(y) = \pm c_o \cdot \exp(k_o y^2) \pm c_b$

Parameter values for the geometry are summarized below:

Parameter	Value (mm)	Description
$c_i$	0.03069	Inner-taper coefficient
$k_i$	0.12585	Inner-taper exponent
$c_a$	0.48535	Inner-taper offset
$c_o$	0.01024	Outer-taper coefficient
$k_o$	0.01058	Outer-taper exponent
$c_b$	0.44442	Outer-taper offset
$w_b$	120.00	Flare opening width
$w$	76.39	Mouth width
$w_{\text{ol}}$ X/Y	3.90/4.00	Overlap width (X-/Y-pol)
$h$	48.60	Antenna height

A small feedline gap $w_{\text{sp}}=0.00$–$0.10$ mm provides the balanced transition for the 100 Ω microstrip feed. Fabrication leverages standard PCB etching and through-hole plating on Rogers 4350B, enabling direct feed junction soldering without impedance discontinuity (Lindvall et al., 20 Nov 2025).

2. Tight Coupling Mechanism

Adjacent AVA leaves on opposite sides of the same PCB are physically overlapped in the flare region by $w_{\text{ol}}=3.9$–$4.0$ mm, introducing strong capacitive coupling. This capacitive reactance at low frequencies effectively increases the overall stored capacitance per element, lowering the resonance (cut-off) frequency and enabling an expanded operational low band.

The mutual impedance network for a two-element slice is:

$$\begin{bmatrix} V_1 \ V_2 \end{bmatrix} = \begin{bmatrix} Z_{11} & Z_{12} \ Z_{12} & Z_{11} \end{bmatrix} \begin{bmatrix} I_1 \ I_2 \end{bmatrix}$$

$$Z_{11}(\omega) = R_0 + j\omega L - \frac{j}{\omega C_0}, \quad Z_{12}(\omega) = j\omega M - \frac{j}{\omega C_{12}}$$

$k(\omega) = \frac{Z_{12}(\omega)}{Z_{11}(\omega)}$

For in-phase excitation $(I_1=I_2)$, the input impedance is $$Z_{\rm in}(\omega) = Z_{11}(\omega) + Z_{12}(\omega)$$. The coupled low-frequency pole position yields a lower cut-off:

$$f_{\text{low,coupled}} \approx \frac{1}{2\pi\sqrt{L (C_0 + C_{12})}}$$

$$\frac{f_{\text{low,coupled}}}{f_{\text{low,iso}}} = \sqrt{ \frac{C_0}{C_0 + C_{12}} }$$

For $C_{12}\approx 0.44\,C_0$ (X-pol), $f_{\text{low}}$ shifts from 3.75 GHz to 3.00 GHz (20% extension); for $C_{12}\approx 0.65\,C_0$ (Y-pol, 4.0 mm overlap), to 2.75 GHz (25% extension) (Lindvall et al., 20 Nov 2025).

3. Dual-Polarized Feed Network

The feed architecture comprises three PCBs: two supporting the orthogonal X- and Y-polarized AVA arrays, plugged perpendicularly into a central feed PCB. The feed PCB (120 × 120 mm, 0.254 mm thick, double-sided copper) routes balanced 100 Ω microstrip feeds from each element through slots, transitioning via 90° turns and plated through-holes to maintain 100 Ω striplines.

Each polarization feed path is fully symmetric and interleaved across the PCB stack, preventing crossover or shorts. At the feed PCB base, a 2:1 wideband surface-mount balun (e.g., Mini-Circuits MTX2-183+) transforms impedance for 50 Ω SMA connectors. Simulated cross-polar isolation ($S_{21}$ from X$\rightarrow$Y) is $<-20$ dB across 3–20 GHz, and the Smith chart trajectory for the 100 Ω balanced section matches smoothly to the balun’s insertion impedance, eliminating the need for additional lumped matching networks (Lindvall et al., 20 Nov 2025).

4. Simulated Performance Metrics

Simulated Total Active Reflection Coefficient (TARC) under in-phase drive for all three central array elements indicates $\leq-6$ dB from 3.0–20 GHz for X-pol and 2.74–20 GHz for Y-pol—compared with 3.75–20 GHz for isolated AVAs. Simulated $S_{12}$ (element–element coupling) is approximately $-8$ dB at 3 GHz (strong at low band), improving to $\approx-20$ dB above 9 GHz. Cross-polar element coupling remains $<-20$ dB throughout the operational band.

E-plane broadside realized gain for the 3×3 array is $\approx$3 dBi at 3 GHz, 8 dBi at 11 GHz, and 5 dBi at 20 GHz; on-axis gain peaks at 5.4 dBi (X-pol) and 5.3 dBi (Y-pol). Simulated total efficiency exceeds $-1$ dB ($\approx$80%) above 5 GHz, decreasing to $-2.5$ dB ($\approx$56%) at 3 GHz. Half-power beamwidths are $\approx$90°/110° (E/H-plane) at 3 GHz and $\approx$15°/16° at 20 GHz. Simulated main-beam cross-polarization discrimination (XPD) remains below $-20$ dB across the full band (Lindvall et al., 20 Nov 2025).

5. Array Architecture and Electromagnetic Behavior

A 3×3 array configuration with element spacing $d=24.1$ mm (corresponding to $\approx0.24\lambda$ at 3 GHz, $1.6\lambda$ at 20 GHz) enables wideband operation. The half-wavelength spacing threshold occurs at 6.2 GHz, allowing grating-lobe-free scanning up to $\pm$30° around this frequency. Strong low-band coupling stabilizes the active impedance against scan-angle variations below 6 GHz, whereas above 10 GHz the array operates more like unconnected Vivaldi elements and scan match degrades more quickly (Lindvall et al., 20 Nov 2025).

6. Prototype Fabrication and Validation Approach

A full prototype is under construction, consisting of the completed feed PCB (with slots and balun mounting) and two antenna PCBs (with 0.254 mm slots for leaf overlap/interleaving). Planned characterization includes:

• Wideband VNA measurements (0.5–20 GHz) with port extension and TRL calibration
• Anechoic-chamber broadside gain, efficiency, and beam pattern assessment
• TARC verification using equi-phase excitation

Expected measurement–simulation agreement is within $|\Delta f_{\text{low}}|<5\%$, $|\Delta\text{gain}|<1$ dB, and S-parameters within $\pm2$ dB (Lindvall et al., 20 Nov 2025).

7. Applications, Limitations, and Future Directions

The TC-AVA’s wideband performance, compact size, and manufacturability render it suitable for UWB positioning (e.g., UWB-RTLS), direction-of-arrival (DoA) sensing, spectrum monitoring, and frequency-hopping communication systems (LPI/LPD). In arrays, the approach is applicable to 3–10 GHz wideband phased arrays for automotive, satellite, and spectrum-surveillance scenarios, as well as dual-pol MIMO links for UWB communications in cluttered environments.

Known limitations include the emergence of grating lobes upon beam steering above 15 GHz due to $d>\frac{\lambda}{2}$, and decreased low-band efficiency attributable to feed and dielectric losses. Potential optimizations are: employing a thicker substrate or dielectric superstrate to enhance low-band efficiency, scaling the array to larger sizes (e.g., 8×8) for increased gain, and incorporating active-impedance compensation for improved scan matching above 10 GHz.

The TC-AVA demonstrates, for the first time, that antipodal Vivaldi elements can be tightly coupled and dual-polarized in a thin PCB stack, achieving an instantaneous impedance match from 3–20 GHz, with high isolation and robust radiation performance. Its overlapping-leaf tight coupling and three-PCB feed design enable new compact dual-polarized wideband array solutions in next-generation sensing and communications platforms (Lindvall et al., 20 Nov 2025).