Figure 6: (a) Illustration of the working principle of an LWA; (b) dielectric-filled waveguide based LWA with doped p-i-n diodes [97]; (c) Modulated LWA on a microstrip guiding structure using varactors for beam-steering [98]; (d) a full 360° beam-steering antenna for a millimeter-wave 5G base-station [99] using three corrugated SIW-based LWAs [100].
Microstrip lines and substrate integrated waveguides (SIWs) are popularly used as the guiding structure for LWA [89], [90]. Microstrip guiding structures tend to be highly lossy because of the dielectric but allows easy fabrication and conformality. Waveguides, on the other hand, tend to be bulky and cannot be miniaturized. SIWs or dielectric filled waveguides (DFW) were introduced as intermediate solution to both the problems, where a low-profile rectangular waveguide is created with a dielectric substrate sandwiched between two conductive planes [91].
LWAs are divided into three categories: periodic, uniform, and quasi-uniform [92]. A uniform LWA has a single uniform slot along the structure and can only scan in the front quadrant. A quasi-uniform LWA is made up of closely placed radiating elements. Since these elements are placed very closely, it can be considered to be almost uniform, and hence, quasi-uniform. A periodic LWA, as the name suggests, has radiating elements arranged periodically. Such LWAs can scan a beam from backward to forward endfire direction [93]–[95]. Conventional periodic LWAs cannot scan the broadside of the antenna because of open stopband suppression [86] as the travelling wave starts acting as a standing wave at the broadside and all the reflections being in phase start creating an impedance mismatch. However, different techniques have been presented in the literature to enable this broadside scanning [93], [96].
Even though LWAs tend to change beam-direction with change in frequency, several innovative techniques have been introduced in the past to actively control this beam-steering using active components such as RF micro-electro-mechanical switches [101], p-i-n diodes [102]–[105], varactor diodes [98], [106], liquid crystals [107], etc. One such work was presented by Yashchyshynet al [97], [108] where a DFW’s substrate material was doped at specific positions on the slots to create p-i-n diodes. The DFW was then controlled using bias lines to manipulate the current distribution on the guiding structure and generate beam-steering of ±45°.
Several such reconfigurable LWAs have been shown in the literature to generate active beam-steering using SIWs as the guiding structure [109]. SIWs are composed of hundreds of vias acting as electrical walls on a planar circuit, which not only makes the antenna rigid, but also costly to manufacture. To further add to it, it is difficult to attain DC biasing on a conventional SIW, and hence, making it difficult to implement diodes and related components. As a solution to this, Eccleston et al [110], [111] introduced corrugations as the replacement for vias in an SIW. The new corrugated SIW (CSIW) works within TE10 mode and can be used as new guiding structures for millimeter-wave antennas. Several iterations of such corrugated SIWs have been shown in the literature to achieve fixed frequency beam-steering. These include straight [112], bent [113], folded [114], half-mode [115] and twisted [116] corrugations. One such work upgraded the corrugations with inter-digitated capacitor design to achieve low transmission loss on the guiding structure and with the introduction of p-i-n diodes, the design demonstrated manipulation of surface currents, which was then used to achieve wide beam-steering in both azimuth and elevation planes [100], [117]. Finally, the new beam-steering CSIW was coupled with new lenses to generate a full 360° beam-steering base-station antenna for millimeter-wave 5G networks [99].
Leaky wave antennas are great planar structures which can achieve wide beam-steering and can be fabricated at low-cost. However, they will always have the inherent problem of frequency-controlled beam-scanning, which can either be used with resource reallocation methods or can be mitigated using new innovative techniques as described earlier in this section.