Abstract
New frequencies can be supported with very effective space use by using the shared aperture antenna This work presents on designing a dual-banddual-polarized (DBDP) S/X-band shared aperture antenna (SAA) for synthetic aperture radar (SAR) applications operating at S-band frequency (3.2 GHz) and X-band frequency (9.65 GHz). The single-layer SAA DBDP S-band antenna is designed in a square-shaped patch with coaxial feeding in both vertical and horizontal polarization. The X-band antenna design is in 1 × 3 vertical series with microstrip feeding and arranged at four corners of the proposed antenna. The S-band antenna is mainly used for airborne applications such as air traffic control and surface ship radar. In contrast, the X-band antenna application is maritime vessel traffic control, defense tracking, and vehicle speed detection for law enforcement. To verify the antenna, a prototype is fabricated and measured with s-parameters. The proposed design exhibits that the gain of the S-band is 7.2 dB and for the X-band is 12.4 dB, and the isolation is achieved more than −35 dB, and for this antenna, we achieved a bandwidth of 0.12 GHz for S-band and 0.27 GHz for X-band. However, the X-band antenna is a multi-input and multioutput antenna that is to be validated by using MIMO characteristic parameters such as envelope correlation coefficient (ECC), diversity gain (DG), channel capacity loss (CCL), mean effective gain, and mutual coupling. The MIMO characteristic parameter of X-band antenna values is found to be in a similar manner to both simulated and measured values. For this X-Band antenna, ECC, DG, CCL, and mutual coupling were achieved as below 0.05, 9.5 dB, 0.5 bps/Hz, and −30 dB to −55 dB, respectively. The total size of the antenna is 100 mm × 100 mm × 1.6 mm.
1. Introduction
For the last several years, research tending toward the multipolarization direction. A single polarized antenna responds only to one orientation of polarization, either horizontal or vertical. Thus, radio waves that are received or transmitted by a single polarized antenna will be either horizontal or vertical polarized. Whereas coming dual-polarized, it can respond to both horizontally and vertically polarized radio waves.
By using strip-line technology and resonant field phenomena, we can get better results in achieving a medium gain of the antenna over satisfied bandwidth [1]. The configuration of the microstrip antenna is in such a way that a combination of microstrip dipoles and square patches with a 1 : 3 frequency ratio [2–4]. To achieve the design of an S/X array antenna, X-band needs to be a series-fed configuration. In contrast, the S-band in other configurations achieves higher radiation aperture and gain by using two different ports, which are orthogonal and result in high isolation [3]. For the S/X-band array antenna, we can increase the impedance bandwidth for the X-band by varying the electromagnetic coupling between prominent and parasitic patches. Whereas coming to the S-band antenna, to increase its bandwidth, we can use modified coupling feeding and perforated patches [5]. For better results, a novel design approach in both S/X-bands to achieve it by frequency ratio is 1 : 3.3, and for a higher frequency, X-Band is used to radiate such high frequency for lower frequency S-band antenna is used [6]. To suppress higher-order modes, we used to integrate microstrip LPF within the patch antenna, which radiates at a lower frequency [7]. We need to use different polarization to achieve high isolation in dual polarized and dual wideband antenna. For S-band, we used to use circular polarization. For X-band, we used to use linear polarization [8]. While comparing a normal antenna and a shared aperture antenna, it shows that the SAR antenna has more advantages [9]. For achieving narrow FBW in S-band and X-band in the SAR antenna, the S-band antenna is in the microstrip antenna, which is a dual-polarized subarray of the antenna used for X-band Radiation [10]. The study tells that for SAR application which is used at 9.65 GHz frequency, a dual -a polarized array antenna is best to achieve high gain by placing six ports. Thus, arranging it in such a way, we can get better impedance matching bandwidth, and −25 dB isolation is achieved between inter- and intraport polarization at all ports [11]. Dual-polarizedseries-fed antenna radiation characteristics can be increased by series feeding, which has an interconnection between patches [12]. To design the radar application and for better results, we used to design the antenna using Chebyshev amplitude distribution (CAD) and uniform amplitude distribution (UAD) [13]. Gain and front-to-back ratio are improved in series-fed linear array antenna, which is useful in Radar-X-band applications [14]. Using the Chebyshev technique with a width‐controlled patch antenna to incorporate both X-band and K-band to achieve better results, we can use linear or circular polarization within a single-layer [15]. A study shows that corporate feeding is introduced at both vertical and horizontal polarization and used at X-band frequency for implementing the shared aperture dual-banddual-polarized multilayer antenna. A cross-shaped element is used to differentiate between the elements [16]. Using a neutralization line can reduce the mutual coupling in the antenna structure by using meander microstrip lines. It provides flexibility for the antenna [17]. For achieving better MIMO characteristics like broader bandwidth and higher isolation in an antenna, the authors use a ring resonator-based coplanar waveguide to feed the ports [18]. To achieve wideband bandwidth and resonates at the lower band (LTE bands 42/43–N77–N78), the higher band (LTE 46) and the intermediate band (N79) antenna were designed in predesigned shape like Hexa decagon polygon not only for that it also achieves better MIMO characteristics [19]. The antenna pattern is more considerable when we analyze the MIMO characters, such as the ground’s height and the antenna’s location, which are directly affected by the radiation pattern [20]. To increase the bore side gain for sub –6 GHz with two port configurations, the authors proposed a circular-shaped antenna with an inner circular slot and a rectangular slot at the edges by obeying the traditional value of <2 W/kg for any 10 g of tissue [21]. By incorporating a varactor diode into its CLL-based NFRP element, a frequency-agile version of a passive fixed-value capacitor in the antenna bandwidth is increased by four times and provides good impedance matching, better stability, and stable and uniform radiation pattern over the frequency range [22]. There is a chance of increase in gain and bandwidth based on antenna arrangements like rectangular, triangular, and hexagonal antennas that can work in different frequencies-based variations of radiation angle [23]. For various LTE bands, the authors proposed an antenna that contains parallel strips at side walls and a metal strip at the top face to reduce the size and achieve better MIMO characteristics [24]. By implementing a nonradiating flooded shorting strip between MIMO antenna elements, there are chances to attain the MIMO characteristics and use a technique called mitigating multipath fading, which helps mobile communication [25]. The authors proposed a multielement antenna in a T-shaped slot antenna that is used to enhance the isolation. Better MIMO characteristics show superior robustness and MIMO capabilities under different hand-grip conditions [26]. In this antenna, an inner square ring resonator is embedded in T-shape substrate and uses the dot walls at radiating patches which helps to enhance the MIMO characteristics [27].
Considering the above-given literature review, we will propose a combination of a coaxial antenna and 1 × 3 series-fed linear array antenna using direct coupling with a quarter-wave transformer presented on 4 sides. The proposed antenna is designed at 3.2 GHz S-band and 9.65 X-band. The proposed antenna is analyzed by performing a full-wave simulation using an industry-standardFFT-based CST MWS software 2021. The proposed antenna is fabricated and tested. The proposed antenna structure and design are discussed in Section 2, and the simulated and experimental result is discussed in Section 3. The analysis is discussed in Section 4. Finally, the conclusions are discussed in Section 5.
2. Structure of the Proposed Antenna Representation on Payload
The SAR representation is shown in Figure 1. The configuration of the proposed antenna is the combination of two antennas that are operated in dual-band frequency, i.e., 3.2 GHz for S-band frequency and 9.65 GHz for X-band frequency. It consists of a square-shaped antenna which is operated at 3.2 GHz and that is fed with coaxial feeding; it is placed in the middle of the antenna polarization that is carried out in two directions which are orthogonal to each order, and it also consists of a 1 × 3 linear array antenna which is operated at 9.65 GHz and that is feed with microstrip feeding or direct coupling, and it is situated at corners of the antenna. To reduce the losses of the antenna while transmission, we have chosen the square shape patches in both S-band and X-band antennas. In this work, an FR-4 material is used as a substrate with 1.6 mm height and its relative permittivity is εr = 4.4 and loss tangent tan d = 0.03. All the metal components which are represented in the work are taken to be copper which is having parameters εr = 1 and μr = 1 and bulk conductivity σ = 5.8 × 107 s/m.
2.1. Structure of the S-Band Antenna
The configuration of the S-band single patch antenna is displayed in Figure 2 with dual polarization (both vertical and horizontal polarizations) with all parameters. The patch is excited using 50 Ω coaxial feeding for both vertical and horizontal polarizations for better impedance matching; inner and outer pin radius is taken as 0.6 and 2.176, respectively. The final optimized parameters of the S-band antenna are shown in Table 1. The frequency response is shown in Figure3 which indicates antenna resonates at 3.2 GHz at both vertical and horizontal ports. The impedance bandwidth covers from 3.18 GHz to 3.3 GHz and achieves isolation of more than −30 dB.
2.2. Structure of the X-Band Antenna
The configuration of the X-band array antenna is displayed in Figure 4 with single polarization with all physical parameters. The array patch is excited using 50 Ω direct coupled microstrip line feeding for the port. The width of the matching transformer section of the direct coupled fed line is MTW = 2.46 to obtain the 50 Ω impendence to improve the impendence characteristics of a quarter-wave transformer (QTW and QTL) that is connected to the matching transformer which is connected with a directly coupled microstrip feeding. After those three-square-shaped patches, connected in a series in between them, a series connection is introduced. The X-band antenna is placed alone in the four corners of the antenna to transmit and receive the information at the same time. The final optimized parameters of the proposed X-band antenna are shown in Table 2. Figures 5 and 6 show the frequency response of all X-band ports. The impedance bandwidth covers from 9.51 GHz to 9.78 GHz.
2.3. Embedding S/X Shared Aperture Antenna
The geometry configuration of S/X-band SAA with the evolution stage is represented in Figure 7. The proposed contains 3 stages. Stage one consists of a single patch antenna which is fed in a coaxial manner both in vertical and horizontal polarizations which resonates at 3.2 GHz (S-band). The array consists of a 1 × 3 vertical series feed presented along four corners of the antenna with the single port which is fed with the direct coupling method and resonates at 9.65 GHz (X-band). The final optimized parameters of the S/X Antenna are shown in Table 3. Figures 8 and 9 show the surface current distribution of S-Band and X-Band SAA at 3.2 GHz and 9.65 GHz, respectively. Figures 8(a) and 8(b) represent the surface current distribution of vertical and horizontal ports, respectively. Figures 9(a)–9(d) represent the surface current distribution of X-Band ports, i.e., XV1, XH1, XH2, and XV2, respectively.
(a)
(b)
(c)
(d)
3. Simulated and Experimental Results and Analysis
The S/X dual band dual polarization array prototype with frequencies of 3.2 GHz and 9.65 GHz for S- and X-bands, respectively, is fabricated and measured to verify the design as shown in Figure 10; the prototype is indulged with S/X-bands. The S-parameters are measured using a keysight microwave vector network analyzer.
The measured return loss S11 of the proposed S/X-band SAA is presented in Figure 11. The results show measured return loss bandwidth from 3.18–3.30 GHz at S-band for both vertical and horizontal polarizations and 9.51–9.78 GHz at X-band for both vertical and horizontal polarizations with a resonant frequency of 3.2 GHz and 9.65 GHz, respectively. The results agree well with simulated and measured antenna parameters. Isolation higher than 25 in both S- and X-bands is obtained. Figures 3, 5, and 6 represent S-parameter result of the S-band which resonates at 3.2 GHz of both vertical and horizontal polarizations as well as it also represents simulated and measured S-parameter results of remaining all X-Band ports, i.e., XV-1 Port (Port 3), XV-2 Port (Port-5), XH-1 Port (Port-4), and XH-2 Port (Port-6).
3.1. SV-Port (Port 1) and SH-Port (Port 2)
SV-Port is used to resonate at a frequency of 3.2 GHz (S-band), and it is vertically polarized for the square shape antenna which is presented in the middle of the antenna. The return loss of port 1 is −22.8 dB at 3.2 GHz and isolation between the ports is −32 dB. Radiation patterns of the antenna system are simulated at either port at both S- and X-bands, as shown as radiation patterns of the S-band antenna at port-1 (V-port) at 3.2 GHz, which is two orthogonal planes (Phi = 0° and Phi = 90°) as shown in Figure 12(a). SH-Port is used to resonate at a frequency of 3.2 GHz (S-band), and it is horizontally polarized for the square shape antenna which is presented in the middle of the antenna. The return loss of port 2 is −22.8 dB at 3.2 GHz and the isolation between the ports is −32 dB. Simulated radiation patterns of the S-band antenna at port-2 (horizontal Port) at 3.2 GHz are two orthogonal planes (Phi = 0° and Phi = 90°) and are shown in Figure 12(b). Frequency versus return loss and isolation of SV and SH-Ports is represented in Figure 3.
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(b)
3.2. XV1-Port (Port 3) and XV2-Port (Port 5)
XV1 Port is used to resonate at a frequency of 9.65 GHz (X-band). It is in series fed with microstrip feeding, and it is situated in one of the corners of the final antenna. The return loss of port 3 is −45 dB at 9.65 GHz. Simulated radiation patterns of the X-band antenna at port-3 at 9.65 GHz are two orthogonal planes (Phi = 0° and Phi = 90°) and are shown in Figure 13(a). XV2-Port is used to resonate at a frequency of 9.65 GHz (X-band), and it is in series fed with microstrip feeding and situated in one of the corners of the final antenna. The return loss of port 5 is −40 dB at 9.65 GHz. Simulated radiation patterns of the X-band antenna at port-5 at 9.65 GHz are two orthogonal planes (Phi = 0° and Phi = 90°) and are shown in Figure 13(b). Frequency versus return loss and isolation of XV1 and XV2-Ports is represented in Figure 5.
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(b)
3.3. XH1-Port (Port 4) and XH2-Port (Port 6)
XH1-Port is used to resonate at a frequency of 9.65 GHz (X-band), and it is in series fed with microstrip feeding and is situated in one of the corners of the final antenna. The return loss of port 4 is −40 dB at 9.65 GHz. Simulated radiation patterns of the X-band antenna at port-4 at 9.65 GHz are two orthogonal planes (Phi = 0° and Phi = 90°) and are shown in Figure 14(a). XH2-port is used to resonate at a frequency of 9.65 GHz (X-band), and it is in series fed with microstrip feeding and is situated in one of the corners of the final antenna. The return loss of port 6 is −44 dB at 9.65 GHz. Simulated radiation patterns of the X-band antenna at port-6 at 9.65 GHz are two orthogonal planes (Phi = 0° and Phi = 90°) and are shown in Figure 14(b). Frequency versus return loss and isolation of XH1 and XH2-Ports is represented in Figure 6.
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(b)
Good agreement between the simulated and measured results is obtained with directional characteristics. It can be found that the peak radiation happens in the broadside direction at these two frequencies. The polarization levels at 3.2 GHz in the Phi = 0° and Phi = 90°-planes in both vertical and horizontal polarizations are below −20 dB. The SLL level at 9.3 GHz in the Phi = 0° and Phi = 90° planes in all ports is below −15 dB. The simulated SLL is at −17.5 dB in both Phi = 0° and Phi = 90°-planes at 3.2 GHz in H-polarization and V-polarization at 9.65 GHz; the SLL remains below −21.5 dB in the Phi = 0° and Phi = 90°.
4. Analysis of MIMO Characteristics
MIMO is a technology related to the antenna for a wireless communication system in which multiple transmitters and receivers are present. The primary usage of the MIMO antenna is to optimize the data speed and minimize errors, and it can also boost the radiofrequency simultaneously, providing the most stable connection. The most significant advantage of having MIMO character is that it decreases packet loss, contains the following characteristics, and presents the analysis of the proposed antenna with respect to the following mentioned characteristics:(1)Envelope correlation coefficient (ECC)(2)Diversity gain (DG)(3)Channel capacity loss (CCL)(4)Mean effective gain(5)Mutual coupling
4.1. Envelope Correlation Coefficient
The envelope correlation coefficient is the correlation between two independent antennas polarized in two different directions, i.e., vertically and horizontally polarized. ECC value is considered while calculating the antenna radiation pattern, polarization, and relative phase of the fields between the two antennas. Equation (1) represents the envelope correlation coefficient present in mathematical form:
Equation (1) shows that ECC can be measured with network analyzer and 2-ports-parameter measurements. And, the values of simulated and measured values of ECC is good in agreement; i.e., both simulated and measured ECC value is less than 0.05 as shown in Figure 15.
4.2. Diversity Gain
Diversity gain is the value talk about the transmission power which can be reduced when a diversity scheme is introduced without a performance loss. Moreover, it is expressed in decibels. Specifically, this is the reduction in the fading margin obtained by reducing the fading with the smart antenna. Moreover, equation (2) represents diversity gain. Here, a diversity scheme is a technique used to enhance the message signal in a more reliable manner by using two or more communication channels with different characteristics. Diversity gain is achieved as below 9.5 dB for all X-band Ports which was represented in Figure 16.
4.3. Mean Effective Gain
Diversity performance analysis mean effective gain is an important parameter and is defined as the mean received power in the fading environment.
Here, K denotes the number of antennas, i represents the antenna under observation, and μirad is the radiation efficiency. For good diversity performance, the practical standard followed is that MEG should be −3 ≤ MEG (dB) < −12.
4.4. Channel Capacity Loss
It was enlisted among the MIMO performance parameters, thereby providing details of channel capacity losses of the system during the correlation effect. The CCL is calculated numerically by equations (4) and (5). Simulated and measured results of channel capacity loss are similar in a manner such that the CCL is obtained as below as 0.5 bps/Hz which is shown in Figure 17.where Rii = 1−(ΣNj = 1|Sij|2) and Rij = −(S ∗ iiSij + S ∗ jiSij)
4.5. Mutual Coupling
Mutual coupling in two different antennas is related to the current distribution on the surface antenna; if the current flows in the same direction on the adjacent sides of both antennas, the mutual coupling increases. Similarly, if the currents are in the opposite direction, the induced mutual coupling is suppressed such that, for all X-band ports, mutual coupling is measured and simulated that was represented in the following figure, and it shows a very good agreement between them. Mutual coupling is obtained in the range of −30 dB to −55 dB as shown in Figure 18.
Table 4 shows the comparison of the author’s work with the already existing works related to the proposed design. The antennas proposed in the article [1–8, 10, 15, 16] are operated in dual-band frequency. Still, all of them achieve low isolation compared to the proposed antenna in which isolation is performed at more than 38 dB. While comparing the bandwidth in [4], the S-Band is relatively low compared to our proposed antenna, which achieves a frequency of 0.12 GHz when compared to X-Band frequency, i.e., 0.27 GHz, which is superior bandwidth compared to the articles [4, 10, 12, 14, 15]. Regarding the application concern, we need to take the antenna size. Size complexity is present in terms of length, breadth, and thickness when comparing the dimensions of the proposed antenna with the antennas of articles [2–8, 10–16]. Our antenna is small in size and produces better compatibility. As we know, the reduction of the side lobe level is the secondary target, but in [2–5, 10, 12, 14] research articles, they proposed an antenna with minimal side lobe level; when comparing to the antennas, our antenna achieves a better side lobe level which helps the transmission works effectively in their respective bands. The use of Lossy Substrate FR-4 with 4.4 permittivity results in less gain and reasonable isolation. It can be enhanced by changing the antenna element with RT-DUROID 5880, which has a permittivity of 2.2.
Table 5 represents the comparison of MIMO characteristics with X-band antenna. Since while comparing to all 5G MIMO antenna which are cited in Table 5 with X-Band, MIMO antenna is achieved better MIMO characteristics such that it is capable to perform multi-input and multioutput operation while used in real time application.
5. Conclusion
The proposed antenna operating at both S and X-bands for SAR applications is included in this paper. In this proposed antenna, single square-shaped element with coaxial feeding is used to operate at the S-band frequency band and the remaining four 1 × 3 series fed antenna is used to operate at the X-band frequency band. Square-shaped coaxial fed S-band antenna is fed in a dual-polarized manner which is orthogonal to each other. The proposed antenna is simulated in computer simulation tool–2021 (CST-2021). Since X-band antenna is MIMO based antenna, we achieved results such as ECC is obtained as <0.05, diversity gain as 9.5 dB, channel capacity loss as 0.5 bps/Hz, and mutual coupling obtained as in the range −30 dB to −55 dB in the desired band. By using RT-Duroid 5880 as an antenna element, we can achieve better return loss in both the S-band and X-band. The total size of the proposed antenna is 100 mm × 100 mm × 1.6 mm which occupies less area.
Data Availability
The data used to support the study are available from the first author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest.