Satellite communication systems are widely used in television, telephone, radio, internet, and military applications. Owing to the rapid development of satellite technology over the years, its applications have increased significantly. Notably, while higher frequency bands typically allow access to broader bandwidths, they are also more susceptible to signal degradation due to the absorption of radio signals by atmospheric rain and snow.

Satellites must be able to operate across a variety of frequency bands to offer various communication services. Some commonly used satellite bands are the L-band, S-band, C-band, X-band, Ku-band, K-band, and Ka-band. Considering this context, circularly polarized (CP) antennas are widely used in satellite communications since they are less susceptible to signal fading caused by changes in the orientation of the receiving antenna or atmospheric conditions [1, 2].

However, most dual-band CP antennas exhibit a frequency ratio of less than 1.5 between two bands, which limits their application in array antennas [3–9]. For instance, in [4], the sequential rotation technique was applied to conduct dual frequency (4 GHz and 6 GHz) operations using only a single aperture. Furthermore, CP characteristics have been generated using a resonator to excite the two orthogonal modes of a patch via two coupling paths with 90° phase difference for the C-/X-band [5]. In addition, several dual-band linearly polarized (LP) antennas characterized by a high frequency ratio have also been reported [10–16]. For instance, an S-/X-band single layer shared-aperture array antenna with a mutual complementary configuration was proposed in [10]. Nonetheless, it is difficult to design CP characteristics structurally.

In this paper, we propose a dual S-/X-band CP shared-aperture microstrip patch antenna for satellite communications. A patch with truncated corners is utilized as the radiating CP wave. Meanwhile, the unit cell of the proposed antenna comprises a single perforated S-band patch and four X-band patches configured in a 2 × 2 array. The S- and X-band patches are interlaced, thus sharing the aperture within the same layer, leading to enhanced isolation between both bands and no degradation in CP characteristics. The proposed antenna features a stacked structure composed of four substrates. The S-band radiating element is printed on the top layer of the upper substrate, while the X-band radiating element is a 2 × 2 array printed on the bottom layer of the upper substrate.

The remainder of this paper is organized as follows: Section II describes the design principle and configuration of the proposed dual-band CP antenna, while the simulated and measured results are discussed in Section III. Finally, the conclusions are presented in Section IV.

Fig. 1 presents the structure of the proposed dual-band CP shared-aperture antenna. The unit cell of the antenna consists of a single perforated S-band patch and four X-band patches configured in a 2 × 2 array. The S- and X-band patches are interlaced, so they share an aperture in the same layer. This enhances the isolation between the bands and helps avoid the degradation of CP characteristics. Notably, in this paper, a trimmed patch antenna is employed to obtain CP characteristics and achieve left-handed CP (LHCP). Moreover, an aperture-coupled feed is utilized to achieve broad bandwidth.

The proposed antenna features a four-layer structure. As shown in Fig. 1, a perforated S-band patch, with a thickness of 2.4 mm, is printed on top of the first layer. The 2 × 2 X-band array antenna, which has a thickness of 1.6 mm, is printed on top of the second layer. The feedline of the S-band is located on top of the third layer. The bottom layer, which has a thickness of 0.8 mm, consists of the ground plane, with apertures for S- and X-band feeding on the top, and the feed network for the X-band antenna located at the bottom. Furthermore, as shown in Fig. 1(c), the feed network for the 2 × 2 X-band array antenna consists of two T-junction power dividers and a quarter-wave transformer. The quarter-wave transformer is added at the 50 Ω input before the first T-junction power divider to split the line into 50 Ω lines. For each split line, a second T-junction power divider is designed to split the line into 100 Ω lines. Moreover, since the feedlines for the patches on the upper and lower sides enter from opposite directions, a 180° phase delay line is included in the feedline of the upper-sided patches to ensure that the four X-band patches are in-phase.

Notably, the truncated corners (S_t and X_t) were optimized to obtain the minimum axial ratio (AR) for each band. The element spacing of the X-band array was maintained at 25 mm, corresponding to about 0.68λ₀. In addition, the dimensions of the feed network for the X-band patches and the element spacing of the X-band array were accounted for to design a scalable array antenna with a large aperture. In addition, the feedline and aperture shape for the S-band patch were designed to minimize the degradation of CP characteristics of the X-band patches. Table 1 summarizes the dimensions of the main design parameters of the proposed S-/X-band dual-band CP antenna. The aperture lengths for feeding the S- and X-band patch antennas were 18 mm and 10 mm, respectively. Moreover, the perforation dimension (P_l) of patches for the S-band had an opposite effect on the gain performance of the S-band and X-band owing to their position and design geometry. Thus, by conducting a full-wave simulation using Ansys High-Frequency Structure Simulator (HFSS), the perforation dimension of the S-band patch was determined to be 14 mm, as shown in Fig. 2.

Figs. 3 and 4 illustrate the full-wave simulated electric field distributions of the proposed dual-band antenna at t = 0, t = T/4, t = T/2, and t = 3T/4. The majority of the electric field distributions are seen around the interlaced patches for each band. Furthermore, time-dependent rotating electric field distributions are clearly observed, confirming LHCP radiation from the proposed antenna.

To verify the feasibility and performance of the proposed antenna, we simulated and measured its characteristics, including its reflection coefficient, AR, and far-field radiation patterns. The measured results were found to be in good agreement with the simulated ones, although slight discrepancies were caused by manufacturing tolerance and the 50 Ω SMA connector. Fig. 5 shows photographs of the fabricated antenna prototype, where the substrate utilized is RT/duroid 5880, with a relative permittivity of 2.2 and a loss tangent of 0.0009. Fig. 6 presents the simulated and measured reflection coefficients and ARs of the proposed antenna. As shown in Fig. 6, −10 dB impedance bandwidths were measured to be 160 MHz (1.96–2.12 GHz) and 1,140 MHz (7.86–9 GHz) at the S-band and X-band, respectively. Moreover, the measured isolations were larger than 36 dB and 25 dB at the operation frequencies of the S-band and X-band, respectively, demonstrating good isolation between the Sand X-band ports. Furthermore, 3-dB AR bandwidths of 40 MHz and 320 MHz were achieved at the S-band and X-band, respectively. Fig. 7 presents the simulated and measured far-field radiation patterns of the proposed antenna at the S-band. The measured broadside LHCP gain attained was 5.7 dBic1.1 dB lower than the simulated result. Fig. 8 shows the simulated and measured far-field radiation patterns of the proposed antenna at the X-band, according to which the measured broadside LHCP gain is 11.6 dBic—0.4 dB lower than the simulated result.

Overall, the simulated and measured results showed good agreement, except for a slight discrepancy in the broadside LHCP gain, possibly caused by losses in the connector and feedline. Fig. 9 shows the simulated and measured broadside gains of the proposed antenna over the operating bandwidth.

Table 2 provides a comprehensive analysis of the proposed antenna in comparison with those reported in previous works, emphasizing that this paper presents a CP dual-band planar antenna of moderate thickness that has a high frequency ratio.

A dual-band CP shared-aperture microstrip antenna with a high frequency ratio is proposed and demonstrated for S-/X-band satellite communications in this paper. The proposed antenna is characterized by a four-layer structure comprising a single perforated S-band patch and four interlaced X-band patches. The perforation dimensions of the S-band patch were optimized to obtain maximum broadside gains of both bands. The structure and dimensions of the X-band feed network were designed with the aim of creating a scalable array antenna. The measured broadside LHCP gains at 2.02 GHz and 8.28 GHz were 5.7 dBic and 11.6 dBic, respectively. In addition, the designed antenna exhibited a high frequency ratio of 4.1, while also being of moderate thickness. Overall, the measured results confirm that the proposed antenna is suitable for satellite communications.