Article below is a part of the assembly process of the UHF duplexer – main article is HERE.
Drawing below shows a [AL100] – Coil 6.5mm.
This part is used in the RX cavities. Please note, that if the repeater has a non-standard, reversed frequency, I mean plus “+” offset then this coil will be a part of the TX cavities, but it is unusual.
Especially the diameter of the coil is important factor to set up the notch distance from desired frequency. The rules: big diameter – bigger split between notch and the desired frequency. Smaller diameter – notch closer to the desired frequency of the cavity duplexer.
More turns in the coil – bigger split, less turns – closer split
Very useful tip – spreading away coil turns brings notch closer to the desired frequency, compressing the coil moves the notch further away form the desired frequency!
The coil brings the notch always to the right side of the desired frequency, where capacitor (LINK to be added later) keeps the notch on the left side of the desired frequency.
A typical chart at the picture below the drawing.