Abstract
Microstrip filters are often used at high frequencies because they are small in size, cost-effective and offer low insertion loss. However, due to their large size, they are inappropriate for frequencies, especially below 1 GHz. In this paper, a compact microstrip bandpass filter for low-frequency applications is designed and its characteristics is studied. The size reduction is achieved by using a microstrip that resonates at a much higher frequency, and with the help of capacitors, the filter is tuned to the required centre frequency. A second-order microstrip resonator loaded with capacitors operating at a centre frequency of 265 MHz is designed, and its performance is analyzed. A reduction in the size of 72.5\(\%\) is achieved compared to the microstrip filter without capacitors at the same resonant frequency. The centre frequency and the bandwidth can be varied using the capacitors without having to redesign the filter. Two filters with different bandwidths are designed using the same filter layout fabricated by varying the capacitor values. The measured insertion loss obtained for 40 MHz and 55 MHz bandwidth filters is 0.9 dB and 0.7 dB with a return loss of 18 dB and 21 dB, respectively. As the bandwidth increases, the insertion loss reduces at the cost of stopband attenuation at the upper cut-off frequency.
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Duraiswamy, P. Compact Capacitor-Loaded Tunable Microstrip Bandpass Filter for Low-Frequency Applications. J. Inst. Eng. India Ser. B 103, 1453–1457 (2022). https://doi.org/10.1007/s40031-022-00763-1
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DOI: https://doi.org/10.1007/s40031-022-00763-1