Circularly Polarized Strip-Crossed Dipole Antenna

• The design integrates driven and parasitic strip dipoles with reactive termination to achieve LHCP and a superdirective gain of 6.14 dB.
• It employs strong mutual coupling and precise geometric tuning, yielding a 23.75% impedance bandwidth and an axial ratio as low as 1.4 dB.
• This low-profile (0.15λ thick) antenna is ideal for sub-6 GHz wireless and sensing platforms, offering simple integration and high polarization purity.

A circularly polarized strip-crossed dipole antenna is a compact radiating structure that employs two sequentially arranged, mutually orthogonal metallic strip dipoles—one driven and one parasitic—with strong inter-element mutual coupling and single-feed excitation to generate left-hand circular polarization (LHCP) and achieve superdirective realized gain in the end-fire direction. The configuration offers a low-profile solution (0.15λ thickness at 3.5 GHz) suitable for integration into sub-6 GHz wireless and sensing platforms, distinguished by its ability to combine polarization purity, superdirectivity, and impedance matching within a geometrically simple form factor (Psychogiou et al., 15 Jan 2026).

1. Geometric Structure and Configuration

The antenna consists of two identical crossed-dipole "strip" elements aligned along the z-axis (end-fire radiation direction) with a center-to-center separation of $d = 0.15\,\lambda_0 \approx 13.7$ mm at 3.5 GHz. Each element is formed by sets of orthogonal strips: x-oriented arms with lengths $l_{x1} = 35.21$ mm and $l_{x2} = 43.83$ mm, widths $w_{x1} = 2.95$ mm and $w_{x2} = 4.14$ mm; y-oriented arms with lengths $l_{y1} = 40.17$ mm and $l_{y2} = 44.64$ mm, widths $w_{y1} = 3.30$ mm and $w_{y2} = 3.29$ mm. The driven dipole is positioned at $z = +d/2$, and the parasitic at $z = -d/2$ in Cartesian coordinates. This configuration ensures strong near-field interaction and supports the necessary orthogonal current distributions for circular polarization.

Element	Arm	Length (mm)	Width (mm)
Driven/Parasitic	x₁ / x₂	35.21 / 43.83	2.95 / 4.14
Driven/Parasitic	y₁ / y₂	40.17 / 44.64	3.30 / 3.29

2. Feeding Scheme and Reactive Termination

Excitation is achieved via a single 50 Ω balanced feed located at the center of the upper (z = +d/2) dipole. The lower dipole (z = –d/2) operates passively, terminated in a purely reactive impedance $Z_L = -j X_L$, with $X_L = 74.96~\Omega$, corresponding to a capacitance $C \approx 0.61~\text{pF}$ at $f = 3.5~\text{GHz}$. Adjusting $X_L$ modulates the phase of the parasitic element's induced current, realizing an approximate 90° phase shift that enhances radiation in the +z direction and suppresses radiation in the –z direction. This arrangement obviates the need for complex multi-feed networks, simplifying integration and matching.

3. Mutual Coupling and Superdirective Radiation

The selected spacing of $d = 0.15\lambda$ amplifies mutual impedance ($Z_{12}$), a critical factor underpinning superdirective performance. While $Z_{12}$ is not reported explicitly, the operational principle adheres to end-fire array theory, wherein closely spaced, strongly coupled dipoles sustain induced currents with phase and magnitude optimized for maximum constructive interference in the forward direction. Tuning both the strip dimensions and reactive load ($Z_L$) sets the mutual currents to simultaneously maximize realized gain and reinforce directivity far exceeding the canonical dipole. This suggests the antenna exploits near-field coupling effects characteristic of superdirective small arrays.

4. Polarization Behavior and Axial Ratio Characteristics

Circular polarization is achieved through the quadrature relationship between the orthogonal currents: the excitation is such that $I_y = +j I_x$, resulting in LHCP in the +z direction. Through strong coupling and precise reactive loading, the parasitic element preserves this phase relation, sustaining high polarization purity. Far-field components at the broadside ($\theta = 90^\circ$) satisfy $E_\theta \propto I_x l_x e^{-jkr}/r$ and $$E_\phi \propto I_y l_y e^{-jkr}/r = j I_x l_y e^{-jkr}/r$$, confirming the required 90° phase lead. The axial ratio, calculated as $$\mathrm{AR}(f) = \frac{|E_{\rm LHCP}(f)| + |E_{\rm RHCP}(f)|}{|E_{\rm LHCP}(f)| - |E_{\rm RHCP}(f)|}$$, is below 3 dB across 3.43–3.57 GHz (4% bandwidth), with a minimum AR of 1.4 dB at 3.5 GHz, reflecting excellent polarization purity over the targeted operating band.

5. Impedance Bandwidth and Matching

The reflection coefficient $|S_{11}|$ remains below –10 dB in the frequency range 3.29–4.17 GHz, giving a 23.75% impedance bandwidth centered around 50 Ω at 3.5 GHz ($|S_{11}| \approx –13.2$ dB, input impedance ≈50 Ω). No external matching network is required beyond the single passive reactance on the parasitic dipole; matching results from synergistic tuning of both the geometry and the reactive load. This suggests a robust compromise between high directivity, polarization purity, and broadband impedance matching is attainable within a straightforward topology.

6. Realized Gain, Electrical Size, and Theoretical Limits

At 3.5 GHz, the antenna achieves a peak LHCP realized gain of $$G_{\rm rea}^{\rm LHCP}(3.5\,\mathrm{GHz}) \approx 6.14\;\mathrm{dB}$$ for an electrical size $ka \approx 1.65$ ($a \approx 22.6$ mm), within 1.8 dB of the theoretical Harrington directivity limit $D_{\max} = (ka)^2 + 2 ka \approx 7.9$ dBi for singly polarized radiators. The profile along the end-fire axis remains $\text{profile} = 0.15\,\lambda_0 \approx 13.7$ mm, underscoring applicability to compact and embedded systems where both spatial and spectral constraints are stringent.

Metric	Value at 3.5 GHz	Bandwidth
Realized Gain	6.14 dB (LHCP)	–
Axial Ratio	1.4 dB	4% (3.43–3.57 GHz)
Impedance BW	–	23.75% (3.29–4.17 GHz)
$ka$	1.65	–

7. Significance and Applications

The single-feed, circularly-polarized super-realized-gain strip-crossed dipole antenna synthesizes high gain and polarization discrimination via compact form factor and minimal feed complexity, rendering it suitable for advanced wireless/SAR sensing platforms operating below 6 GHz where integration density, polarization purity, and directivity requirements are prominent. Its reliance on strong mutual coupling and precise reactive loading showcases potential for broader applicability in low-profile directive arrays subject to constraints on feed count and package thickness (Psychogiou et al., 15 Jan 2026). This suggests utility in emerging communications and sensing paradigms prioritizing miniaturization, spectral efficiency, and polarization control.