Over on YouTube user Mile Kokotov has uploaded a video showing his home made low loss helical bandpass filter for 145 MHz, but also tunable from 110 MHz to 160 MHz. Bandpass filters are useful for the RTL-SDR as often strong out of band signals can cause overload, causing poor reception. A bandpass filter blocks all signals outside of its pass band. A helical bandpass filter is simply a coiled wire enclosed in a conductive container that can be tuned with a variable capacitor made out of two plates.
In his video Mile shows the inner construction of his helical filter, explains the parts and shows what calculations he used for construction.