# Capacitively and Differentially-Fed Shared Tx/Rx Antenna With Shielding Cavity for IBFD Systems

Daniel Chaya, Marc Le Roy, Raafat Lababidi, André Pérennec, Roland

Gautier, Anthony Fiche, Phillipe Pouliguen, Olivier Roncière

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Daniel Chaya, Marc Le Roy, Raafat Lababidi, André Pérennec, Roland Gautier, et al. Capacitively and Differentially-Fed Shared Tx/Rx Antenna With Shielding Cavity for IBFD Systems. 2025 IEEE Conference on Antenna Measurements and Applications, IEEE, Nov 2025, Antibes Juan-Les-Pins, France. hal-05450476

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# Capacitively and Differentially-Fed Shared Tx/Rx Antenna With Shielding Cavity for IBFD Systems

Daniel Chaya

Lab-STICC

University of Brest, France

daniel.chaya@univ-brest.fr

Marc Le Roy

Lab-STICC

University of Brest, France

marc.leroy@univ-brest.fr

Raafat Lababidi

Lab-STICC

ENSTA | Institut Polytech. Paris

raafat.lababidi@ensta-bretagne.fr

André Pérennec

Lab-STICC

University of Brest, France

andre.perennec@univ-brest.fr

Roland Gautier

Lab-STICC

University of Brest, France

roland.gautier@univ-brest.fr

Anthony Fiche

Lab-STICC

University of Brest, France

anthony.fiche@univ-brest.fr

Philippe Pouliguen

AID, Agence Innovation Défense,
Paris, France

philippe.pouliguen@intradef.gouv.fr

Olivier Roncière

DGA MI,

Bruz, France

olivier.ronciere@intradef.gouv.fr

**Abstract** — Electronic warfare (EW) systems demand unprecedented integration of sensing, jamming, and communication capabilities within Size, Weight, and Power (SWaP) constrained platforms. This paper presents a novel cavity-backed cruciform (plus-shaped) patch antenna designed for in-band full-duplex (IBFD) operation in EW applications. The proposed design addresses critical limitations of existing IBFD antenna solutions by employing a cruciform radiating element with differential feeding, capacitive coupling, and a metallic cavity structure for enhanced Self-Interference Cancellation (SIC). The cruciform geometry enables improved near-field cancellation through orthogonal current distributions, achieving over 100 dB SIC under ideal conditions and ~80 dB SIC under 5° phase imbalance. The cavity-backed architecture supports compact phased-array integration with 0.5λ element spacing, demonstrating improved inter-antenna isolation in a 1×2 array configuration. Full-wave electromagnetic simulations validate high efficiency and stable radiation characteristics across array configuration. By combining high isolation, compact integration, and robustness to feed imbalance, the proposed antenna represents a transformative solution for next-generation IBFD-enabled EW systems, where concurrent jamming and communication are essential.

**Keywords** — Full-Duplex, electronic warfare (EW), shared antenna, Self-interference, Near-field cancellation, capacitive and differential feeding, cavity isolation.

## I. INTRODUCTION

The evolution of modern electronic warfare (EW) systems is driven by the need to integrate sensing, jamming, communications, and intelligence within SWaP-constrained platforms [1] [2]. Future defence systems must also operate in congested and contested electromagnetic environments, where spectrum scarcity and adversarial agility demand highly integrated and adaptable solutions. In-band full-duplex (IBFD) technology, which enables simultaneous transmission and reception in the same frequency band, has emerged as a transformative approach [3]. Initially developed for commercial networks for its potential to double spectral efficiency, IBFD offers even greater advantages in EW and integrated sensing and communication (ISAC) applications. It enables concurrent jamming and interception, supports shared-aperture operation, reduces hardware duplication, and enhances situational awareness through improved spectrum utilization.

The primary challenge in IBFD systems is self-interference (SI), where the transmitted signal can be several orders of magnitude stronger than the desired received signal,

<figure><img src="image_2.png" /><figcaption>Fig. 1. Illustration of an EW IBFD antenna array with simultaneous Tx/Rx (SI: self-interference)</figcaption></figure>

potentially saturating the receiver and masking weak incoming signals. Effective self-interference cancellation (SIC) requires a multi-stage strategy that combines passive antenna isolation, analog/RF suppression, and digital cancellation. Among these, the antenna stage is most critical, as strong passive isolation directly helps maintaining the expected dynamic range and linearity requirements of subsequent active stages, a key consideration for SWaP-constrained IBFD EW platforms [4]-[5]. This concept is presented in Fig. 1, showing a full-duplex EW antenna panel able to perform simultaneous transmission-reception or jamming-reception, highlighting self-interference between the TX and RX paths.

Recent research has proposed multiple approaches to improve Tx/Rx isolation in IBFD antennas. A common limitation of early solutions is their reliance on large antenna separations or bulky decoupling structures, which restrict scalability to phased arrays and constrain essential functions such as adaptive beamforming and null steering. To overcome these challenges, several strategies have been investigated, including orthogonal polarizations [6], structural decoupling [7], and differential feeding for Near-Field Cancellation (NFC) [8], which have demonstrated effectiveness at the single-element level. In particular, NFC has emerged as a particularly promising passive method for reducing Tx/Rx coupling. Monostatic shared-aperture implementations, where transmit and receive paths coexist in the same physical aperture, are especially suited to SWaP-constrained platforms, offering tighter integration than bulkier bistatic alternatives [9] [10]. However, in phased arrays and MIMO antennas, mutual coupling between adjacent elements can degrade both self-interference cancellation (SIC) and cross-interference cancellation (NFC) [8].
cancellation (XIC), posing a major challenge for array scalability.

To address integration and mechanical limitations in shared-aperture IBFD antennas, this work presents a capacitively and differentially-fed shared Tx/Rx design to maintain excitation symmetry and improve robustness. A shielding cavity surrounding the array is further introduced to suppress inter-element coupling, enabling dense integration with high isolation and efficient radiation. Together, these features support compact, scalable IBFD arrays suitable for SWaP-constrained platforms.

## II. ANTENNA LAYOUT AND DESIGN METHODOLOGY

### A. Differentially-Fed Rectangular Patch

The foundational stage of the proposed full-duplex antenna architecture is a rectangular microstrip patch excited via differential feeding to implement NFC, a key technique for passive SIC. Two symmetrically positioned feed points are driven 180° out of phase, establishing a balanced mode distribution across the patch. This arrangement produces destructive interference in the near-field region, where self-interference is strongest, while preserving constructive combination in the far-field to maintain efficient radiation. [11].

The theoretical behaviour of passive SIC in this configuration can be derived from the principle of wave superposition [12]. Considering two isotropic sources separated by a distance d, each driven with equal amplitude A and a phase difference φ = 180°, the total electric near-field at an observation point P is expressed as the superposition of the individual contributions:

$$ 
\begin{aligned}
E_{total} &= E_1 + E_2 \\
E_{total} &= \frac{A}{r_1}e^{j(\omega t - \beta r_1)} + \frac{A}{r_2}e^{j(\omega t - \beta r_2 + \pi)}
\end{aligned} 
\quad (1) $$

where β = $\frac{2\pi}{\lambda}$ is the phase constant, and r₁ and r₂ are the distances from the two sources to P. For a receiver positioned along the boresight axis at the antenna's centre (r₁ = r₂ = r), the expression simplifies to:

$$ E_{total} = \frac{A}{r}e^{j(\omega t - \beta r)}(1 + e^{j\pi}) = 0 \quad (2) $$

This confirms that perfect NFC can theoretically occur along the central axis. However, in practical implementations, small deviations from equal amplitude or an exact 180° phase shift, arising from fabrication tolerances, feed-network imbalance, or component mismatches, inevitably limit the achievable cancellation.

Building upon the foundational work of our research group [9] which demonstrated the feasibility of closely-spaced differential feeds for achieving high SIC in a single element, the current design implements this approach in a 5 GHz monostatic shared-aperture configuration and addresses key limitations identified in the previous study. At this frequency, achieving effective NFC requires sub-wavelength feed spacing at around $\lambda/20$, which inherently forces the receive ports into close proximity with the transmit feeds [10]. This proximity increases intra-element mutual coupling between Tx and Rx ports and introduces significant challenges in fabrication, connector integration, and mechanical tolerance

<figure><img src="image_3.png" /><figcaption>Fig. 2. (a) Front view of the cruciform shared-aperture structure: g=0.4mm, Lp=16.65mm, Wp=3.95mm, Wsm=3.7mm, Lla=4.1mm, Ls=27mm, Ws=27mm, (b) side view, (c) Single-to-differential feeding schematic.</figcaption></figure>

in PCB layout and assembly. To overcome these constraints, the design was fundamentally revised to decouple feed placement from the patch geometry, allowing greater physical separation between the Tx and Rx ports without compromising NFC. This design reduces inter-port coupling, enhances mechanical robustness, improves design flexibility, and prepares the antenna for future array integration and potential bandwidth optimization.

### B. Cruciform Monostatic Shared-Aperture Architecture

The antenna architecture evolves into a cruciform (plus-shaped) shared-aperture configuration employing capacitive feeding, as illustrated in Fig. 2(a). Operating at 5 GHz, it consists of two orthogonal rectangular patch arms forming the cruciform geometry, with one arm dedicated to transmission (Tx) and the other to reception (Rx). Capacitive differential feeding serves as the primary mechanism for NFC by decoupling port placement from the patch edges, allowing the Tx and Rx ports to be separated by more than $\lambda/4$. This increased physical separation enhances mechanical feasibility, simplifies fabrication, and maintains effective SIC for each channel.

Each patch arm is excited through capacitive (proximity) coupling via a strip separated by a gap g from the patch edge. Differential excitation is realized with a 1-to-2 balun that generates the required 180° phase difference as illustrated in
Fig. 2 (c). The capacitive feed allows effective control of impedance matching through the coupling gap g and strip dimensions, eliminating the need for external matching networks. In addition, relocating the feed network (baluns) to internal PCB layers clears the radiating surface of obstructive traces, facilitating dense array integration and reducing parasitic inter-element coupling [13] [14] [15].

Beyond addressing port isolation requirements and strengthening the NFC technique, the capacitive-fed architecture offers a scalable platform for future enhancements. Its inherent reactance compensation allows thicker or lower-permittivity substrates to be used, reducing the quality factor Q and broadening impedance bandwidth. While a simple air gap could achieve this, its mechanical fragility limits practical deployment. A cavity-backed implementation, however, would allow the creation of an optimized air gap beneath the substrate (between the non-metallized bottom layer of patch-PCB and the bottom of the cavity), offering enhanced bandwidth, structural rigidity, and RF shielding in array environments. Furthermore, strategically placing the capacitive feed near the patch edge could allow a single strip to serve adjacent elements, simplifying routing and supporting compact, shared-aperture layouts [16].

### C. Simulated Performance and Validation

The antenna was designed and optimized using FDTD full-wave electromagnetic simulations in CST Microwave Studio. The simulated S-parameters, shown in Fig. 3, indicate well-matched impedance at the target frequency of 5 GHz, with a -10 dB bandwidth of 98 MHz for both Tx and Rx ports and a Tx-Rx isolation over 100 dB under idealized conditions (0 dB amplitude imbalance and 180° phase difference). This high isolation results from the near-field cancellation mechanism enabled by the symmetric capacitive differential feed. The simulated total efficiency of 88%, and the realized gain reaches 5.9 dBi at 5 GHz, confirming that the design maintains high radiation performance while achieving strong self-interference suppression.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Transmission and reflection coefficients with realized gain of the proposed antenna</figcaption></figure>

## III. CRUCIFORM 1×2 SHARED-APERTURE ARRAY

Scaling a single self-decoupling shared antenna element into a multi-element array is a critical step in realizing

practical in-band full-duplex (IBFD) systems. Phased arrays provide key capabilities for IBFD, including spatial isolation through beamforming (steering the transmit beam away from the receive beam), higher gain for extended range, and adaptive spatial nulling to actively cancel self-interference across different angles of arrival [17] [18]. However, array scalability introduces challenges that can significantly affect system-level performance. In particular, inter-element coupling increases with proximity, and practical imperfections in the differential feeding network, especially phase imbalance which can degrade NFC and reduce SIC [19].

<figure><img src="image_5.png" /><figcaption>Fig. 4. 1x2 Antenna array configuration (a) without cavity, (b) with cavity: $L_t=66$mm, $W_t=33$mm, $Sep_D=33$mm, $C_{VLdc}=27$mm, $C_{vw}=33$mm, $C_{lw}=5$mm, $C_{ow}=2.5$mm, (c) Differential Feeding schema with 1-to-4 baluns</figcaption></figure>

To investigate these effects, the proposed cruciform antenna was scaled into a 1×2 shared-aperture array and its performance compared to those of a cavity-backed structure introduced to enhance isolation and reduce sensitivity to feed imperfections.

### A. Array Configuration and Practical Feed Imperfections

The 1×2 array shown in Fig. 4 (a) consists of two cruciform elements, each with its capacitive feeding strips and via-fed ports. As illustrated in Fig. 4 (c), differential excitation is defined as follows: for the first element, Tx1 combines ports 1 (0°) and 3 (180°), while Rx1 combines ports 2 (0°) and 4 (180°); the second element follows the same convention with
Tx2 and Rx2 using ports 5/7 and 6/8, respectively. The global Tx and Rx ports are obtained by coherently combining the differential outputs from both elements.

To comprehensively assess system-level performance, a two-stage simulation methodology was implemented, first isolating intrinsic array behavior and then accounting for practical feed imperfections. In the first stage, ideal differential feeding was applied to establish a baseline and characterize the array’s inherent performance. In the second stage, practical effects were incorporated by considering a commercial balun (Mini-Circuits BLJC1-542R+) exhibiting amplitude and phase imbalances of approximately ±0.5 dB and ±2.5-5°, respectively [20]. The simulation emphasizes phase imbalance, using a 5° deviation, as it predominantly affects NFC, whereas amplitude variations are expected to play a smaller role. Finally, the full 8-port S-parameters (four ports per element) were de-embedded into a 2-port network representing the global Tx and Rx channels, enabling a comprehensive evaluation of system-level self-interference cancellation (SIC).

## B. Cavity Integration

To mitigate inter-element coupling and reduce sensitivity to feed imperfections, a metallic cavity was integrated around the 1x2 cruciform array (33 × 33 × 14.8 mm), as shown in Fig. 4 (b), fully enclosing the elements while leaving the top aperture open. The cavity suppresses parasitic surface currents and reflects impinging fields, thereby reducing near-field coupling between elements. Unlike electromagnetic band-gap (EBG) structures, which are inherently frequency-selective and provide isolation only over narrow bands, the cavity offers broadband isolation enhancement without modifying the substrate, facilitating integration into a multilayer PCB stack-up. Its dimensions were optimized through full-wave simulations to balance reflection coefficients across both Tx and Rx ports. Analytical calculations further confirm that the cavity’s fundamental mode lies below the operating band, ensuring it primarily functions as a near-field shield rather than a resonant radiating structure.

The cavity dimensions were primarily constrained by the array periodicity (~0.5λ at 5 GHz, yielding L = W = 33 mm) to preserve proper element spacing. The height was optimized through full-wave simulations, ranging from 7.5 mm to λ/4 (~15 mm), with a final value of 14.8 mm selected to achieve balanced reflection coefficients across Tx and Rx ports. Potential cavity resonances were analytically validated using the standard rectangular cavity equation:

$$f_{mnp} = \frac{c}{2} \sqrt{\left(\frac{m}{L}\right)^2 + \left(\frac{n}{W}\right)^2 + \left(\frac{p}{H}\right)^2} \quad (3)$$

where L, W, and H are the cavity dimensions, m, n, and p are mode indices, and c is the speed of light. For the closed cavity, the two degenerate fundamental modes TE₁₀₀ and TE₀₁₀ occur at ~4.55 GHz, below the 5 GHz operating band. The next mode, TE₁₁₀, appears at ~6.43 GHz. However, the open-top aperture renders these eigenmodes strongly leaky, resulting in a low loaded quality factor Q and preventing sharp in-band resonances. Consequently, the cavity primarily serves as a near-field shield rather than a radiating structure [21] [22].

Furthermore, the cavity complements the capacitive feeding technique. While capacitive feeding may be combined with air gaps between the substrate and the ground plane to reduce effective substrate permittivity and enhance impedance bandwidth, the cavity provides mechanical support to maintain these gaps together with inter-element isolation enhancement. This combination brings possibilities for both improved bandwidth and decoupling, ensuring robust performance in dense array configurations.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Array Isolation performance (a) Inter and Intra element Coupling pair, (b) Global SIC under ideal and 5° phase imbalance</figcaption></figure>

## C. Results and Discussion

Using the previously established two-stage simulation framework, the 1x2 array was initially evaluated under ideal differential feeding, providing a baseline for intrinsic isolation and self-interference suppression. Intra- and inter-element coupling were then analysed through key S-parameters, as shown in Fig. 5 (a). These include: S₂₁ represents intra-port coupling within element 1; S₆₁ indicates cross-element inter-port coupling from element 1 to element 2 (cross-polar); S₂,Tx1 and S₆,Tx1 describe the differential contributions from Tx₁ toward the Rx ports of elements 1 (port 2) and element 2 (port 6), and Sₓ₁,Tₓ₂ and Sₓ₂,Tₓ₁ capture the corresponding cross-element differential paths. These results demonstrate that the cavity effectively acts as a local Faraday cage, consistently reducing both intra- and inter-element coupling and preserving high isolation across the array.
<figure><img src="image_7.png" /><figcaption>Fig. 6. Realized Gain Patterns</figcaption></figure>

Fig. 5 (b) illustrates the system-level transmission coefficients between the global Tx and Rx ports of the 1x2 array for four scenarios: with the cavity (WC) and without the cavity (WOC), each under ideal differential feeding (PH = 180°) and a 5° phase imbalance (PH = 175°). The blue curves correspond to the cavity-backed array (WC), while the red curves represent the array without the cavity (WOC).

Under ideal 180° feeding, the intrinsic isolation of the array exceeds 100 dB, with the cavity providing an additional improvement of approximately 6.2 dB compared to the WOC case. For the 175° phase imbalance scenario, global SIC degrades to around 80 dB, with the cavity again enhancing isolation by about 6.06 dB. Although these differences are subtle in the plotted curves, they clearly indicate the cavity’s role in mitigating inter-element coupling and reducing sensitivity to practical phase deviations, thereby stabilizing near-field cancellation. The vertical dashed line at 5 GHz highlights the frequency of interest where these isolation improvements are quantified.

Realized gain patterns, presented in Fig. 6 confirm that the cavity preserves radiation performance with a peak gain of ~8 dBi, Total radiation efficiency decreases from 85% to 77% due to ohmic losses, a manageable trade-off in IBFD systems where SIC is critical. Impedance bandwidth remains around 90 MHz across all scenarios, demonstrating consistent performance both with and without the cavity.

## IV. CONCLUSION AND PERSPECTIVES

This paper presented a cavity-backed cruciform patch antenna as a compact shared-aperture solution for IBFD EW systems. By combining cruciform element geometry with near-field cancellation and capacitive differential feeding, the design achieves high Tx/Rx isolation even under practical feed imbalances while remaining scalable to phased arrays. The integrated cavity enhances isolation, mitigates the impact of feed imperfections, and maintains robust radiation performance, demonstrating the feasibility of dense, high-performance IBFD arrays.

The proposed architecture establishes a strong foundation for high-density IBFD arrays, enabling advanced EW

functions such as simultaneous jamming and signal intelligence. Future work will focus on gain and bandwidth enhancement, for example through the integration of an NFC-compatible superstrate, and experimental validation of larger array implementations, paving the way toward compact, multifunctional, and scalable IBFD front-ends for next-generation EW and ISAC systems.

## ACKNOWLEDGEMENT

This work is supported by FullSSAT ASTRID project, funded by the French Defense Innovation Agency (DGA/AID).

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