UHF duplexer issue

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK6RPX, Nov 15, 2019.

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  1. KK6RPX

    KK6RPX Ham Member QRZ Page

    Hi All,
    A fellow ham recently sent me a couple of Cellwave UHF duplexers so I could tune them for a GMRS repeater he is working on. One is a TDN-7407A and the other is a 633-6A-2. The 7407 tunes up great. The 633 tunes up fine on the high side but not on the low side. I wish I could attach a screen shot of the spectrum analyzer, but the gist of it is that the 633 low side has a wider notch and by the time you get to the 5mHz above frequency the signal is still down by about 15dB. The high side is fine. Any ideas what could cause that?
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Can you post a picture of the bad tuning duplexer? That would help to diagnose what is wrong.
  3. KK6RPX

    KK6RPX Ham Member QRZ Page

    Ok, I figured out that one can't post .bmp files. I converted screen shots from my analyzer.

    633Hi.jpg 633Lo.jpg

    All settings are the same in both shots.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    AFAIK the Mot duplexer you have and the Celwave being tested are the same product.

    Can you sweep "return loss vs. frequency" on the low-frequency side? I suspect something isn't matching well and creating a problem.

    Remember most of these are "surplus" for a reason. Your plots indicate about 70 dB isolation at 5 MHz, and their spec is 75 dB, so you're not all that far off, but the slightest bit of oxidation or corrosion internally can really destroy performance, and I've seen that. These "mobile duplexers" are just that...intended for low-power mobile operations. Last I can recall, they're only rated 50W TX power, which is an indication of lossy conductors and connections from the start. The "bigger" can-sized duplexers can handle several hundred watts.
  5. K7JEM

    K7JEM Ham Member QRZ Page

    Are all the tuning rods on each side about the same distance out? Have you adjusted each one to make sure it is tuning correctly? Did you check it before you tuned it up? There could be internal damage or a cracked joint or other problem. If you have one rod that just isn't tuning right, the problem is probably with that cavity. There isn't much going on inside these types of duplexers, you might take it apart and look for problems.
    AI3V likes this.
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    Coax lengths do mater.
  7. AI3V

    AI3V Ham Member QRZ Page


    Any problem will be easy to find, and if this is the first time you have taken one apart you will be supprised how simple they are.

  8. W6KCS

    W6KCS Ham Member QRZ Page

    When you say it tunes fine on the "High side", are you referring to the side marked "High" on the duplexer? That path should notch the low frequency, and the low side should notch the high frequency. The way you phrased the question makes me think you might be expecting the high side to notch the high frequency and the low side to notch the low freq, which isn't the case. It's not likely but I wanted to rule that out as the problem. K9JEM's advice is good.

    Edit: The more I look at the sweeps the more I think this isn't the case, but I can't delete this post. These Rigols sure impress me for the cost. I wish they'd build a VNA.
    Last edited: Nov 17, 2019
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Can you even dismantle these mobile duplexers?

    It's been a while since I worked with one (probably ten years or more). Are they closed with just screws, or are they riveted or heliarc welded to enclose them?
  10. W6KCS

    W6KCS Ham Member QRZ Page

    yes, usually hex socket screws.

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