Duplexer Tuning

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD9LSZ, Jun 3, 2021.

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  1. KD9LSZ

    KD9LSZ Ham Member QRZ Page

    Planning to purchase a mobile repeater system, I understand how duplexers are tuned and have seen it done, but I don't want to spend $30k on a scope capable of 70cm. Is there any reason I couldn't simply hook up the antenna and then run the high/low side of the duplexer into a cheapish spectrum analyzer and reference the background noise level to notch out the frequency? Surly this would not be as accurate, but for a 50W max mobile system it would get the job done no? Please steer me straight if I'm wrong, just doesn't seem like it needs to be so complicated with a fancy scope and a noise generator.
  2. K7JEM

    K7JEM Ham Member QRZ Page

    Depends on the type of duplexer. If it is a "flat pack" notch only style, it is easy to tune with a receiver and a signal generator, or using a service monitor. In any case, you need a decent signal generator capable of output from -125 dBm to at least -30 dBm or more.

    Lots of people out there can tune them for you, if you don't have the equipment.
    W6KCS likes this.
  3. KD9LSZ

    KD9LSZ Ham Member QRZ Page

  4. K7JEM

    K7JEM Ham Member QRZ Page

  5. KD9LSZ

    KD9LSZ Ham Member QRZ Page

    Could you explain in electrical terms what the issue would be with a noise generator? No disrespect, but when I hear something like that my mind goes crazy and I lose sleep wondering why. If the goal is to be able to observe a dip in the signal response of the system, then what does it matter the source of the signal? Further, why not just observe the dip in the natural background noise using the intended antenna instead of introducing a piece of equipment that won't be there during normal operations, which could potentially contaminate the results.

    The problem is that being a mobile system, the frequencies may need to change to coordinate around a local environment. Therefore having a small, portable, economic solution to retune a duplexer in the field is desirable. Again, not looking for lab results, but what will get the job done?
  6. K7JEM

    K7JEM Ham Member QRZ Page

    I don't know if that noise generator will output enough signal level to adequately tune the duplexer. If you are using a 6 cavity flat pack duplexer, your noise output would need to be 80 dB above the lowest signal level, or around -40dBm. Also, what are you measuring the noise, or background noise with? This takes some specialized equipment, like a very sensitive spectrum analyzer. You can't use background noise as a source, since it is down around -122 dBm to start with, so any attenuation would make the noise floor fall below whatever measurement equipment you are using.

    Here is a link on how to tune it using a signal generator and an FM RX, like a handie talkie or common mobile radio:

  7. K7JEM

    K7JEM Ham Member QRZ Page

    One other thing:

    Most of these duplexers will be wide enough in notch to cover several adjacent channels with sufficient notch depth. IOW if it is tuned to 443.000 and 448.000, it will probably work OK from 442.9 to 443.1, give or take. The isolation won't be quite as good at slightly offset frequencies, but should still work OK.
  8. KD9LSZ

    KD9LSZ Ham Member QRZ Page

    Understood, thank you this clears up everything and is exactly what I needed.

    Thank you!
  9. K7JEM

    K7JEM Ham Member QRZ Page

    Also, if you are building a portable repeater, your best bet is to run as low of output power as is needed. 5W should typically be enough, but I wouldn't run more than 25 watts maximum. Your repeater will work better at the lower power, and usually high power is not needed or desired. The biggest problem with repeaters is "getting into" the repeater, not usually a problem with hearing it in the mobile.
  10. AA5MT

    AA5MT Ham Member QRZ Page

    I've watched it done using nothing more than an swr analyzer and a dummy load.

    It is very easy, or at least it appeared to be. Each cavity was removed and put into the dummy load. Each cavity was either peaked or dipped as required. It took more time to change the cavities than it did to tune it. This system was tuned about 15 years ago and still works great.


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