What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier?
Why A ferrite bead slipped over a lead acts as a small inductor whose core material is deliberately lossy at VHF and UHF, so it turns unwanted high frequency energy into heat while adding almost no impedance at HF. That kills the tendency of a high gain transistor to oscillate parasitically well above its intended operating band. Because the bead is only a few ohms at HF, the desired signal passes through essentially untouched.
Watch out Butterworth filters are designed, tuned passband filters for shaping a response, not the broadband lossy element you want for parasitic suppression; steel cores are for low frequency or power transformer use and are useless at VHF/UHF.
Ferrite bead = broadband VHF/UHF energy sponge: lossy up high, invisible at HF.