14.4 Adjusting the in- and output couplings of a cavity.
14.4.1 Turn-out the finetuning bolt completely, then turn 3 turns in and lock it.
Change the transmitter-frequency, and find the resonance-frequency of the cavity (max output / min VSWR).
If nessacery, connect a couplinglink to the inputplug, and find the resonance with a dipmeter.
14.4.2 Now move the resonance of the cavity towards the wanted frequency, by tapping down or lift-up the course tuning rod, until it is near enough to the final wanted frequency (+/- 10 KHz).
14.4.3 Measure the VSWR on the resonance frequency.
14.4.4 Change the position of the coupling-link connected to the transmitter just a little (1-2mm) by lowering it (tightening the coupling).
Retune the signal generator to resonace.
If the VSWR now is better then before (nearer to 1:1), the coupling was under-critical before the lowering of the link. You should move the coupling link in the same direction if nessacery.
If the VSWR is worse then before, the coupling was over-critical. You should move the coupling link up.
14.4.5 Change the coupling little by little, until VSWR is at the lowest point (very near 1:1).
Keep the link in parallel to the central conductor.
14.4.6 Check every time for resonance (lowest value of VSWR), by retuning the transmitter, as the resonance could change a bit after repositioning a link.
14.4.7 Swap transmitter/VSWRmeter and powermeter/dummyload on the cables.
Repeat the adjustments in 14.4.3 to 14.4.6 now with the other link.
14.4.8 Swap instruments again, and measure the VSWR.
Keep correcting alternately both links until BOTH sides have an INPUT-vswr of 1:1.
14.4.9 Note how much the marking of each link is above topplate "A".
This gives an idea how much of the link is in the cavity. It helps adjustment to final VSWR and finding the pre-adjustment for the other cavities.
14.4.10 When BOTH links have a VSWR of 1:1, and the position of the marks on the links is noted:
loosen the coupling of both links a little (lift-up the links).
Measure and correct alternately VSWR in both directions, until the VSWR on BOTH sides is equal and abt. 1:1.2.
14.4.11 Check the insertion-loss of the cavity. It should be less then 1 dB.
IMPORTANT: THE ADJUSTED COUPLING HAS INFLUANCE ON ALL PROPERTIES OF THE CAVITY. FROM NOW ON DO NOT CHANGE THE POSITION OF THE ADJUSTED LINKS UNDER ANY CIRCUMSTANCES. THE POSITION NOW (A LITTLE UNDER-CRITICAL TO A VSWR OF 1:1.2) IS OPTIMAL!
14.4.12 So, before going on, LOCK THE LINKS IN PLACE, by fixing the "hot" side of each link to topplate "A" with slow hardening 2-component glue. Harden in abt 30 min. by applying some heat.
This cavity is now ready to be used as a bandpass-cavity. It can be changed to a bandpass-notch filter by adding an extra component.