---
title: Tightly-Coupled Dual-Polarized AVA
url: https://www.emergentmind.com/topics/tightly-coupled-dual-polarized-antipodal-vivaldi-antenna-tc-ava
type: topic
---

# Tightly-Coupled Dual-Polarized AVA

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 [2511.16472].

## 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 [2511.16472].

## 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) [2511.16472].

## 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 [2511.16472].

## 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 [2511.16472].

## 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 [2511.16472].

## 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 [2511.16472].

## 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 [2511.16472].

Source: https://www.emergentmind.com/topics/tightly-coupled-dual-polarized-antipodal-vivaldi-antenna-tc-ava