Review on fixed-frequency beam steering for leaky wave antenna

This paper aims to survey the efforts of researchers in response to the novel and effective technology of control radiation pattern at a fixed frequency for leaky wave antenna (LWA), map the research landscape from the literature onto coherent taxonomy and determine the basic properties of this potential field. In addition, this paper investigates the motivation behind using beam steering in LWA and the open challenges that impede the utility of this antenna design. This paper offers valuable recommendations to improve beam steering in LWA. The review revealed the development and improvement of several techniques of beam scanning LWA. However, several areas or aspects require further attention. All the articles, regardless of their research focus, attempt to address the challenges that impede the full utility of beam scanning and offer recommendations to mitigate their drawbacks. This paper contributes to this area of research by providing a detailed review of the available options and problems to allow other researchers and participants to further develop beam scanning. The new directions for this research are also described.

circularly polarized LWA, a periodic HW-LWA, a CRLH SIW LWA, a coupled HW-LWA EH1-mode antenna, a HW-LWA with periodic short circuits, a substrate integrated composite right left hand (CRLH) LWA with two elements [15]. Since the electric field is bound strongly between the microstrip line and the ground plane, the uniform microstrip line's fundamental mode lacks radiation. Some of the microstrip's higher-order modes radiate as leaky waves. Electric-field is null at center and phase is reversal in first higher-order mode This paper aims to analyses the fixed frequency beam steering of MLWAs a topic that has not been systematically reviewed. This paper aims to provide valuable insights for researchers by comprehending the options that are available and unlocking key features that characterize this potential line of research. The following section explains the coherent taxonomy. This particular topic has been discussed in many manuscripts, and a systemic review is necessary to summarize all the latest research findings. The method of this paper is highlighted by discussing the motivations of previous studies. This also aids new researchers in developing their respective studies.

Beam Steering Techniques
A map of taxonomy categorizing articles following goals and the respective study direction of contribution is provided in Figure 1. Seven main classes are included in this categorization. Research associated to the control of radiation pattern classification is the first class. A beam steering is indicated by the results of articles within this category via metamaterial technique. Seventeen articles are in this set (17%). Fourteen articles representing studies about loading with varactors are in the second class (14%). Eight articles representing the studies on beam steering by applied electric and magnetic field to the liquid crystal are in the third class (8%). Six articles representing studies on beam steering by altering capacitor capacitance, connected at the HM-SIW free edge side, are in the fourth class (6%). Six articles representing studies of beam steering via the utilization of gap capacitor are in the fifth class (6%). Two articles representing studies on beam steering via stub loaded are in the sixth class (2%). Six articles representing studies on other techniques are in the seventh class (6%). The last class includes four articles that represent review and survey articles about LWAs (4%). The next subsection provides a detailed elaboration on the articles linked with each category. the 63 articles produced by the final set. All selected articles were linked to beam scanning for MLWAs at fixed frequency, with 4 articles review and survey.

Review and Survey Articles
Surveys and reviews of literature related to beam steering make up the last and smallest group of articles (4) in our taxonomy. A review and summary of latest developments for LWAs and the basic physics and operating mechanisms is done by researchers in [16]. Some of the latest developments in the LWA field have been linked to latest developments in the metamaterials area, which has particularly inspired new designs. They have also analyzed  [17], which includes attenuation by objects, antenna misalignments and sever path loss. Explanations are also provided on the way beam steering can solve this issue, as recommended in literature. Distinct beam steering techniques sourced from literature are presented and compared in terms of figures of merit (instead of LWA) in this review. The basic practical applications and theoretical principles of electromagnetic leaky waves are reviewed in [18], with few connections linked to microwave, engineering and optical physics. In addition, discussions are made on the way the theory of leaky wave has a significant role in understanding different phenomena, including some of the anomalous optical impacts that are unexplainable by traditional geometrical optics and diffraction theory. Researchers in [19] have looked into the evolution of CRLH-LWAs to produce few new CRLH LWA. The introduction of the concept of CRLH-LWAs led to the discussion of the concave and convex passive CRLH LWAs along with the conformation impacts in each case. To compensate for the conformation impact and thus redirect focus onto the main radiation beam, dispersion engineering system is introduced. Moreover, reviews are made about the planar and conformal varactors based on CRLH-LWAs that are electronically controlled.

Beam Steering by Using Metamaterials
The initial segment of the research is directed the use of metamaterials to regulate radiation pattern. The type of metamaterials and control main beam technique were classified by utilizing articles listed in this group. This class holds 19% of the selected studies under the classification category. A dielectric film fitted with metal strips and fed by the image line composed of high dielectric material constant was utilized by researchers in [20]. A simple structure is achieved by utilizing a 10.8 relative dielectric constant image line, wide and symmetrical beam-steering angle. A particular weakness of this design system is that it is non-reconfigurable, thus large cells are not operable for other configurations following fabrication. As illustrated in Figure 2, the design in [21] is made up of varactor diodes with unit cell in shunt and series configuration, with the waveguide's β being electronically altered through two DC voltages (VS, VSH) from left hand to right hand. The future work of this design is can switching between the two input ports the beam can be steered from broadside to negative and positive angles, but the gain for each pattern is low between (3-4) dBi.

Loaded Varactor-based Technique
Due to the fact that their capacitance is easily changeable through the bias voltage, provide reconfigurable surface impedance by varactor diodes are integrated into the LWA, thus allowing fixed-frequency beam scanning. Subsequently, the leaky wave's radiation direction can be steered in an electronically way at a fixed frequency. Researchers in [22] are given fixed-frequency beam steering abilities in this communication. A shunt and series capacitive tuning elements load the antenna, thus providing two degrees of freedom which allows manipulation of its dispersion properties. As illustrated in Figure 3, the suggested design of the LWA in this article has shown to have excellent potential for fixed operating frequency applications that need a wide range of beam scanning.

Liquid Crystal (LC)-based Technique
Recently, the LC was coined to the microwave field, from the optical field to serve as a novel and electrically unable material [23]. Orientation by bias voltage of the liquid crystal molecules is possible, which varies its dielectric constant. LWAs with LC usually have characteristics of low profile, linearly tuning, high gain and excellent compatibility with most of materials [24]. The LC may be magnetically tuned or either electrically. Nonetheless, the liquid crystal does have several disadvantages which include a small tuning range when it is utilized in controlled electrically for microwave devices. In addition, a key issue that remains unresolved is its complex configuration.

Loaded Capacitor-based Technique
In this method, alteration of the capacitance of capacitors that are linked along the antenna aperture's length enables steering of the main beam. The β is changed by alterations in capacitance, which then changes the main beam's direction by equation No.1. A HMSIW LWA which digitally scans its primary beam at fixed frequency of 26 GHz was proposed by researchers in [25]. Alteration of the PIN diodes' status allows steering of the antenna's primary beam. The antenna body is linked to one end of the diode while the other end is linked to a printed rectangular patch, which functions as a capacitor. A half width microstrip line's free edge is loaded with several interdigital capacitors in [26].

Loaded Stub-based Technique
A reconfigurable stub-loaded HW-MLWA is presented by authors in [27]. As illustrated in Figure 4, the HW-MLWA with periodic stubs on one side is fixed with a set of periodic patches that may be selectively linked to the ground by utilizing PIN diodes. The microstrip line's effective β is altered by changing the periodic loading on the microstrip line. Thus, regulating the PIN diodes' states (off and on) allows steering of the beam. By observing the radiation patterns for all switches is off and all is on higher capacitance i.e. a smaller reactance results in a main beam directed away from endfire and smaller capacitance i.e. larger reactance results in a beam directed towards endfire. Further, this observation leads to conclude that turning the PIN diodes on results in a relatively smaller effective β compared to the effective β when all PIN diodes are off.

Loaded Gap Capacitor-based Technique
A digital method to scan the periodic MLWAs' beam in steps at a fixed located frequency, using only two DC bias voltage values is approaching in [28]. The control radiation pattern is achieved without altering the operating frequency and need for lumped capacitors by loading the microstrip line's free edge with periodic gap capacitors and regulating their connection to the ground plane by binary PIN diode switches. This method assists in running of analysis and design of reconfigurable periodic structure in a more systematic manner. The prototype antenna's measured scanning range is 29 o at 6 GHz. The measured peak gain at 6 Hz was 12.9 dBi. Its variation inside the range of scan was only 1.2 dBi. Digital switches and narrow gaps that act as capacitors were utilized by [29]. The ease of having a wide range of switch combinations is possible by having each consisting of several digital switches. The main beam can be directed towards several distinct directions via alteration of the switch configurations.

Comparison of Techniques
An overview of each scanning technique found in literature and characteristics following the tables discussed in section 2 is presented in Table 1. Continuous steering based on the active device utilized can be achieved by beam steering by using metamaterials. The antenna's overall size is larger than the metamaterial surface compared with the original antenna. The standard quality of Chemical vapour deposition (CVD) graphene results in low levels of ohmic, as revealed by the first realizations of graphene-based devices [30]. The varactor technique, which is commonly loaded on LWAs, has lower cost of production and can be independently controlled [31]. However, network biasing is commonly complicated, and the varactor can only be utilized at frequencies below 10 GHz [32], thus limiting its application. The MEMS can switch between two distinct states, but it possesses a small and discontinuity tuning range as a tunable mean [33]. MEMS, ferrites and diodes, are mainly utilized in the LWA as tunable means. Ferrites possess light weight, high tuning speed, high permittivity and permeability [34], and can easily achieve wide scan angle and miniaturization without lowering gain [35]. However, they entail a high cost of production and strength magnetic field and have bigger effects on microwave band devices [36]. The lumped element (capacitor) technique may not be suitable for space borne applications because they can result in losses. Owing to its low insertion losses and performance that is not degraded at any point, the gap capacitor beam steering technique may be ideal for beam steering.

Recommendations
We provide brief recommendations for beam steering for LWAs at a fixed frequency that is dependent on technique utilized based on literature, as illustrated in Figure 5.

Recommendations Related to the using Metamaterials Technique
Researchers in [37] made recommendations on the general performance of this simple antenna structure, citing it as extremely promising for integration into future version of all-graphene reconfigurable THz sensors and transreceivers

Recommendations Related to the Use of the Varactor Diode Technique
Researchers in [38] recommended further improvements of the pattern by increasing the width of the host transmission line's ground place (this enhancement is limited by technical requirements to the structure's maximum overall width). From the perspective of all antenna parameters, the matched load terminated structure is the best design.

Recommendations Related to the Use of the Liquid Crystal (LC) Technique
Researchers in [23] suggested that improvements can be made by using LC materials with higher anisotropy to increase the difference between the two tuning states. Improved R&D LC mixtures, which are only available in small quantity now demonstrate almost two times higher tunability than the LC utilised for demonstration.

Recommendations Related to the Use of the Loaded Capacitor Technique
The introduction of interdigital capacitors allows the control of the reactance profile at the free edge, whereas digital switches allow the control of their connection at the ground plane [26]. Authors in [39] recommended two significant effects of adding a capacitive load.

Recommendations Related to the Use of the Stub-based Technique
The authors in [27] suggested that a good impedance match is possible by using tapered transmission line at the feed port and the matching port.

Recommendations Related to Use of the using Gap Capacitor Technique
This approach was suggested to assist in the design and analysis of a reconfigurable periodic structure in a systematic manner by researchers in [40]. The capacitance of the gap between the patch and microstrip edge influences the beam scanning range of the proposed design antenna

Conclusion
This review covers the key item of this research, that is, MLWAs' beam steering. A systematic review protocol is designed based on articles using three key terms. The Methods section describes the rationale of this selection. The identified exclusion and inclusion criteria underwent three iterations of article screening and reading. Several beam steering methods obtained from the literature are compared and loaded gap capacitor technique beam steering is identified as the ideal form of beam steering because it does not involve insertion losses. This research method is highlighted by discussing the motivation of previous studies. It also assists new researchers in building their respective research works. Analyses assist researchers in developing more research experiments that are within the scope of the research. The gaps highlighted by the distinct analysis type of beam steering techniques can be addressed by these experiments. For instance, they help identify techniques that have not been tested in our applications. The analysis of previous studies' technique configurations also enables this taxonomy to assist researchers in designing their studies. The knowledge of the taxonomy patterns of previous research also enables researchers to identify possible directions for future research.