# Application of Compact Folded-Arms Square Open-Loop Resonator to Bandpass Filter Design
## 7 Conclusions  
A fifth-order (that is, 5-pole) bandpass filter, based on the compact microstrip folded-arms square open-loop resonator, has been proposed, analysed, designed, implemented, fabricated, and measured. The filter device has been characterised using a Vector Network Analyzer, with the experimental responses used to validate the simulation responses. The presented measurement and EM simulation results indicate decent agreement. The EM simulation responses achieved a minimum insertion loss of 0.8 dB and a very good channel return loss of 22.6 dB. The measurement results, on the other hand, show a minimum insertion loss of 0.9 dB insertion loss and return loss of better than 19.2 dB. The designed filter component footprint is 36.08 mm by 6.74 mm (i.e., 0.26 kg \(\times\) 0.05 kg), and \(\lambda\)g is the guided wavelength of the 50 Ohm microstrip line at the centre frequency of the proposed fifth-order bandpass filter. This type of filter has popular applications in wireless transmitters and receivers. Its main function in the transmitter is to limit the bandwidth of the output signal to the band assigned for the transmission. By this effect, the transmitter is prevented from interfering with other stations. In the receiver, a bandpass filter permits signals within a certain band of frequencies to be received and decoded, while stopping signals at undesirable frequencies from getting through. The bandpass filter, reported in this paper, achieved an additional 20% physical device size reduction when compared to filters implemented using the conventional microstrip square open-loop resonator. The size reduction is due to the folded-arms of the FASOLR. The proposed compact filter would be very essential in radiofrequency (RF) front end of cellular radio base station transceivers as it would ensure the overall miniaturization of an entire system. Also, considering the fact that filters are used as building blocks for ubiquitous microwave components including multiplexers (e.g., diplexers), filtering antennas, filtering power dividers/combiners, etc., one can see the benefit of striving to make the filter design as compact as possible. Therefore, our proposed filter design is of huge benefit in terms of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \(f_{\mathbf{0}}\) & \multirow{2}{*}{**Filter Order**} & **Footprint** & **IL 1** & **RL 2** \\  & **(GHz)** & & & **(dB)** & **(dB)** \\ \hline
[16] & 3.5 & 3 & 0.17 \(\times\) 0.16 & 0.6 & 13.0 \\
[17] & 4.5 & 2 & 0.28 \(\times\) 0.09 & 1.6 & 10.0 \\
[19] & 3.4 & 5  
footprint, being of great interest to RF and microwave engineers and technicians alike. Practical technologies that would impact directly by the compactness of the proposed filter design technique include wireless communication systems. Take space crafts for example, and it may be seen that compact filters are employed in the design of diplexers used in the satellite communication systems of space crafts. This is because space crafts are built to an effective reduced mass and volume, as highlighted in [3]. Other receiver applications of the proposed filter include radar, drones, Wi-Fi, etc. All these technologies need filters for appropriate frequency selection.  
Conceptualization, A.O.N.; methodology, A.O.N.; software, A.O.N.; validation, A.O.N.; formal analysis, A.O.N.; investigation, A.O.N.; writing--original draft preparation, A.O.N.; writing--review and editing, E.R.O.; visualization, E.R.O.; supervision, A.O.N.; project administration, E.R.O. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.