Notch or Band Pass

[petnrdx]

Here is how I do it.
Dummy load on RX port.
Generate signal into TX port at RX freq.
"Measure" that signal at antenna port.
Adjust for reject, as signal gets weaker, increase the generated signal keep adjusting reject to as great as possible.
Switch dummy load to TX port.
Switch generator to Rx port, and TX frequency.
Change freq on "measuring" receiver or whatever you are using to Tx freq.
Adjust for reject as above.
Repeat at least three times.
Should be able to get 75 or more dB of reject on each "half" if not you either are mis-tuned or might be beyond the range the duplexer will tune.
If you have a VNA you can also make adjustments to the "coupling" via the small access holes to get the device to really be 50 ohms, and get the insertion loss, reject, SWR at your frequencies to be what you really need.
Those interconnecting semi-rigid coaxes between the cavities are frequency critical, so if they are the wrong length, you won't be able to get the thing to tune properly.

[maxkelley]

Also, I haven't seen this mentioned yet, but *DO NOT* adjust the duplexer tuning under power, aka, do not twist the screws while you're transmitting into it, as you have the possibility of arcing inside the cavity and ruining the duplexer. Make a tiny adjustment, key up, look at the meter, release the PTT, and adjust again, etc.

[g3ueq]

Originally Posted by petnrdx

I am sorry, this is likely to start a storm, but...
You really won't get good results tuning a reject duplexer with a watt meter and transmitter.
I have worked on dozens of these "flat pack" duplexers and hundreds of duplexers, filters and combiners.Its kind of what I do for a living.
And I am sure other people on here have done more than me. I am a bit surprised others have not joined in here...
I am not convinced there is a "standard convention" for labeling. Sometimes I have found the "High" and "low" labeled ports to be the PASS, and other brands the REJECT.
Of course the same holds true for TX and RX.
So, I always check them for actual reject and determine which should be which.
MOST of the time you will find the interconnecting semi-rigid coax between the individual "cavities" (resonators if you like) will be LONGER on the lower frequency half. Shorter on the higher frequency half.
MANY of the duplexers brought to me were "tuned" upside down.
You will never get one to operate in specs when tuned upside down.
I use a vector network analyzer or a tracking generator to tune these things.
MUST you have a VNA or TG to tune one?
NO you can use a simple signal generator and tunable receiver.
The VNA or TG are just faster and maybe a little more accurate. Not enuf to matter in most cases.
So, if I were you, I would find someone that has an old Signal generator and use a programmable RX as the receiver.
Then tune each "half" for the greatest REJECT of the opposite frequency.
As another poster stated, terminate the unused port with a good dummy load. ( you could use your antenna, but the antenna is rarely a real 50 ohm load so it is not recommended)
Never tune for maximum transfer of power ( best pass ) on this type of duplexer.
When you try to use a transmitter, you WILL get stray coupling direct from the transmitter ( thru the case ) direct to the receiver ( again, thru the case ) and not be able to "see" the reject notch thru your device and cabling.
Bear in mind that you are looking for a reject measurement in the 70 dB and greater range.
If you had radios that had 80 or 90 dB "isolation" or "shielding" you might be able to get a duplexer of this type to partially work, but I am doubtful of the results.
You might get something to "work" but it won't be near as good as you would get with 30 minutes of tuning with a simple sig gen and a tunable ( programmable ) receiver.

I have just bought a chinese duplexer, it is pre-tuned to my repeater frequencies TX 430.9625 and RX 438.5625MHz, trying to get technical info from them is a nightmare cos of the language barrier.
Mine is made by Jiesai GuangZhou, trying to find out if bandpass or notch, it has longer cables on the low side and shorter on the high side, it has six cavities, I am snowed in at the themoment, so hope to get to work soon to put on spectrum analyser.
rgds
Andy

[petnrdx]

If it is the size of a book, it pretty much has to be a reject only.
If it is the size of a toaster, it could be either.
All the china duplexers I have seen are just simple reject duplexers.

[g3ueq]

Thanks for that, it is a book size one, I managed to fire up the spectum analyser/generator and it seems pretty well to spec, insertion loss on each port is about 1db and isolation is of the order of 85 db, so I am quite happy with this. I have bandpass single cavity so I might put this in the receive path to bring RX isolation up to around the 95+ mark