Adjusting the coupling of each individual cavity after assembly.
About in- and output coupling.
14. Here after, "DOWNLINK" and "UPLINK" refers to the 2mtrs-repeater-frequencies as used in The Netherlands. We use here an "UPSHIFT" of 600 kHz. So our repeater-transmitters are tuned 600 kHz higher than our repeater-receivers. Interprete this fact to your own situation!
14.1 Amount of coupling.
Experiments showed that:
To tight coupling (over coupling) of the links in the cavities results in a less deeper notch, "pulling" of the pass-frequency when adjusting the notchfrequency, and vise-versa, less selectivity, and lower insertion-loss.
To loose coupling (under coupling) results in a deeper notch, higher selectivity, bad VSWR and much higher insertion loss.
So, the correct amount of coupling of each link is essential!
14.2 Conclusion after all experiments
Coupling should be just a little under-critical, resulting in an acceptable VSWR of abt. 1:1.2, good notch, only little pulling between notch and pass-frequencies, and not to much insertion-loss.
Correct coupling must be measured and adjusted BEFORE notch-components are installed on the cavity. It must be done for every cavity SEPARATELY in BOTH DIRECTIONS (both links in turn acting as input and output).
The coupling-links are then about 32mm inside the cavity.
The exact position of the coupling-links is critical, and MUST be found (WITHOUT the notch-components on the cavity), by changing the couplings, and measuring the VSWR in BOTH directions.
Filterproperties and the ease of further adjustment depends fully on this adjustment!
Adjusting the coupling.
14.3 Place the cavity upright.
Connect to both couplinglinks temporarely an ELECTRICAL 1/2 wave long pieces 50 ohms coax (abt. 68cm RG58) with a coaxplug on one end.
Solder the braid of the free end AS SHORT AS POSSIBLE to the mass-side of the coupling link, and the inner conductor AS CLOSE AS POSSIBLE to the toplate "A" on the "hot" side of the coupling link.
At this stage, do NOT use other cable lengths than an electrical 1/2 wave long for testing.
Connect at the (input-) cable a good VSWR-meter and a transmitter. This VSWR-meter must be designed for measuring low powerlevels (a few watts).
The transmitter wich is used as a signal generator, should be best of the synthesizer type, have tuningsteps of 2.5 or 5 kHz and have an output power of several watts. No counter is needed then.
Connect at the other testcable an outputmeter and a good dummyload. The VSWR of this meter/dummyload combination must be GOOD and must handle full transmitterpower for long periods.
BEWARE: The input VSWR of the cavity can become very high or low, when it is not tuned to the transmitter frequency. The transmitter should then automatically reduce its outputpower to prevent damage to the transmitter.
Notice that in such circumstances the HF-voltage into the cavity can become different from nominal. Bare this in mind while measuring!
A transmitter with higher power can also be used. Connect it via an attennuator. This attennuator must be connected directly to the 1/2 wave cable, or to the in/output recepticle.