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SWR meter oscillates with cavity filter?

Discussion in 'General Technical Questions and Answers' started by KM6LYW, Jan 20, 2021.

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  1. KM6LYW

    KM6LYW Ham Member QRZ Page

    I have an aprs cavity filter and an mfj swr meter. the swr needle quickly oscillates (4hz) from 1.4 to 5.0 when its on the pass frequency. sometimes the needle will settle down and hold at 1.4 . the needle indicates high swr when not on the pass frequency as expected.

    is this normal?
  2. AI3V

    AI3V Ham Member QRZ Page


    Sounds like a loose connection in the cavity.

    Take the cavity apart (they are very simple inside) and make sure everything is clean and tight.

  3. AA5MT

    AA5MT Ham Member QRZ Page

    If the analyzer is steady on a dummy load, you may have rfi from a broadcast station. If the swr meter fluctuates like a VU meter, this may be the case, if it is connected to the antenna. It could be an am broadcast station, or you may be near an aircraft transmitter which is also am modulation.

  4. W1VT

    W1VT Ham Member QRZ Page

    That may also indicate that the final is oscillating.

    The high Q cavity is designed to reflect out of band signals. Which re-enter the amplifier to combine with the output. And is sent back out again. If the gain of this process is equal to 1 an oscillation results!

    Zak W1VT
    W1BR likes this.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Are You testing with CW or a modulated signal ?
  6. KM6LYW

    KM6LYW Ham Member QRZ Page

    testing with an mfj analyzer, at very small airport... ill listen on AM next time
  7. W1VT

    W1VT Ham Member QRZ Page

    An SWR analyzer uses a very small signal. You may have a signal from a nearby transmitter that upsets the SWR reading.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Probably tells the story. Use an SWR bridge with a transmitter that runs more than the 10 mW of the analyzer and you'll likely see the SWR isn't "modulated" as described.
  9. KM6LYW

    KM6LYW Ham Member QRZ Page

    ok, yah, i have an inline swr meter too. I'll test it with more power. I have a dummy load too.

    so nobody has ever seen a swr needle oscilate on a cavity?. other than with a nearby transmitter?
  10. W1VT

    W1VT Ham Member QRZ Page

    The other "gotcha" is the use of a clever AC/DC techniques on the coax. Such as the Ameritron RC-4 remote coax switch. The decoupling capacitor can let enough charge through to modulate the sensitive detector used by test gear.

    Zak W1VT

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