VHF Notch Duplexers work @ 600KHz?

[Tim]

Hi Folks,

Been pulling my hair out with these duplexers, but desense is still present.
Using directional coupler in the receiver line with the 'antenna' port connected
to a dummy load. No antenna attached.

Using 8 cavity DB products SP-1894. No data available from mfg.

4 cavities in RX & 4 in TX. They are notch type - single SO-239 in top
of cavity. Has adjustment shaft. The 4 cavities on the TX side have
an additional 'T' connector which has a short stub connected. These
stubs raise the high side of the notch. Evidently, the cavities have
a better 'left side' slope than the 'right side'.

Here is a composite view of both notches (combined from separate
pictures). You can see how the notch on the upper frequency looks
without any stubs. The lower notch is the one that has the stubs.

http://img.villagephotos.com/imageview.aspx?i=25438337

Each bank shows about 90-93dB notch per Spectrum analyzer/trk gen.
The picture is misleading - to actually see the depth of the notch, I have
to adjust the reference level, and when that's done, you can't see the
entire notch. It is 90-93dB.

Using Quantar running 60 watts. I've heard rumors that Quantars only
like 'quality' duplexers. (i guess a BpBr).

Anyhow, the original question: do notch cavity duplexers like these
work with a 600khz split?

Thanks,

Tim W5FN

[xmo]

Do notch duplexers work at 600 KHz spacing?

If you have one that was sold for that application it certainly will work fine - assuming it is healthy. Decibel's notch duplexer family had models that work down to 300 KHz spacing.

The notch performance is really all that matters in close-spaced applications. A quick look at the manufacturer's curves for BP-BR models should convince you that the quasi-bandpass characteristic makes virtually no contribution to the duplexer's isolation at your radio's operating frequencies.

The quasi-bandpass does afford some attenuation of other in band frequencies which can sometimes be useful.

Your measurements appear to show that you have enough isolation. Since you are experiencing problems - your duplexer or cabling must have an issue. The first assumption is that you are using only high quality double shielded coax such as RG142, RG400, or RG214 - everywhere - from the radio to the duplexer, the jumpers on the duplexer itself, from the duplexer to the dummy load, and the test equipment cables. Braid over foil cable such as Times 400 should be avoided in close-spaced duplex applications.

Also, all connectors should be high quality, e.g. silver plated with gold center pins. There should be no adapters in use.

If all those conditions are met you should suspect that somewhere in the chain there is a poor connection such as a cold solder joint in a connector or cavity loop that is breaking down under application of RF power and causing generation of wideband noise.

As far as the Quantar - until someone produces a legit Motorola PSB/SRN or FRB document - don't buy into "rumors". Quantar is a premium infrastructure product that will duplex at 600 KHz spacing on 2 meters with less isolation that most other repeaters. Try it yourself . Connect some other repeater to your duplexer - that won't solve your issue - you will probably see more desense.

[Tim]

Hi xmo,

Thanks for the reply.

The duplexer was originally up at 166/164 MHz, so it wasn't
in a close spaced environment.

All of the cables are original RG-9, about 8 1/4" in length between
cans. The cables seem to have some kind of sticky substance on them,
about the consistency of maple syrup. Not much mind you, but something
is there just the same. I found one cable that does have some green
goo coming out of the junction of the coax coating and the PL-259.
The rest of the duplexer & cabinet are clean, so it doesn't appear to
be a general corrosion issue. (like on a offshore drilling platform)

Not using any adapters, and all cables to/from any test equipment are also
double shielded.

I suppose it is possible that there are crummy ground paths, although the
connector's braid is soldered to the PL-259.

I did have to repair 4 internal links - they were not soldered to the SO-239's
pin. It just had a blob of solder on the end of the pin!

A friend of mine has a Sinclair Q-202G, and I am going to borrow it to
see if it works. Might be a better route than trying to replace all of the
inter-can links with new 'clean' ones, & still not being sure that it will fix
the problem.

The guy I knew used to work for Mot, & the problem they had was on a
VHF Astro Digital system. He said it took them 2 years to finally fix
the thing correctly - by buying the 'recommended' duplexer.

Anyhow, thanks for the info, & I'll report back when I get the Q-202
working.

BTW, checked the cables on the Quantar, and the Mini-UHF that they
used for the receiver input had the crimp portion actually swiveling
around on the center portion. Didn't figure that was a good idea, so
soldered the crimp barrel to the other piece. Unfortunately, that
didn't fix it!

Thanks,

Tim

[N4DES]

Being the duplexer was originally a 164/166 bandsplit I'm going to say that the interconnect cables between the cans are the wrong length.
Put a wattmeter between the output of the Quantar and the input to the duplexer and I bet you will se a high VSWR even though they seem to tune correctly.

I have seen this with a number of duplexers that were taken out of a public safety environment and re-tuned for an amateur radio system.
You have to change the lengths of the cables to make it work properly and I did that with a Sinclar 4 cavity duplexer about 10 years ago.

Mark

[Tim]

Hi Mark,

Thanks for the info - I did put the bird between the two, and the SWR was
really pretty good - about 1.5 : 1.

I'll be checking out the sinclair duplexer tonight, visually it appears
pretty rough, but supposedly it's ok electrically.

Depending on how things go, I may just buy a couple of fistfulls of
PL-259s & some RG214 cut to the right lengths & replace all
the links. Although more time intensive, it would be somewhat
cheaper than the Sinclairs. (I checked the length of the harness, and
it is 'correct' for the ham bands.

Thanks,

Tim

[PETNRDX]

I am going to assume the cables are all PL-259's.
If you happen to have some UHF ( PL-259 ) "elbows" you can insert them to make the harness appear "longer".
I would not recommend this as a permanent solution, but I regularly use whichever is needed (type N or UHF) elbow connectors
to "lengthen" a harness for testing.
Its a good quick and dirty method to find the critical length needed for making new cables.
And, yes notch dupes work fine at 600 khz.
Better to have BpBr, but sometimes you have to use what you have.
I have moved many of different ones to the ham band.
If it came from the 160's it nearly always will work better ( more isolation and less insertion loss )
with correct length cables.