Introduction
Characteristic modes (CMs) have been massively deployed in the microwave and antenna community since the concept of a characteristic mode was first proposed by Garbacz [1]. Tremendous applications have been developed using CMs, addressing electromagnetic radiation, scattering, and coupling problems [2, 3]. CM-based applications in antenna design have huge advantages in terms of understanding of the electromagnetic properties of an actual physical structure. Characteristic currents, value, and far-field radiation and scattering have close relationships with specific physical objects. The inherent attributes of the specific physical object can be controlled by adjusting the object structure to control the electromagnetic performance.
Meanwhile, the reduction of electromagnetic scattering in antenna engineering has attracted widespread attention. Antennas that have a low radar cross-section (RCS) while maintaining excellent radiation performance are widely used in the electromagnetic stealth field. Typically, RCS reduction is most significant for electromagnetic stealth technology.
There are several RCS reduction methods. First, resistors can be utilized to absorb the surface current generated by plane waves [4, 5]. An integrated switchable absorber was proposed in [4], and the absorber cell consisted of a metallic triple loop, 12 lumped resistors and 4 pin switches. The proposed structure realized a 10-dB RCS reduction in the frequency range from 3.8 GHz to 9.3 GHz. A similar microwave absorber technology with four resistors was utilized in [5]. Even though the absorbers in [4] and [5] could effectively reduce the RCS, the complex structural design and additional resistors required for these absorbers would increase the difficulty of the design as well as the cost.
Second, metasurface structures have been utilized to cancel out scattering energy by controlling the phase, amplitude, or polarization modes of the scattering field [6–9]. In [7], a slot antenna that was surrounded by a large area with a periodic rectangular patch artificial magnetic conductor realized an RCS reduction of more than 6 dB. The irregular periodic structure of this design was used to cancel the reflection energy. In [9], a mushroom-like electromagnetic band-gap structure, which was integrated onto a radiation patch reduced in-band RCS by a maximum of 8 dBsm. These methods achieved the goal of RCS reduction but did so by increasing additional dimensions or profile height.
Third, superfluous radiation structures have been optimized by observing the radiation current distribution to cut down the physical dimension for scattering performance [10–13]. In [11], two slots were introduced in specific locations of a proposed T-shaped slot antenna, and an RCS reduction of approximately 5 dB was achieved by disrupting the current path. This simple and effective method is attractive an approach to RCS reduction.
To date, RCS reduction has mainly been achieved by attaching complex and bulky scattering structures, causing antennas to lose their advantages in terms of low profile, miniaturization, and light weight. Therefore, new technological methods are needed to reduce RCS without adding structures to the antennas.
One study [14] pointed out that the main scattering modes, which contribute the most to the scattering performance of an antenna, can be obtained through characteristic mode analysis with a plane wave source irradiation in a certain polarization direction. High-order modes within the operating frequency band have been successfully removed through slotting and short-circuit loading on a metasurface antenna. Therefore, the scattering and radiation characteristic modes of antennas can be controlled to reduce RCS through the use of slots or holes.
The use of the theory of the characteristic mode (TCM) to reduce RCS has the following advantages. First, it provides comprehensive understanding of the physical characteristics of the analyzed antenna and lays the groundwork for optimizing the proper feeding position to excite the desired modes. Second, by calculating the mode weighting coefficients (MWCs), the contribution of each mode to antenna scattering can be accurately displayed. Third, by observing the characteristic current distributions of different modes, the positions of characteristic currents with each scattering mode can be accurately located, and corresponding methods can be taken into consideration to suppress scattering modes and reduce RCS.
In this study, the radiation performance of a half-H shaped slot antenna and scattering modes and scattering performance were analyzed based on the TCM. This approach enabled the observation of the distribution of the scattering currents that contributed the most in the scattering mode. Using slit-loading, the scattering currents were eliminated and controlled to achieve RCS reduction. The simulation results for the radiation and scattering performance of the original and slitted antenna were compared, and they showed that radiation performance was not affected when the RCS was reduced using slit-loading.
To achieve further RCS reduction, the scattering current distribution at different locations was observed again, and the scattering performance was further suppressed by loading another slit. Compared to the original antenna, the slitted antenna achieved a maximum RCS reduction of 10 dB; however, the radiation performance of the slitted antenna remained unchanged compared to the original antenna.
Two models were fabricated to verify the simulation results. The measured results were found to be consistent with the simulation results, indicating that the RCS of the original antenna was effectively suppressed by the slit-loading, while the radiation performance and bandwidth performance remain unchanged. This paper also summarizes the steps to suppress RCS based on CM analysis and provides guidance for future research.
Characteristic Mode Analysis and RCS Reduction
This section provides a detailed explanation of how to achieve RCS reduction by using TCM for scattering analysis and control. Subsection A proposes the use of a half-H shaped antenna structure as the original radiation object. The modal significance (MS), current distribution, and corresponding far-field performance of each desired characteristic mode are analyzed and excited for radiation performance with a specific feeding structure. This subsection also discusses how the prototype was fabricated and measured to verify the simulation results, and how it was used as an object for comparison with the RCS-reduced antenna.
Section II-2 describes how, by observing and analyzing the characteristic scattering currents distribution with the TCM, four slits were loaded in particular positions, and the RCS was successfully reduced by an average of 3 dB within the operating frequency band. The original antenna and the slitted antenna are also compared.
Section II-3 discussed how an additional slit was applied for further RCS reduction. It also discusses how the prototype was assembled. Furthermore, it compares the radiation and RCS performance of the original and five-slit-on antennas. It shows that the measured results agreed well with the simulated results, with an additional decrease in the average RCS, reaching a maximum reduction of 10 dB. Additionally, the steps for a CM-based scattering analysis and RCS reduction are summarized.
1. Original Antenna Design and Performance Analysis
Slot antennas are garnering more and more attention in the modern communication system. The slots are usually formed by cutting over a ground plane, and they are usually considered the major radiation aperture. Inspired by the structures in [15] and [16], an improved half-H shape was developed in this study, as shown in Fig. 1(a), The metal surface was printed on a dielectric substrate with a thickness of h1 = 1.5 mm and a relative dielectric constant ηr = 2.2. Altair FEKO 2017.1 electromagnetic simulation software was used to conduct the CM analysis and explore the electromagnetic characteristics of the improved structure.
The model significance curves of the antenna are shown in Fig. 1(b). M1' and M2' were the main modes of the proposed half-H shape, and the MS value of M1' was higher that of M2'–M6' within the analyzed frequency band range of 2.5–3.5 GHz. Stimulating the M1' mode was significant and valuable because the MS value was greater than 0.707, while other the MS values of the other modes were 0.4 or less [17].
The characteristic magnetic current distributions and the farfield radiation patterns of M1' and M2' at 2.8 GHz, which was the resonant frequency, are shown in Figs. 1(c)–1(f). Observing the magnetic current distribution of M1' revealed that the magnetic currents were mainly distributed at the horizontal upper edge (x-direction) of the structure. Therefore, capacitance coupling feeding could be used to excite M1'. A reasonable capacitive-coupled feeding structure can raise the desired radiation pattern with ease.
As shown in Fig. 2(b), the half-H-shaped antenna with coupling feeding structure was simulated, processed, assembled, and measured. The measured port reflection coefficient S11 was less than −10 dB in the frequency band range from 2.17 GHz to 3.2 GHz, with an overall relative bandwidth of 38.36%. These results were essentially consistent with the simulation results.
Fig. 2(c) and 2(d) show the simulated and measured maximum realized gain and radiation efficiency, respectively. The simulated and measured realized gain results were in good agreement. The highest radiation efficiency was near 2.5 GHz, which was about 98%. The simulated and measured far-field radiation patterns of the original antenna at 2.25 GHz, 3 GHz, and 3.25 GHz were compared and are presented in Fig. 3.
As shown in Fig. 4, the RCS results of the simulation of the half-H shaped antenna, in which a planar wave source in the x-direction polarization was applied directly above the antenna (+z direction), ranged from −13.5 dBsm to −12 dBsm. The measured RCS results followed the same trend, ranging from −13 dBsm to −11 dBsm. The very slight differences between the results were caused by the inevitable errors introduced during fabrication and assembly.
2. Scattering Modes Analysis and RCS Reduction
Fig. 5 shows the MWCs of the original antenna structure with a planar wave source in the x-direction polarization irradiation. Only Modes 1 and 4 were successfully excited throughout the entire frequency band by the TCM. These modes were considered the most significant contributors to the scattering performance in the x-polarization direction of the antenna. The other scattering modes were almost negligible with MWC values below 0.001.
The characteristic current distributions of Mode 1 at 2 GHz and Mode 4 at 3 GHz are shown in Fig. 6(a) and 6(b), respectively. The significant characteristic currents for the scattering performance were mainly concentrated at the top and bottom edges, as marked with white circles. It can be inferred that the RCS can effectively be reduced by suppressing and weakening these characteristic currents and keeping the radiation mode of the antenna effect-free.
As shown in Fig. 6(c), four vertical slits were employed on the top and bottom edges of the antenna. The MWC curves of the new antenna with the vertical slits are shown in Fig. 6(d). The MWC values of the two effective scattering modes (Modes 1 and 4) were significantly reduced. Therefore, the scattering performance in the x-polarization direction was effectively suppressed throughout the entire band. An RCS reduction of approximately 6 dB at 3 GHz was achieved, compared to the original structure, as shown in Fig. 6(e).
The comparison of the original and slitted antennas (cut_1) in S11 and the realized gain performances are shown in Fig. 7(a) and 7(b), respectively. The slitted antenna had the same or similar S11 and radiation characteristics as the original antenna. This indicates that the loading of slits on the top and bottom edges can effectively suppress an antenna’s scattering performance, while keeping radiation performance unchanged.
3. Scattering Modes Analysis and Further RCS Reduction
Although the slits played a role in suppressing the scattering modes, there were still significant characteristic currents at the bottom edge of the half-H shape, as shown in Fig. 8(a). Investigating the current distribution at different frequencies of the antenna structure showed that the surface current was mainly concentrated at the upper edge of the half-H-shape along the y-axis. Therefore, slits on this edge will inevitably have a certain impact on the radiation performance of an antenna. As shown in Fig. 8(b), a slit with a length of 18 mm and a width of 0.5 mm was made on the upper edge in the center position.
Fig. 8(d)–8(e) show that the simulated and measured results after the second slit was processed and assembled were basically consistent. The measured S11 of the antenna was less than −10 dB in the frequency band ranging from 2.15 GHz to 3.32 GHz, with a relative bandwidth of 42.78%. Fig. 8(d) shows that the measured and simulated realized gain results were basically consistent. Fig. 8(e) shows that the highest radiation efficiency was near 2.5 GHz, up to 98%, and the efficiency-frequency variation curve shows that the radiation efficiency was greater than 80% in the whole frequency band from 2.15 GHz to 3.32 GHz.
The TCM analysis of the new structure revealed that the MWC values of Modes 1 and 4 were lower after cut_2, compared to after cut_1, as shown in Fig. 9(a). Fig. 9(b) shows that, within the entire bandwidth, the simulated and measured average RCS results decreased, compared to the results for the original antenna, with the maximum RCS reduction reaching about 10 dB around 3 GHz.
The simulated and measured far-field radiation patterns at 2.25 GHz, 3 GHz, and 3.25 GHz are listed in Fig. 10(c)–Fig. 10(h). The comparisons of S11 and realized gain are shown in Fig. 10(a) and 10(b), respectively. The results showed that the trends in simulated and measured realized gain for the three proposed antennas were nearly the same. For all of them, the peak realized gain was more than 5.2 dBi. Moreover, the measured and simulated far-field radiation patterns were in good agreement. At the same time, by comparing these patterns with the original structure patterns in Fig. 3(a) and 3(f), it can be concluded that the radiation performance of the two antennas was basically unchanged.
The steps for the CM-based scattering analysis and the RCS reduction are summarized as follows.
(1) A specific antenna structure and a suitable feeding structure should be determined. An antenna with the desired radiation modes is required before achieving scattering suppression and RCS reduction. A well-designed feeding structure can stimulate the expected radiation modes.
(2) Modal decomposition should be used to analyze the contribution of each CM to the radiated or scattering field. The radiation and scattering modes should be distinguished. The identified scattering modes should be suppressed while the desired radiation performance is retained.
(3) Measures should be undertaken to suppress the scattering modes. Slits were employed in this study. Other applications, such as loading lumped inductors and slots, can control scattering modes in similar ways. Through this process, a new antenna structure is formed.
(4) The radiation and scattering performance of the new and original antenna structures should be compared. The new antenna structure should be studied using the TCM. The scattering performance or RCS reduction should achieve a desired or acceptable radiation level. If not, the applications should be optimized to meet the requirements for RCS reduction and maintain good radiation performance.
These steps can be used to make RCS reduction and scattering control based on CM analysis easily accessible. Furthermore, the TCM can be used to gain clear physical insights into the scattering mechanism as well as guidelines for scattering control. Table 1 compares the proposed antenna with antennas developed in previous studies.
Even though the technology in [5–8] and [13] can realize RCS reduction to a different degree. However the proposed antenna in this manuscript has the lowest profile height (0.011λ0, where λ0 is the vacuum wavelength at the lowest frequency), compared to other designs. Furthermore, the proposed antenna can reach a maximum RCS reduction of 10 dBsm without adding electromagnetic structures or electronic components, such as metasurfaces or resistors. This significant advantage means that it does not require complex or bulky scattering structures, and it can continued to realize the benefits of a low profile, miniaturization and a light weight.
Conclusion
Slits were adopted on a specific antenna based on the TCM for RCS reduction by suppressing the scattering modes that contributed the most to the antenna’s scattering performance. By observing the characteristic current distribution of the scattering modes on the antenna, four slits were used for RCS reduction, and another slit was added for further RCS suppression. A maximum RCS reduction of about 10 dB around 3 GHz and an average 5 dB reduction over a wide frequency band from 2.1 GHz to 3.3 GHz were achieved using the slits, and radiation performance was maintained with no deterioration.
Two prototypes were fabricated and assembled to verify the simulation results. The measured S11, directional pattern and the RCS results were in good agreement with the simulated results. The TCM was used to efficiently design an RCS suppression method for this study.
Overall, this paper discusses a specific case of RCS reduction and provides guidelines for applying CMs to scattering problems. An antenna with a low RCS that maintain an excellent radiation performance can be widely used in the electromagnetic stealth field and electronic warfare application systems.
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