Batboard

I am building a small light-duty repeater from two Maxtrac radios side by side. After hooking into the duplexer and antenna, I test the radio and find that the tranmitting radio appears to be interfering with the receiving radio, ever though a duplexer is installed. When the transmitteer is keyed, the receiver still acts as though there is a carrier present, but the signal is muted. The system works fine if the power level in the RSS is adjusted to level 25 (which I found online to correspond with just over 2 watts) but nothing higher.

I know other people have build repeaters from Maxtracs before, so I'm looking for a little guidance. Could there be some RF leaking from the mini uhf connectors at the radio point? Am I not using the quality of cable (RG-58) that I should be? Or perhaps the radios are just simply too close to each other.....

The transmit and receive frequencies are over 6 mhz apart. I have Included a picture. Thanks in advance!

My first guess is improper duplexer tunning, sounds like your tx is getting into your rx. and when you lower your output power you lessen the desense. Is it possible that you've got the high side of your duplexer connected to the low side by mistake? It happens been there done that.
Doug

The duplexer came pre-tuned by the shop I bought it from. I double-checked the high side and the low side and they match up.

You didn't say what the model number on the Maxtracs was, or what the rated power output was. If you reduce the power output on a 45W unit, for example, too far it can yield some pretty crazy spurious output(quite far away from the programmed output frequency), and if you are using a band reject type duplexer that only rejects the programmed tx frequency that could spell trouble.

The receive radio is a 25 watt Maxtrac, model D33MJA7DA5_K.
The transmit radio is a 45 watt Maxtrac, model D43MJA73A5_K.

The duplexer is a 6-cavity mobile/miniature type design for low power. An example can be found here: http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0108968970

I never thought about the transmitter emitting spurious transmissions when the power is reduced but I suppose it would make sense, the power would have to go somewhere.

Well if the duplexer you have is rated for 35 watts, and you are pumping 45 into it... that could be your problem.. Try backing the power down to 25 watts and see if that helps.

Nope. The transmitter hasn't been placed at full power in this setup at all ever. The highest I've put it has been about 30 watts. I don't want to run it at full power as a repeater anyway because then I would risk burning it up from the heavier duty cycle. Even at 5 watts this problem occurrs.

My two bits worth:
If I was trouble shooting this set up, the first thing I would do is to ensure that all shields are in place.
Inspect the cables very carefully. Broken shields can raise havoc.
I would then sweep the duplexer to be sure that is has not gotten mis tuned.
I would then set up the repeater using an iso tee into a known quiet dummy load.
If I still was experiencing desensitization, I would connect my analyzer to the receive port of the duplexer and measure the isolation on transmit frequency.
Look at the output of the transmitter to be sure it is not spurious.
I have seen spurs caused by burnt solder on the PA collector.
Repair this with "Silver bearing" solder only.
If the frequency is trimmed to an extreme you may see spurs.
If the either VCO is near the end of its lock range it can go noisy.
No reason why you cannot make this set up work with NO desense.

There are a number of possible reasons for the RX problems.
Bernie has come up with some of the more common ones.

I like to point out that MOST people that tune duplexers,
don't terminate all the ports with at least 10 db pads while
they are playing with it. This has a result to end up with the
tuning not being where you thought it was. Not knowing
just how the shop did the duplexer adjustments, I would
start with asking the questions about the pads.

Not using the pads generally causes the cavities to get
set wrong. When you connect up the radios, it gets even
worse.

When I tune cavities, I do all my adjustments through the
pads. Leave them on till your totally done with the adjustments.
If you don't have enough signal for the scope or analyzer
to see, then you might have to use a low powered portable
in TX to get enough output to see. When you get into the
-90 db range, it can't be seen by the average piece of
test equipment.

Jim

"...Am I not using the quality of cable [RG-58] that I should ..."
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RG-58 is a single shielded cable with much less than 100% shield integrity and should never be used in duplex operations.

You should make your cables from high quality double-shielded [silver plated] cable such as RG-142 or RG-400. You should also be sure to use top quality connectors, preferably silver plated with gold plated pins. Inferior nickel plated connectors with tinned pins and cheap dielectric have very poor PIM charactersitics, i.e. they are Passive InterMod generators.

The other basic first step to perform is to terminate the transmitter ditectly to a dummy load [using a proper double shielded cable], key the transmitter and verfy that there is no recevier desense. Any problems here [e.g. poor shielding between the two radios] must be resolved before you move to the next step which is to verify desense free operation through the duplexer using the dummy load and iso-T.

I believe I located the source of the problem. I finally took a look at the power supply I was using and it appeared to be experiencing a heavy voltage drop at anything over 3 amps. Something must have been busted. I replaced the power supply and the system seems to be working ok now.

Thanks to everybody for your input!