Why A helical filter uses a helical resonator, a coil wound inside a shielded cavity, which behaves like a high-Q quarter-wave resonant line in a much smaller package than a straight coaxial cavity. That high Q at VHF and UHF gives sharp band-pass or notch skirts, so helical resonators are the standard front-end preselector and duplexer element in VHF/UHF radios. At those frequencies the physical coil size is small enough to be practical, which is why the technique dominates there rather than at HF.