Helical Filter for Satellite Reception

This page details the design and construction of a helical filter for 144 MHz satellite reception, specifically to mitigate strong local interference. The author, VE3NEA, experienced severe overloading of SDR receivers (RTL-SDR, RSP1a, Airspy Mini) from Land Mobile Radio Network signals between 141 MHz and 143 MHz, making 2m band reception impossible. The article explains the theory of helical resonators, referencing Anatol I. Zverev's "Handbook of filter synthesis." It covers the equivalent circuit of a two-resonator passband filter, the role of stray capacitance (C3) in determining the coupling coefficient (K), and the importance of critical coupling (K=1) for optimal filter performance. The author clarifies misconceptions about tap positions and impedance transformation. The filter aims to provide narrowband separation for satellite signals in the 145.8 - 146 MHz range, with only 3 MHz spacing from interference.