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.