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LC filter, LNA w/SAW, or something else?

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  • LC filter, LNA w/SAW, or something else?

    I'm getting ready to put up a set of antennas at my house for ADS-B and AIS monitoring, as well as ham radio use (HF/VHF/UHF). I'm going to have vertical separation between the RX-only antennas and the RX/TX antennas to reduce interference, but I'd still like to make sure I really minimize the amount of unwanted RF coming into my receivers for ADS-B and AIS. I can make up some standard LC bandpass filters, but SAW filters seem to be much more efficient. They're a lot harder to come by than capacitors and inductors though. I also don't know if I should add an LNA up at the antenna, or if I should just stick with filters. I'll be feeding these antennas into RTL dongles. Thoughts? (for an LNA/SAW combo I'd prefer something commercially available in the US...all the ones I've found so far are for sale in Europe)

  • #2
    I have gone with a simple coax stub.
    68mm of RG6 with the free end shorted (measured for the centre of the tee)

    My ADS-B TV dongle was completely swamped whenever the 2M repeater in my shed fired up. After fitting the stub, there is still some desense but nothing to worry about.
    The stub also helps cut out the various commercial transmitters a few kilometers away.

    You should get the filter going before considering a preamp.

    Jim VK7JH

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    • #3
      Can you post a pic of the stub please?

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      • #4
        This is my current setup. The dimensions are more accurate than in my first post.

        I don't have any decent test gear for 1090MHz (yet) so there is no easy way to fine tune the dimensions.
        I tested the RG6 coax I am using and it has a velocity factor of 85%

        The calculated free space 1/4 wave length is 68.8mm. With a VF of 85%, we need 58.5mm
        If your coax has 66% VF the required length is 45.5mm
        The hard part is determining the VF of the tee adapter. When I have better test gear, I will see how close I am.
        The free end of the stub I used a bit of inner cable bent into a U and pressed into the coax shield space.
        A dob of solder ties it to the centre.

        The protruding centre conductor can be tied to earth for more protection.
        Stub1090.jpg
        I used F-Type connectors because they suit the RG6 (and are cheap) but it could be hard to find a F-type tee.
        MY first attempt used BNC connectors but that left a very short bit of coax to play with.

        Most of the time, I see no difference when I plug the stub in but if the 2M repeater is active, without the stub, there is no reception at all.
        The same happens if I transmit on 50MHz.

        I will built an inter-digital filter when time permits but for now the cheap and simple stub does a good job for me.

        Jim

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        • #5
          Keep me informed Thanks

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