Article below is a part of the assembly process of the UHF duplexer – main article is HERE.
Drawing below shows [AL100] – Copper M5 Rod TX
The M5 copper rod – it does a final tuning for the desired frequency – not for the notch (kind of…) – please read explanation below!!!
Also please note all RX and TX things refer to a standard offset for the repeater (minus offset on your radio). If your repeater uses reversed frequencies (plus offset on your radio) then you need to reverse everything what I said below 🙂
With no copper rod and the main conductor sticking out 156mm the resonance frequency is 447.000MHz***, so adding the rod inside of the cavity adds extra capacity and virtually increases the length of the conductor. In my case moving the rod 73mm inside of the duplexer cavity brought resonance frequency 447.000MHz down to 439.7125MHz.
*** I will probably soon exchange the copper TX conductor with the new one, longer 3mm, which will be 159.4mm, to get the resonance closer to 439.7125 MHz.
The desired frequency graph is not very sharp (see pic below) so I use the rod to tune the pass frequency and then I play with capacitor to get the notch frequency as accurate as I can.
Playing with the capacitor does NOT change the pass frequency – capacitor sets up ONLY the offset of the notch from the resonance frequency! If the main frequency moves, the notch mover together with it – when main conductor length changes (in our case it is fixed) or when the tuning rod moves.
So then I do the very final tuning for the notch (!) using the copper rod again – yes, it changes the pass frequency, but as you can see below the pass frequency is not very sharp at all, so it does not really matter, however it is much easier than doing the final tuning using just the capacitor!