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Thread: How much desense on a repeater is OK ?

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  1. #1
    Join Date
    Jan 2002
    Kansas City, MO


    Let's get one thing straight right off the bat, ... I am NOT an expert on repeaters. Still, I have our local 147.330 ARES county repeater in my shop on our tower and it's starting to act differently. We started to notice degraded performance several months ago. The antenna SWR is about 1.2:1 so I don't think there is a problem with the antenna or feedline. I can bring up another repeater 30 miles away with a 1 watt HT on this antenna. When I disable the repeater transmitter and couple a signal into the receiver (through another antenna on the tower) the COR activates at about -102 dbm. When I re-enable the repeater transmitter and repeat the sensitivity test, the COR now activates at about -95 dbm. 7 db seems like a bit much to me. And,... the COR sometimes "chatters" badly in the region in between -95 and ~-100 dbm. My initial thought is that the Wacom WP 639 "4 pack" duplexer is either detuned or defective. I do not have a manual on this duplexerand have never tuned one. Any thoughts on this? Tom WA4ILH

  2. #2


    Tom -

    I don't like any desense on a repeater's receiver, but it can happen. #I usually used a 6 cavity duplexer for 2 meters. #The Wacom duplexer is factory tuned to the exact operating frequencies prior to shipment from the factory (marked on unit). No further field tuning or adjustment is normally required. If it becomes necessary to change the operating frequencies of the duplexer, it can be field tuned if a spectrum analyzer with a tracking generator is available. In addition a 50 ohm load will be required.

    I would suggest the Repeater Builders web site. #

    This has most of the information you will require, including Field Instructions for Tuning the Wacom WP- 639 duplexer (REMEC/Wacom still located in Waco, TX):

    They also have a YAHOO user group with over 2,600 members!

  3. #3
    Join Date
    Jan 2000
    Montréal, Québec


    Try to determine if the desense is caused by overload of the receiver or by too much noise generated by the transceiver.

    Sometimes a bad transceiver generates noise that will affect the receiver even with a perfectly well tuned duplexer. Before suspecting the duplexer or trying to tune it (it's not hard but it's critical and has very little tolerance) try to change the tranceiver.

    If it's doing it on the antenna only and not on the dummy load, it could be the antenna, a connector or the cable itself. Even if the antenna shows no SWR and works fine with a simplex radio, it could very well induce noice that'll affect the reception of a repeater, it's not unusual at all.

    Think about it, if you feed 20W into an antenna and any contact has so much of a little corrosion, it could create a super tiny spark that would be inaudible... except for a receiver connected to that very antenna at that moment.

    Let us know!
    I believe every problem has a solution.
    Hence, if there's no solution, there's no problem.


    God is our creation, not our creator.

    73 de HI8/VE2NSM

    Montréal, FN35gm

  4. #4
    Join Date
    Jan 2002
    Kansas City, MO


    Thanks guys, (and David, ... for the email info) From preliminary reading, I infer that duplexers do not normally need re-tuning. I will inspect the cables carefully but I suspect now that the problem is somewhere else.
    Another problem that I forgot to consider is that we have a new low power repeater (1/2 watt) on that tower in the 162. Mhz range. It has a cavity on the TX side but it's antenna is only about 10 feet away from the Amateur repeater antenna. I've turned that off for a while while I run further tests.
    Tom WA4ILH

  5. #5
    Join Date
    Apr 2002
    Rochester, MN


    We had an ongoing problem on one of our repeaters for quite a while that eventually turned out to be a bad cable between the duplexer and the repeater. We purchased new cables from a local 2-way radio supplier and they provided what appeared to be commercially made mini-hardline cables. These were worse than the ones we replaced! It turned out that the supplier had cut off the factory connectors and soldered on PL-259's. I made up some cables using Belden 9913 with it's 100% shielding, and that temporarily cured the problem until we could get some new commercial cables with factory connectors.
    EchoLink, IRLP, Allstar and DSTAR linking - adding interest to repeaters worldwide 24X7

  6. #6
    Join Date
    May 2003
    Point Pleasant Beach NJ


    Hi ILH and all,

    Resonant cavity diplexers are mechanical devices and are rather finicky about thier environment. Standard calibration temperature is 25C so departure can result in a slight detuning. Duplexers in unheated, uncooled environments often need seasonal tweaking and the more (cans) the merrier. That's one reason why most commercial and public service systems employ climate controlled equipment shacks at the base of the tower.
    I don't suffer from insanity, I enjoy every minute of it.
    73 de Warren KB2VXA

  7. #7


    First, you should expect about 2-3 db loss in the cavities, and then you have to deal with the desense problem.

    Any desense is detrimental to the operation of the repeater, as desense is a product of the sideband noise from the transmitter. Tube transmitters tend to be cleaner than solid-state, since they use a higher Q output network which acts as a bandpass filter.

    You may have to install some sort of suck-out cavity on the receiver side to notch out the transmitter frequency to get the desense down to a minimum -- none being the ultimate goal.

    Gary WA7KKP

  8. #8


    The test you ran isn't perfectly scientific, but it's close, and I agree 7 dB "desense," along with your observed degradation of repeater performance, is too much to tolerate if you expect the repeater to be of any use in fringe areas.

    I normally get my repeaters right down to "zero" desense, i.e., you can listen to a very weak signal that's barely breaking the squelch, and turn the repeater TX on and off (completely) and notice absolutely no difference in S/N of the weak signal. That's the ultimate test, and any decent repeater system should pass it.

    The WaCom duplexer is a good one, and it shouldn't drift with temperature changes unless those changes are huge. It uses Invar slugs to tune the pistons inside the cavities, and these don't change over a very broad temperature range.

    However, the duplexer can still degrade. One thing that degrades them faster is moisture/humidity/condensation. If you have a "shack" that gets warm during the day and cold at night, condensation can form inside the cavities (they're sealed, but not hermetic) and play havoc with internal connections. I've had this happen a few times, myself. Had to open the cavities up and blow them out with dry air, bake them in the sun a while, and then plug them back up. Alternatively, leaving them "open" in a warm, heated room that won't get cold also works.

    Antennas can also degrade, as can coaxial cable and its connectors. It doesn't take much to cause "desense" if the repeater transmitter power is more than a few Watts. Degradation of "amateur" antennas (ones not specifically designed for repeater service and sold into the commercial 2-way field) happens routinely, and sometimes surprisingly fast. As you likely know, real "repeater" antennas cost several hundred dollars each and are worth it because they really are quite different from ordinary "amateur" antennas or ones that can be homebrewed. They use more expensive materials, and all active connections are soldered or welded, never clamped or bolted.

    A bit of moisture inside a coaxial connector can cause problems. Very well-sealed connectors can still take in moisture by condensation. It's sometimes better to let them breathe, or use pressurized cables and connectors and let some nitrogen gas trickle through them. At least that eliminates the condensation problem.

    The "new" antenna you describe is certainly suspicious and could cause problems. It's in the very near field of your repeater antenna and could be generating noise and IMD products simply by coupling to your antenna and being physically present so close by. Especially if there are metal clamps used to install the new antenna and those clamps are made of a different material than whatever they're clamping to (presumably a tower).

    I tackle desense point by point, changing stuff one by one and then re-testing for desense. The easiest test, which requires no test equipment or instrumentation at all, is to simply use a very weak far-field test signal and turn the repeater transmitter on and off. I can always find a test signal from a repeater user; the object, of course, is to be certain that signal does not change in amplitude over time, so it cannot be a hand-held or anything like that.

    I've built VHF crystal oscillators (like the Pletronics programmable ones, which are only a few dollars each and are available up to a couple hundred MHz) with a small battery to power it into a well-sealed aluminum case having a BNC connector -- it provides in the range of 1 mW to 10 mW signal at 50 Ohms -- and asked a distant user to simply connect this to his home station (permanently mounted) antenna and let it run. The 1 mW signal from 5 miles away or whatever is usually weak enough for a desense test. If it's not weak enough, find somebody a bit farther away!

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