VHF high-power tunable low-pass filter based on nested coupled helical inductor

In this paper, a novel VHF high-power tunable low-pass filter based on nested coupled helical inductors is reported. By analysing the theory of nested coupled spiral inductors, a proposed VHF low-pass filter based on a set of nested coupled spiral inductors is realised, which is equivalent to a third-order low-pass filter with a transmission zero. The resulting filter offers advantages of compact structure, low insertion loss, high power and wide adjustable range. As a demonstration, a new VHF high-power tunable low-pass filter with two sets of nested coupled spiral inductors is designed and fabricated. The measured results show that the filter having a size of 30 mmx20 mmx20 mmx an insertion loss of less than 1.0 dB, a tuning range of 50% from 150 MHz to 250 MHz, with a band rejection greater than 10 dB, and that can withstand a power of at least 47 dBm.