Build a Dual Coaxial Stub Notch Filter

Discussion in 'Homebrew and Kit Projects' started by KB1THM, Mar 29, 2020.

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  1. KB1THM

    KB1THM Ham Member QRZ Page

    This is how I built a dual stub coax notch filter for under $30 using RG-6 coax. Enjoy. Comment any corrections if there are any mistakes. Use during field day or contests to reduce interference between stations operating close together. Use in conjunction with a Band pass filter to even further attenuate adjacent bands.

    Click the attached PDF writeup

    Attached Files:

    AI3V and N1OOQ like this.
  2. NK7Z

    NK7Z XML Subscriber QRZ Page

    Thanks, interesting, and useful...
  3. AA3EE

    AA3EE Ham Member QRZ Page

    Have you done any work on the placement of the stubs along the coax. For example, if I have a radio and amp (assume a resonant antenna), does it matter if its at the amp end of 10' of coax?

  4. KB1THM

    KB1THM Ham Member QRZ Page

    Dave, thanks for the question! Preface this by saying I am not an expert on this subject and I have not done any testing at different points, just read about what others have done. My understanding is that it should go after the amp if you are using one. As to how many stubs you can use, it seems you can employ different combinations of shorted and open stubs depending on what you are wanting to supress. Another consideration is how much feedline you want to use, and how many stubs you want to use. Better results can be had with more feedline and more stub combinations but this can start to get pretty expensive. Especially obtaining 1/4 wave lengths on the low bands.

    Take a look at this article by K9YC,

    Also K2TR if you google k2tr coax stub filters, he has a nice writeup and he says spacing two stubs a quarter wave apart can improve results but not by that much.

    Let me know your thoughts, I find this topic fascinating.
  5. AI3V

    AI3V Ham Member QRZ Page

    I especially like your idea of adding connectors for the "adjusting stub".

    KB1THM likes this.
  6. KB1THM

    KB1THM Ham Member QRZ Page

    Yes it sure helps to make the filter work as best as possible on all parts of the stop band. Unfortunately the notch is creates is so sharp that it will work well to attenuate only one part of a certain band such as 40m ssb or cw if you are on 80m. The difference can be as much as 10dB! Other stubs I've seen people make with one length of coax may not be getting optimal performance if you have no way to adjust the length slightly. I did make a mistake in the article saying that the pass band is very sharp, no it is not. It is the stop band that is sharp in the adjacent bands to 80m. The 80m pass band is actually pretty broad and changing out that stub doesn't affect it much at all.

    Crimping on F connectors was really easy and quick. The tool and connectors can be found on ebay or at home depot for very cheap.

    The NanoVNA really helps you to build this. I'm sure there are other measurements you could make with other tools other than a vector network analzer but I think it would be really hard to visualize with anything else. I picked up the NanoVNA on ebay for less than $40!. Great tool. Make sure to calibrate correctly though.

    Currently I am working on building the W3NQN band pass filters! What a learning experience, I wen't into this knowing almost nothing about filters at all!

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