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I pulled a nice big external AM ferrite bar antenna off a working stereo. 5.5" long.

it has 4 wires coming off of it. red must be the main wire, because checking that with an LCR meter connected to the other wires I get (uH) blue, 310 black, 310 white, 372

so I connect the red and white wires to an air gap variable capacitor gang. 125, 250, and 250 pF. I can use any combination.

I put the antenna on a radio, right along where I know its ferrite bar antenna is.

the only thing that happens is when the VC is about midrange the signal gets nulled. not sharp at all, there is a wide range of VC that makes it happen.

it's not the wrong antenna orientation, because I had already chosen a weak signal with the radio oriented to pick it up.

why is it degrading reception instead of improving it?

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    $\begingroup$ Welcome! How do you connect the radio to the antenna? Is C in parallel with the antenna, or series? You know L, have you calculated the resonant frequency you expect from L&C? You can edit your question to add detail, or even a circuit diagram. $\endgroup$
    – tomnexus
    Commented Oct 14, 2023 at 19:33
  • $\begingroup$ Null? You do know these are directional, right? Did you find a null by changing antenna orientation rather than by changing the VC? $\endgroup$
    – user10489
    Commented Oct 16, 2023 at 11:20

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