Antennas are the key components in wireless communication systems. Their main function is to transmit and receive signals. Further to the increasing trend concerning simplicity and miniaturisation of communication systems, it is necessary to merge antennas and filters into a solitary device that simultaneously achieves both radiating and filtering functionalities. This integrated device is known as a filtering antenna (or filtenna). The filtenna lessens the pre-filtering condition and advances the noise performance of the system.
Substrate-integrated waveguide filtering antennas are currently receiving a wide range of interest and have been achieved using different design techniques. Studies [
84,
85,
86] proposed single band SIW filtennas with controllable radiation nulls. The radiation nulls enhance the radiation features of the filtering antennas. Electric and magnetic combined coupling structures and basic modes were used to generate two radiation nulls as explained in [
84]. The two radiation nulls may be independently operated to attain elevated selectivity and flexibility in the suggested filtenna, as explained in [
85]. Dual-band SIW filtering antennas have also been reported [
87,
88,
89]. These type of filtennas have the added advantage of improved out-of-band suppression, with a multifunctional single slot. The etched multifunctional slot helps to improve the out-of-band suppression and the radiation gain. SIW filtennas have also been proposed and designed for millimetre-wave applications [
90,
91,
92,
93]. A performance-improved filtenna was exploited in [
90] to act as the source of a 60 GHz Fabry–Perot Cavity antenna. This greatly improved the filtering function of the proposed filtenna. Compact size and low insertion loss was achieved in [
91] by using eight-mode SIW cavities fully shielded by metallised vias. Multilayer SIW structures have also been employed in the design of antennas as reported in [
92]. The structure consists of an SIW feed with a coupling slot, differential-fed L-shaped probes, and radiating patches. The authors of [
93] proposed an antipodal linearly tapered slot filtenna with diverse split-ring resonators. The proposed design achieved a lower band suppression between 23.5 and 27.5 GHz. An SIW antenna based on negative order resonance is reported in [
94]. The major difference between the antenna reported in [
94] and those reported in [
84,
85,
86,
87,
88,
89,
90,
91,
92,
93] is the absence of the integrated filtering property in [
94]. This simply means that, while the antennas in [
84,
85,
86,
87,
88,
89,
90,
91,
92,
93] will radiate energy as well as filter out unwanted frequencies, the antenna reported in [
94] will only radiate energy, while depending on separately designed filters for frequency filtering.
6. SIW Sensors
Sensors and wireless identification are modern technologies with a wide range of applications. Some popular utilisations include indoor and outdoor tracking, sensing, operation of tags attached objects, human bodies, etc. Sensing and wireless identification of people and physical objects has enabled them to become smartly connected. This has led to scientific breakthroughs in various fields of human endeavours including healthcare, health monitoring, disaster monitoring, logistics, social networking, smart environments, security services, etc.
Substrate-integrated waveguides have been widely employed in achieving humidity sensors [
95] and rotation sensors [
96] due to their improved performance and accuracy. Microstrip-based sensors [
97] normally exhibit poor quality factors and moderate sensitivity. This explains why their use is limited to testing of only medium to high-loss dielectric materials as explained in [
98,
99,
100,
101]. The SIW sensor reported in [
98] was excited using an external coupling topology incorporating a transition offset. The sensor exhibits yielding at high sensitivity of 20 MHz which is equivalent to 0.67% in terms of normalised sensitivity [
98]. An SIW-based sensor with negative order resonance is reported in [
102]. The sensor is reported to be “compact dielectric-permittivity of liquid samples”. The sensor was implemented using SIW and achieved a compact footprint of (0.25 × 0.42)
λo, where
λo is the wavelength in free-space at the sensor’s operating frequency.