Introduction

The development of satellite communication systems has created a significant demand for high-gain circularly polarised (CP) antennas. These CP antennas are gaining increasing attention because of their ability to prevent polarisation mismatches and mitigate multipath effects1,2,3,4,5,6,7,8. To meet this demand, several methods have been explored for designing high-gain CP antennas9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24. Metasurfaces have emerged as efficient solutions for generating CP waves among the approaches. These structures offer unique capabilities for polarisation manipulation, allowing precise control over the amplitude and phase of both the reflected and transmitted waves. This capability has paved the way for developing effective metasurface-based polarisation converters25,26,27.

Several configurations have been explored for enhancing the performance of CP antennas using metasurfaces. One widely studied approach involves Fabry–Pérot resonators, where an air gap between a linearly polarised (LP) radiator and a metasurface polarisation converter enables constructive interference at specific frequencies. This setup effectively increases the CP gain and extends the axial ratio (AR) bandwidth, as shown in various high-gain CP antenna designs9,10,11,12,13. In9the authors presented a high-gain Fabry–Pérot CP antenna. The design featured an LP patch antenna and a unit-cell polarisation converter comprising a combination of an LP square patch on the bottom substrate layer and a truncated patch on the top substrate layer of the polarisation converter, with both patches connected by a via. Although the antenna achieved a very high gain, it exhibited narrow impedance and AR bandwidths, as well as low efficiency. In10the authors designed a Fabry–Pérot antenna for C-band satellite communication. The LP radiator was a truncated patch, and a Z-shaped metasurface was used for linear polarisation (LP: this acronym can be used interchangeably with linearly polarised) to circular polarisation (CP: this acronym can be used interchangeably with circularly polarised) conversion. They achieved high gain and wide impedance bandwidth but yielded low AR bandwidth, low efficiency, poor radiation patterns at higher frequencies, and a thick profile size. In addition, the authors of11 presented a high-gain Fabry–Pérot CP antenna utilizing a patch array excited by a feeding network and chessboard polarisation converter. Although they achieved high gain, the design had limitations, including narrow impedance and AR bandwidths. Moreover, the radiation patterns obtained were poor, and the antenna had a bulky profile size. In12the authors presented a dual-CP high-gain Fabry–Pérot antenna based on a dual–polarised LP patch radiator and a metasurface polarisation converter comprising a circular patch etched with a slot oriented at 45° for LP-to-CP conversion. They achieved high gain, albeit with extremely narrow impedance and AR bandwidths. In addition, the efficiency was low, and the radiation patterns were poor. In13the authors designed a high-gain CP Fabry–Pérot antenna based on a double-differential-feed-patch LP source and highly reflective metasurface polarisation converter comprising two corner-cut truncated patches with orthogonal slots and twelve shorted pins. They achieved high gain with extremely narrow impedance and AR bandwidths, as well as a thick profile.

Other studies have explored alternative approaches for high-gain CP antennas with polarisation converter metasurfaces. One method involves placing a transmissive metasurface polarisation converter above an LP source with an air gap14– 15while another uses a reflective metasurface polarisation converter beneath the LP source with an air gap16,17. These configurations enable the metasurface to convert LP waves into CP waves, enhance CP performance, and support improved bandwidth and gain characteristics.

Both designs using the Fabry–Pérot technique and placing reflective metasurfaces with an air-gap separation between the LP source and converter have limitations of poor radiation characteristics, bulky profile sizes, and narrow impedance and AR bandwidths, necessitating further study in this area. To this end, cavity-backed antennas are promising candidates for high-efficiency and high-gain wireless communication systems. In18the authors proposed a double-cavity structure for high-efficiency, high-gain CP antennas. The LP source was placed on the top side of the feeding cavity at the lower side with a feeding slot, and 3 × 3 and 8 × 8 periodic radiating cross slots were positioned on the top side of the upper cavity to convert the LP waves to CP waves. Although the structure achieved high gain and efficiency, it resulted in a narrow AR bandwidth and bulky profile owing to its double-cavity design.

This paper presents a novel approach to achieve high gain, high efficiency, wide impedance, and AR bandwidths by incorporating a modified dipole antenna with a wideband complementary-shaped LP-to-CP converter metasurface and cavity reflector. The antenna comprises a modified dipole structure with two parasitic patches in a coplanar configuration on the bottom side of the substrate and a metasurface with four modified square metal patches arranged in a 2 × 2 layout with a centrally positioned cross-shaped strip on the top side of the substrate. A modified cavity reflector featuring truncated corners and four triangular extensions at the top corners was introduced beneath the dipole to focus energy in the broadside direction. The proposed antenna achieves a global bandwidth of 4.0–6.64 GHz, simultaneously satisfying impedance matching, AR, and gain requirements. Specifically, it offers an AR bandwidth of 49.7% (4.01–6.73 GHz), an impedance bandwidth of 67.3% (3.50–7.05 GHz), a 3-dB gain bandwidth of 61.1% (3.68–6.92 GHz), a peak gain of 10.8 dBic at 5.2 GHz, and a radiation efficiency exceeding 92%. With compact dimensions of 80 × 80 × 15.508 mm3 (1.36 × 1.36 × 0.26 \(\lambda _{o}^{3}\) at 5.1 GHz), the antenna is suitable for wideband applications in satellite and wireless communications.

Antenna geometry

The geometry of the proposed antenna, as illustrated in Fig. 1a, incorporates a cavity structure along with a metasurface and modified dipole fed by a tapered balun. The antenna is designed on a Rogers AD250C substrate with dimensions of 80 × 80 × 15.508 mm3corresponding to 1.36 × 1.36 × 0.26 \(\lambda _{o}^{3}\), where λo is the center frequency of the AR bandwidth at 5.1 GHz. This substrate has a relative permittivity (εr) of 2.5 and loss tangent (tanδ) of 0.0015, supporting high-frequency operation with minimal loss. The metasurface, located on the top side of the substrate, comprised four modified square metal patches arranged in a 2 × 2 layout with periodicity Pm. A cross-shaped strip with dimensions Wc and Lc for its width and length, respectively, was placed at the center of the 2 × 2 metasurface structure to improve impedance and AR matching, as shown in Fig. 1b. Each unit cell comprises a metal patch modified by two diagonal C-shaped slots into the metal to create a chiseled pattern. These slots, with widths g1 and g2, are separated by d1 and d2, providing a 90° phase shift and equalizing the amplitude between the orthogonal components of the incident LP wave, thereby enabling CP radiation. The gap of each metal patch is denoted as gm. The primary radiator, which is a modified dipole with stepped-width arms and vertical stubs, was positioned at the bottom of the substrate in a coplanar configuration, as shown in Fig. 1c. The structure comprises modified dipole arms with a fork-like stub for impedance matching and two parasitic patches that improve both impedance matching and the AR bandwidth. The dimensions of the modified dipole arm include Lf (length) and Wf (width of the feeding part), and Ld and Wd represent the length and width of the entire arm, respectively, which together determine the resonance frequency of the antenna. Additionally, the fork-shaped adjustment at both ends of the dipole, with length Ln and width Wn, acts as a notch, further improving impedance matching and contributing to the wideband performance of the antenna. A vertical rectangular structure of length Lst and width Wst was added to each dipole arm, aiding in current distribution and impedance matching. The two parasitic patches have widths of Wp and lengths of Lp, which further aid in improving the impedance matching. Figure 2a shows the standard cavity reflector, and Fig. 2b depicts the modified cavity reflector with truncated corners and four triangular extensions at the top corners of the cavity. Figure 2c shows a 3D view of the metasurface dipole antenna incorporating a modified cavity reflector. The cavity was placed beneath the metasurface dipole antenna, as shown in Fig. 1a and 2d. The dimensions of the cavity are W × W × hcav, where the lengths and widths of the cavity are the same as that of the substrate of the metasurface dipole antenna, but the cavity height hcav is different from the substrate height hs, as shown in Fig. 1a. The cavity enhances the gain by focusing the radiated energy and reducing the backward radiation, resulting in a more concentrated broadside radiation pattern. This is achieved by suppressing surface wave propagation and unwanted modes, thereby improving the radiation efficiency and polarisation purity. With its truncated corners and triangular extensions, the modified cavity further refined energy confinement and redirection, enhancing the front-to-back ratio and reducing the scattering effects. These modifications improved the surface current control and polarisation characteristics, resulting in superior gain and AR performance compared to those of the standard cavity. The antenna is excited using a tapered microstrip-to-parallel stripline balun4which transforms the unbalanced input from the coaxial feed into a balanced differential signal across the dipole arms,

Fig. 1
figure 1

Antenna geometry: (a) side view of dipole antenna incorporated with the metasurface and cavity, (b) top view of the top side of the metasurface, (c) top view of the modified dipole antenna, and (d) microstrip-to-parallel strip tapered balun.

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Fig. 2
figure 2

(a) Conventional cavity reflector, (b) unfolded 2D structure, (c) modified cavity structure with truncated corners and triangular extensions at the top corners, and (d) 3D view of the proposed structure.

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as shown in Fig. 1d. Specifically, the inner conductor of the coaxial cable connects to one dipole arm via a vertical stripline, while the outer conductor is soldered to the opposite dipole arm through a wide vertical ground plane, achieving balanced excitation. This structure ensures effective impedance matching and current symmetry, which are essential for achieving circular polarisation over a wide bandwidth.

The antenna was optimised using an ANSYS high-frequency structure simulator (HFSS) to obtain a wide AR bandwidth, good impedance matching, and a low profile. The optimised design parameters are as follows: W = 80 mm, Pm = 15.6 mm, gm = 1.0 mm, g1 = 1.8 mm, g2 = 0.2 mm, d1 = 0.2 mm, d2 = 9.2 mm, Ld = 24 mm, Wd = 2 mm, Lf = 8 mm, Wc= 0.6 mm, Lc =6 mm, Wf = 1 mm, Lst = 8 mm, Wst = 0.5 mm, Ln = 2 mm, Wn = 1 mm, Lp = 6 mm, Wp = 4 mm, hs = 0.508 mm, and hcav = 15 mm.

The design guidelines of the proposed antenna are as follows:

  1. 1.

    The modified dipole arms, placed in a coplanar configuration on the bottom of the substrate, established the resonant frequency of the antenna. A vertical stub was introduced to improve impedance matching.

  2. 2.

    A complementary-shaped wideband LP-to-CP conversion metasurface was designed with periodicity Pm and placed on the top of the substrate to convert the LP waves from the dipole antenna into CP waves. The diagonally oriented C-shaped slot had widths g1 and g2 separated by d1 and d2. These slots were designed to create a 90° phase shift between the orthogonal polarisation components and achieve a near-equal amplitude, thereby improving the CP over a wide frequency band.

  3. 3.

    To enhance both the impedance matching and AR bandwidths, two parasitic patches were introduced close to the modified dipole arms. These patches have a width Wp and length Lp, positioned strategically to strengthen the coupling and ensure optimal current distribution. Additionally, a cross-shaped strip was centrally placed within the 2 × 2 metasurface structure to further improve the impedance matching and AR performance.

  4. 4.

    A modified cavity reflector was positioned beneath the metasurface dipole-antenna structure. This reflector enhances the gain by reducing backward radiation and focusing energy in the broadside direction. The modifications, including truncated corners and triangular extensions, further improved the energy confinement, front-to-back ratio, and overall radiation performance.

  5. 5.

    Finally, the performance of the antenna, including the bandwidth, gain, and efficiency, was optimised through careful fine-tuning of the metasurface polarisation converter, modified dipole structure, parasitic patches, tapered balun, and modified cavity reflector. This comprehensive optimisation ensures wide AR and impedance bandwidths, enhanced gain owing to the cavity structure, and high efficiency with minimal loss across the operating frequency range.

Antenna design evolution

The design evolution was thoroughly analysed across four antenna configurations (Ants I, II, III, and IV) to provide comprehensive insights into the operating mechanism of the proposed high-gain CP antenna. Ant I comprises only a modified dipole LP source without parasitic patches, as shown in Fig. 3a. Ant II comprises a modified dipole incorporated with a 2 × 2 complementary-.

Fig. 3
figure 3

Antenna design evolution: (a) Ant I, (b) Ant II, (c) Ant III, and (d) Ant IV.

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shaped LP-to-CP polarisation converter metasurface without parasitic patches, as depicted in Fig. 3b. Ant III includes a modified dipole LP source with a complementary 2 × 2 metasurface structure28,29 and two parasitic patches along with a centrally positioned cross-shaped strip, as illustrated in Fig. 3c. Ant VI includes Ant III, which has a modified cavity reflector with truncated corners and four triangular extensions at the top corners of the cavity reflector, as shown in Fig. 3d.

Comparative analyses of the reflection coefficient (|S11|), gain, and AR characteristics were conducted for various configurations (Ants I, II, and III) and the proposed antenna (Ant IV). Ant I achieves a limited impedance bandwidth ranging from 4.46 to 5.21 GHz (15.5%), a peak gain of 2.2 dBic, and an AR > 70 dB, as shown in Figs. 4a,b and c, respectively. Ant I does not generate CP because it operates solely as an LP source. Ant II was incorporated into a 2 × 2 complementary-shaped LP-to-CP converter metasurface on Ant I, as shown in Fig. 4b. This modification introduces additional resonances, resulting in multiple impedance frequency bands: 3.18 to 3.58 GHz (11.8%), 4.15 to 4.67 GHz (11.8%), and 7.64 to 9.58 GHz (18.8%), as shown in.

Fig. 4
figure 4

Antenna performance comparison: (a) reflection coefficient, (b) axial ratio, and (c) gain.

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Figure 3a; and an AR < 7 dB across 3 to 6.8 GHz and an AR < 3 dB between 4.1 and 4.39 GHz (6.8%), as illustrated in Fig. 4b. The modified antenna, Ant II, achieves a peak gain of 3.2 dBic, as shown in Fig. 4c, which corresponds to an approximately 1-dB gain improvement compared to that of Ant I. Additionally, the CP performance of Ant II is enabled by the introduction of the polarisation converter metasurface, which facilitates a 90° phase difference and near-equal amplitude between the two orthogonal polarisation components of the incoming LP wave from the dipole antenna, resulting in CP radiation. Despite these enhancements, the impedance and AR bandwidths remained relatively narrow, because most of the structures were not fully matched across the entire frequency range. To address the limitations of narrow impedance and AR bandwidths in Ant II, parasitic elements were strategically positioned to interact with the main radiating structure and improve impedance matching. This improvement occurred because the strategically positioned parasitic elements30,31 acted as secondary resonators coupled with the primary radiating structure. This coupling introduced additional resonant modes near the main resonance, effectively broadening the impedance bandwidth by creating multiple resonant paths. Additionally, a cross-shaped strip was introduced to enhance the AR matching and obtain Ant III. The additions of the parasitic elements and cross-shaped strip led to significantly improved performance with an impedance bandwidth from 3.52 to 8.42 GHz, corresponding to an 83% fractional bandwidth, a wide 3-dB AR bandwidth from 4.2 to 6.5 GHz (43.39%), and a peak gain of 3.4 dBic, as shown in Figs. 4a, b, and c, respectively. Although the aforementioned antenna design, Ant III, demonstrates good impedance and CP performance, it exhibits very low gain, limiting its suitability for applications requiring high-gain CP. To address this limitation, a cavity reflector was introduced to improve the gain and overall performance of the antenna. The standard cavity reflector was modified by truncating the corners and adding four triangular extensions to the top corners to obtain the proposed Ant IV structure. First, truncating the corners alters the current path along the edges, preventing the formation of unwanted surface currents. This effectively reduces energy loss and scattering, allowing more energy to be confined and directed towards the broadside direction, thereby improving the gain and radiation efficiency. Additionally, the triangular extensions at the top corners help fine-tune the current distribution on the cavity reflector, further stabilizing the radiation characteristics across the operational frequency band. Thus, the proposed antenna (Ant IV) achieves an impedance bandwidth of 68.8% (3.46–7.09 GHz), a wide AR bandwidth of 49.2% (4.01–6.64 GHz), and an improved peak gain of 10.9 dBic, as shown in Figs. 4a, b, and c, respectively. Overall, the proposed antenna exhibited excellent performance compared to Ants I, II, and III, achieving high-gain and wideband CP response.

Experimental results

To validate the performance of the proposed design, a low-profile dipole antenna incorporating a metasurface and cavity reflector was fabricated and measured. The fabricated prototype is shown in Fig. 5. S-parameter measurements were performed using an Agilent N5230A network analyser with a 3.5-mm coaxial calibration standard (GCS35M). Far-field measurements were performed at the RFID/USN Center in Incheon, South Korea, within a fully anechoic chamber using an Agilent E8362B network analyser. A standard wideband horn antenna was used as the transmitter, and the proposed metasurface-based antenna with a cavity reflector was used as the receiver. The antennas were positioned 7-m apart to satisfy the far-field criteria. During measurements, the proposed antenna was rotated from − 180° to 180° at 0.5°/s with a 1° scan angle, while the horn antenna remained stationary. The AR was determined by recording the received power over the full rotation and computing the ratio of the major to minor axes of the resulting polarisation ellipse. The 3 dB AR bandwidth was defined as the frequency range over which the AR remained below 3 dB, indicating effective circular polarisation. The measured results aligned closely with the simulated data, with only minor discrepancies, likely caused by slight fabrication misalignments. Figure 6a shows the measured and simulated reflection coefficients of the fabricated antenna. The measured impedance bandwidth for |S11| < − 10 dB was 3.48 to 7.08 GHz (68.8%), which is similar to the simulated impedance bandwidth of 3.50 to 7.05 GHz (67.3%). Figure 6b shows a comparison of the measured and simulated AR bandwidths of the proposed antenna. The simulated AR bandwidth was 4.03–6.64 GHz (49.2%), and the measured AR bandwidth was 4.05–6.73 GHz (49.7%). The measured and simulated gain performances of the proposed antenna are shown in Fig. 6c. The simulated peak gain reaches 10.9 dBic at 5.2 GHz, whereas the measured peak gain was slightly reduced to 10.8 dBic in the broadside direction. The measured and simulated radiation patterns are shown in Figs. 7 and 8, and 9. As shown in Figs. 7 and 8, and 9, respectively, the measured and simulated radiation patterns at 4.1, 5.1, and 6.4 GHz show unidirectional radiation with LHCP in the broadside direction. Furthermore, the cross-polarisation levels remained significantly low, indicating the effective suppression of the right-hand CP components and confirming the high purity of the LHCP pattern across the operating frequencies. These results validate the suitability of the proposed antenna for applications requiring stable unidirectional LHCP radiation.

Fig. 5
figure 5

Fabricated sample of the proposed antenna.

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Fig. 6
figure 6

Simulated and measured: (a) reflection coefficient, (b) axial ratio, and (c) gain.

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Fig. 7
figure 7

Radiation pattern at 4.1 GHz: (a) xz-plane and (b) yz-plane.

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Fig. 8
figure 8

Radiation pattern at 5.1 GHz: (a) xz-plane and (b) yz-plane.

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Fig. 9
figure 9

Radiation pattern at 6.4 GHz: (a) xz-plane and (b) yz-plane.

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Performance comparison

Table 1 provides a detailed performance comparison between the proposed antenna and other notable high-gain CP designs based on metasurfaces reported in the literature9,10,11,12,13,14,15,16,17. The selection criteria for the comparison table included the antenna profile size, − 10-dB impedance bandwidth, 3-dB AR bandwidth, 3-dB gain bandwidth, peak gain, and efficiency. In10the authors achieved a high-profile structure (0.51 λo), whereas the proposed design is approximately half the size with a profile size of 0.26 λo. The impedance bandwidth of the proposed design (68.8%) is similar to the 64% bandwidth presented in10. The AR bandwidth of the proposed design (49.7%) was significantly greater than the 18% reported in10. The peak gain achieved in10 (12.88 dBic) is higher than the proposed design by 10.9 dBic, but the 3-dB gain bandwidth of 30% is narrower than that of the proposed design by 61.1%. The authors in13 achieved a profile size of 0.58 λo, which was thicker than that of the proposed design. Additionally, the proposed design achieved a wider impedance and AR bandwidths of 68.8% and 49.7%, respectively, compared to that in13 (11.3% and 8.6%). Although the gain in13 13.7 dBic is higher than that of the proposed structure.

Table 1 Performance comparison of the proposed antenna with other antenna designs.
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(10.9 dBic), the structure in14 has a relatively narrow 3-dB gain bandwidth (13.8%). The proposed design achieves a lower profile size compared to that presented in14 (0.60 λo). Additionally, the impedance bandwidth in14 was only 8.6%, which was considerably narrower than that of the proposed design. The AR bandwidth of the proposed antenna (49.7%) was also significantly wider than the 7.1% in14. The peak gain in14 (11.5 dBic) was only 0.6 dB higher than that of the proposed design (10.9 dBic). In15the design achieves a better profile size (0.12 λo) compared to the proposed design (0.26λo). However, its impedance bandwidth is much narrower (15.7%) than that of the proposed design (68.8%). Additionally, the AR bandwidth achieved in15 has extremely narrow (13%) compared to that of the proposed design (49.7%). Moreover, the peak gain achieved in15 (9.6 dBic) is lower than that of the proposed design (10.9 dBic), and its 3 dB gain bandwidth (28.5%) is also narrower than that of the proposed design (61.1%). The proposed design also exhibits a higher radiation efficiency of 92% compared to the 85% reported in15. The authors in16did not specify the profile size, but the proposed design offered a wide impedance bandwidth of 68.8% compared to 63% in16. Additionally, the dual-band AR bandwidths achieved in16 of 8.2% and 10.7% were considerably narrower than the 49.7% AR bandwidth of the proposed design. The peak gain in16 (8.0 dBic) is 2.9 dB lower than the proposed design of 10.9 dBic. Additionally, the.

gain variation in16 is severe compared to that of the proposed design. The proposed antenna provides a lower profile (0.26 λₒ) compared to that reported in17 (0.32 λo). Additionally, the impedance bandwidth of the proposed design (68.8%) is also significantly broader than that of 14.3% in17. Furthermore, the AR bandwidth in17 was only 8.0%, which was significantly narrower than that of the proposed design of 49.7%. The design in17 achieved a peak gain of 8.03 dBic, which is slightly lower than that of the proposed design at 10.9 dBic. Lastly, the antenna design in17 had a profile size of 0.52 λo, which is twice as large as that of the proposed design. Additionally, the impedance bandwidth in18 is extremely narrow (1.6%) compared to the proposed design. Furthermore, the AR bandwidth in18was only 0.5%, significantly smaller than that of the proposed design. In17the peak gain of 12.6 dBic is higher than that of the proposed design (10.9 dBic). However, the 3-dB gain bandwidth in18 is about 2%, which is extremely narrow compared with the 61.1% offered by the proposed antenna. Additionally, a slightly higher efficiency of 94% is reported17whereas the proposed design achieves 92%.

Overall, the proposed design provides a lower profile size, wider impedance and AR bandwidth compared to those of the antennas in10,13,14,16,17,18. Although the design in15 provides a lower profile size and smaller lateral size compared to the proposed design, it suffers from limited impedance bandwidth, narrow AR bandwidth, low gain, and a reduced 3-dB gain bandwidth. The gains of the designs in10,13,14,18 are higher than that of the proposed design, however, the proposed structure offers a wider 3-dB gain bandwidth. These exceptional characteristics make the proposed design suitable for applications requiring compact size and wideband performance.

Conclusion

In this study, a low-profile dipole antenna incorporating a polarisation converter metasurface and cavity reflector was designed and analysed for wideband CP and high-gain performance. The proposed antenna achieves excellent performance characteristics, including a global bandwidth of 49.7% (4.05–6.73 GHz), satisfying impedance matching, AR, and gain requirements. It offers a wide AR bandwidth of 49.7% (4.05–6.73 GHz), an impedance bandwidth of 68.8% (3.48–7.08 GHz), and a 3-dB gain bandwidth of 61.1% (3.68–6.92 GHz). Including a modified cavity reflector with truncated corners and triangular extensions enhances the gain by minimising the unwanted radiation and focusing energy in the broadside direction. This results in a gain increase of approximately 7 dB. Its compact design, with dimensions of 80 × 80 × 15.508 mm3 (1.36 × 1.36 × 0.26 \(\lambda _{o}^{3}\) at 5.1 GHz), makes it well suited for wideband applications, including satellite and wireless communications. The effective incorporation of the metasurface and cavity reflector validated the ability of the proposed design to achieve wideband CP and high-gain performance.