Close by 2m repeater is desensitizing transceiver

Discussion in 'General Technical Questions and Answers' started by AJ4AJ, Oct 13, 2017.

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  1. AJ4AJ

    AJ4AJ Ham Member QRZ Page

    I have a 2m repeater close by, the tower is 550 feet from my back porch. This is great when I am talking on that repeater however it attenuates other repeaters/frequencies that are anywhere close to its frequency. Is there a filter that I can put on my input to block out this repeater when I want to use other frequencies? I use a Yaesu FTM-3100 for my base. I mostly use a 13 element beam but get this with my a small mobile whip also. The signal is so strong that I can remove the antenna from my HT, put the HT in the freezer, close the door and still hear the radio.

    Any help would be appreciated, thanks...AJ4AJ
  2. AI3V

    AI3V Ham Member QRZ Page

  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Sounds like Iceotropic.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    I think that you have to identify the actual mechanism behind the desensitation before doing anything.
    There are two major causes; receiver overload and transmitter sideband noise.

    In case of receiver overload, a narrow filter between the antenna and your transceiver might help,
    but in the case of transmitter noise, nothing can be done at the receiver, except using a directional antenna
    with a null in the direction of the repeater site.

  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Ask your handheld if the light really goes out when you close the door.

    Actually, with the repeater that close I think there's really nothing you can do. Even an expensive cavity filter with a very high-Q is unlikely to be of much help, and if you used one it would not only notch the repeater's output frequency, but greatly attenuate other frequencies within 100-200 kHz of that frequency and a single cavity can't provide more than about a 30 dB notch of the target frequency to which it's tuned.

    In my experience under similar situations, that wouldn't be enough.

    To make matters worse, ham gear isn't well shielded so even if you attenuate the repeater's frequency 30-40 dB via the antenna line, the repeater's signal is likely to just blast right through the enclosure (going "around" the antenna connection) unless you add extra shielding to the receiver circuits.
    K4MSM, KA9JLM and K7JEM like this.
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    You can make a coax stub cut to notch the offending frequency, No need for expensive filters.

    Did you try putting the radio in your microwave oven ?
    Last edited: Oct 13, 2017
    N1OOQ likes this.
  7. WR2E

    WR2E Ham Member QRZ Page

    Turn the audio down.
    K3RLD likes this.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    That won't do anything. A notch cavity is effectively a coax stub, just a very big one usually made of silver-plated copper or brass of much larger diameter to provide 100x higher-Q than ordinary coax could have -- and even it can only provide about a 30 dB notch per cavity. A coaxial stub notch will provide less than that and have far lower Q, so the notch would cover pretty much the entire 2m band.

    Coaxial stubs are good for notching an "entire ham band" at a time, but not a specific frequency within that band. If the user wants to notch the repeater frequency only and let the rest of the 2m ham band come through, a coax stub can't do that.
    AG6QR likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Yep, They are broad, But would be better than operating the radio in the freezer. :D
  10. K8JD

    K8JD Ham Member QRZ Page

    a Single Coax stub as a filter would have , relatively, very little attenuation and be very broad banded. :eek:
    A hand held radio is very portable and I think taking it a mile away from the offending repeater would be an excellent fix for the problem. :rolleyes:

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