An electronically tunable multi-passband filter using one varactor diode is implemented based on transmission
line stub method for passing several most favourable applications through multiple operating bands. In this paper,
the filter is designed on Rogers RT/Duroid 5880-substrate and its input and output ports are terminated by 50 Ω
microstrip feed line. The filter passbands consist of low pass filter (LPF) with tunable cut-off frequency which can
reach 0.94 GHz, then several tunable bandpass filters (BPFs) that can cover the following frequency ranges BPF1
(1.94 – 3.33 GHz), BPF2 (3.83 – 4.23 GHz), BPF3 (4.53 – 5.56 GHz) and BPF4 (6.83 – 7.48 GHz) with insertion
loss (IL) of |S21| ≤ 3 dB. The designed filter is the binomial type with 3 elements that are implemented in three
shunt stubs with the middle stub being shorted. A parametric study was conducted for the optimum location of the
varactor diode and an external DC biasing circuit introduced to produce the required reverse biasing for the
varactor diode and its effect was considered. The demonstrated filter is investigated using the high-frequency
structure simulator (HFSS). The measured scattering parameters’ S11 (reflection coefficient) and S21 (transmission
coefficient) results show good agreement with the simulated values.
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