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My Android phone (as many others) have FM radio receiver on-board. But apparently, it has no internal VHF antenna at all, since there is no signal at all w/o headphones plugged in and sound quality varies drastically depending on the headphone cable currently in place.

So, I'm curious how to build dedicated antenna, particularly, which pin(s) of 3.5mm connector FM receiver uses to get a signal from. Thanks!

Finally, found my worst headphones, less strong station and the longer conductor and did the clever experiment suggested by @Kevin again. However, I'm not exactly sure how to interpret the results:

With headphones connected reception improves when an additional conductor attached to all three pins (can't really hear the difference if any)

Without headphones (open circuit) I've got the definite results, reception have most clear treble frequencies at Tip pin (Left), noisy treble the Ring pin (Right) and the worst at Sleeve pin (Ground).

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The “pinout” for this purpose could be anything.

Considering the perspective of the equipment designer: If you have a multi-conductor cable of any sort and you want to use it as an antenna element, it doesn't matter at all which conductor you use.

(You might think that if the headphone cable is shielded then the shield would prevent it from functioning as an antenna, but that is only true if the shield is also connected through to the radio receiver. Any single wire is as good as any other.)

Here's a procedure to find out what the actual implementation is:

  • Get a headphone splitter (Y cable), and plug in headphones to one side. Coil up the cord so it doesn't work very well as an antenna.
  • On the other side, plug in an unwired 3.5 mm plug with the shell off, or whatever else you have handy as a breakout.
  • Touch a piece of wire or a clip lead to each of the three contacts, and see which one makes the most difference to radio reception. (Since re-radiation and crosstalk are possible, the results can be fuzzy.)

It's even possible that the circuit is designed to pick up RF from any of the three wires.

Also, you might like to read this existing question about improving reception in this type of receiver.

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  • $\begingroup$ For this experiment I'm having an exactly opposite problem - reception with cluttered headphones is still too good to hear the difference. $\endgroup$
    – Free Consulting
    Jun 7, 2017 at 20:42
  • $\begingroup$ Hey, Kevin, thanks for telling me how to figure this out! I'm pretty sure phone uses L pin as antenna now. Not sure how other conductors matter when circuit is closed thru 36 Ohm earbuds, tho. Pleas check out the addition. $\endgroup$
    – Free Consulting
    Jun 7, 2017 at 21:29
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    $\begingroup$ @FreeConsulting Updated my answer a bit — fuzzy results are to be expected. $\endgroup$
    – Kevin Reid AG6YO ♦
    Jun 7, 2017 at 21:32
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Combine all 3 wires and connect to a soda can tab, aluminum of course. Works like a charm. For me anyways, result could be different for u.

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