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Planar structure provides a simple structure and allows flexible filter design, and it is easy to integrate tuning with it. Cho, Y. H. et al. [17] proposed reconfigurable filter, which changes from bandpass to bandstop by changing the coupling coefficient of resonators controlled by RF MEMS switches and varactors. Bouyge D. A. et al. [18] proposed size reduction using split ring resonators (SRR) and frequency agility using vanadium dioxide switches. Different combination of switches adds a pole to the bandstop filter to change configurations of bandwidth and centre frequency. Bandwidth or center frequency switching is also possible with P-I-N diodes. Miller A. et al. [19] designed tunable bandpass filters with switchable bandwidths using short circuit coupled lines and short circuit stubs with P-I-N diodes. When the P-I-N diodes are in the ON mode, it achieves around 35% fractional bandwidth (FBW), while in OFF mode FBW is reduced to 16%, at a centre frequency of 1.9 GHz.

High-temperature superconducting (HTS) filters are popular due to their low insertion loss and they can be easily realized with higher orders. A HTS-based tunable bandpass filter with dual-mode resonator and Gallium arsenide (GaAs) varactors was proposed in [20]. The said work is designed for L-band tuning range from 0.784 GHz to 0.918 GHz (16%) while having small variation in fractional bandwidth. Hu Jiang et al. [21] proposed quasi-elliptic coplanar waveguide (CPW) tunable bandpass filter using BST varctors in Ka band. The filter exhibits area reduced by 35% with constant fraction bandwidth (FBW).

Filter size miniaturization is a demand in modern communication systems. Surface Integrated Waveguide (SIW) structure has low losses, higher Q value and is easy to integrate with other planar technologies. Bin You et al. [22] proposed tunable filter based on Half-mode SIW (HMSIW). HMSIW has the advantages of SIW along with size reduction to 50%. Bin You et al. [22] utilized reduced space to load JDV2S71E varactor (manufactured by TOSHIBA) as a tuning element. Designed HMSIW filter provides both center frequency and bandwidth tuning capabilities.

Filter size can be reduced in different ways in microwave circuit design. One of them is by using metamaterial. A metamaterial-based tunable is proposed in [23] for size reduction and ease in integration for FM receivers. A dumbbell-shaped slot metamaterial along with varactor diode is used to change center frequency from 9.1 GHz to 11 GHz. Another way to reduce the size of the filter is to use Defected Ground Structure (DGS) with fractal geometry. Research activity has increased to use DGS in planar filters as it offers better performance and compactness. If a higher iteration level of fractal DGS is utilized, the bandpass filter becomes more compact [24].

The next section proposes a tunable bandpass filter utilizing hairpin structure along with fractal DGS for size reduction. Tuning is proposed by varactor diodes used in the geometry of fractal DGS.