A compact helix structure implementation and associated design formula of lumped element second-order bandpass filter circuit for high power frequency-hopping filter are proposed in this paper. The filter schematic provides one, two or three finite transmission zeros (Tzs), and these Tzs locates in the upper stopband to improve the rejection above the center frequency, especially the suppression of second harmonic with two Tzs. The filter schematic is built on a common grounded helix coil of inductive coupled resonator tanks whose suspectance is tunable. Due to the parasitical capacitance of the helix coil, the filter has a feedback capacitor between input and output. Its working mechanism is revealed both mathematically and graphically. The measured results have a good agreement with the 3D full-wave electromagnetic simulation responses. The experimental filter has a insert loss < 1.5 dB , return loss > 15 dB, a 3-dB bandwidth of 5%~8.5% over entire operating range with the power handling capability greater than 49 dBm and the suppression of second harmonic better than 56 dB.
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