Index
14.4 Adjusting the in- and output couplings of a cavity.


14.4.1 Turn-out the finetuning bolt completely, then turn in 3 turns 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 Change the resonance of the cavity towards the wanted frequency. Tap 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.
14.4.4 Change the position of the coupling-link connected to the transmitter a little (1-2mm) by lowering it (tightening the coupling). If the VSWR now is better then before (nearer to 1:1), the coupling was under-critical before the lowering of the link. If the VSWR is worse then before, the coupling was over-critical.
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 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 adjustment 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), and 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: 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! THE ADJUSTED COUPLING HAS INFLUANCE ON ALL PROPERTIES OF THE CAVITY.

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 heath.

This cavity is now ready to be used as a bandpass-cavity. It can be changed to a notch-filter by adding an extra component.

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