Duplexer quandry

[windmillsforsale]

Hi..I'm new to the forum and have been wanting to join a group for some time.

I'm putting an old uhf Motorola Micor on the air and the duplexer that came with it was a two can bandpass/reject. I believe it to be a T1507. Each can has a pass rod and for reject are a set of screws on the side for adjustment. My first inclination was to get rid of it. Then I came up with another 2 can duplexer just like it. So made a four can duplexer with two high and two low freq. I tested each can individually and was getting about a db loss on the pass and about 55db on the reject after adjustment. Then I hooked both high together and after a small tweak was getting about two db loss on the pass and about 100db on the reject. Did the same for the low pass. Now I hook everything together with a tee for the antenna port using existing short RG 174 cables and ran into a problem

For test equipment I programmed a mobile radio with two chs.for the high and low freq. From inside the radio I brought out a wire that measures signal strength from the radio's discriminator. It's fairly linear from -60 to -120 dbm. I monitor that with a DVM. I set my HP 8640b for a -100dbm and adjust the DVM for .1vdc. To calculate loss for both pass and reject. I simply adjust the generator output until I get the .1vdc which means the receiver is getting the exact same amount of signal as before and I read the amount of db change from the generator dial.

When I check the low freq. pass I insert the generator on the low side, the receiver set to the low ch., on the ant. port and a 50 ohm load on the high side. I am getting about 1.5 db loss. However, when I check the high freq. pass side by inserting the generator on the high side and the receiver set to the high ch. on the ant port with a 50 ohm load on the low side I now get a 16db loss.

To summarize: The cans check out individually. They also check out in series. When all hooked together the low side is not materially affected by the high side. But the high side is loaded down 16db by the low side. Removing the low side cable from the tee reduces the loss to 6db. I also replaced the tee with no change.

Soooo. guys what do you think?

Dave..

[jistabout]

Well, sounds to me like the length of the interconnect cables (between cans and to "T" connector is wrong. In a 4 can T1500 series duplexer, there should be 4 cables of very specific length, dependant upon the desired frequency range. There is an excellent PDF article with everything you need to know about these at http://www.repeater-builder.com, under Motorola Information. Also, 1db loss per can sounds a bit high to me. Check the inner (fixed) coupling loops on each cavity and if necessary, set them to the inner (.5db) positions. Hope this helps

[xmo]

jistabout has pointed you in the right direction. After you have re-checked your configuration against the Motorola documentation - if you still have a problem you should be able to find a local resource to assist in looking at the issue - John [CMC] in Omaha, Ed [RPT] in Lincoln, or Tom [TIK] in Nebraska City.

[windmillsforsale]

After viewing Motorola's information sheet on the duplexer and reading replys to my post I tried a 3" longer coax cable from the low side to the ant port and bingo I got back almost all of the 16 db loss. Thanks for the help.

[bernie]

My two bits worth:
These cavities were built by the parts department in Scaumburg,
in a previous era when manufacturing was considered an honorable persuit.

There have been a few problems:
The individual parts come from various machine shops, plastic moulding companies, etc.

Seems that Parts gave an injection moulder the special plastic for the posts used in the probe cavity.
This was a dirty gray color. Seems that the plastic really fowled the drills,
so at no extra cost provided some nice white plastic that could be easily machined.
Incoming inspection had no problem, the color was not specified.
Then we started to find failed repeaters, with melted plastic posts in the duplexer.
I had to open every one in Honolulu.
There was also a problem with one of the jumpers, the green one, I think, where some were cut to the wrong length.

After 25 years or so, it might be reasonable to presume that the copper plunger in the cavity has become tarnished.

I have found that it is a good idea to completely diss assemble the cavity.
Clean the threads on the Invar rod with a wire brush, never attempt to turn a rusted thread, it will lock.
Remove the lock nut.
Diss assemble paying attention to washer locations.
brighten the plunger with a "Scotch Brite".
Remove the loop probes, brighten the can where the loop contacts the wall. Be careful, this is only copper plate.
Touch up the end of the loop probe, I use a polishing stick.
Rub Caig D-5 on the wall where the loops contact, as well as on the plunger with a cloth or tussue.
Use D-5 or light oil on the threads.

[jistabout]

Ditto on everything Bernie said. . I did that to mine about 2 months ago.

[windmillsforsale]

Thanks guys for all the help. I did open two of the cans and one was still bright and the other was dull in color. All four are extremely touchy on the reject adjustment. But I needed to get the repeater up to check out the rest of the system. I have 160' of 7/8" feeding a DB 411 side mounted. I need 180 deg. coverage to the north and this seemed one way to do it. At a later date I may want to do the duplexer clean operation and now I know what to do. Thanks again.

[bernie]

My two bits worth:
The touchy notches is why I do the wall of the cavity, and lubricate.

[richyradio]

I forget weather it was the loops or the probes, (whichever one was the flat paddles that actually touched the can) but the surface area was so large, that as you tightened the knurled nut, the reject would jump around... sticking a little star washer (no split washer!) under there would limit the contact area and make tuning a little easier....