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Bluetooth FM Radio antenna extension

Discussion in 'Riding Gear and Bike Accessories/Parts' started by nick1234, Apr 10, 2014.

  1. Hi,

    I have a Motorbike Bluetooth Handsfree (ebay $50 no brand cheapy) and it has a built in FM Radio. It mounts to the side of the helmet.

    Combined with some (not so cheap) custom earplug headphones, it seems to work quite well, although due to the rain, I am yet to test how well others can hear me on the phone while riding at speed. I chopped the 2 speakers off, and soldered a $2 headphone jack (from Jaycar) onto one of the speaker wires. I then removed the microphone boom, and glued the microphone to the front of the helmet, near my mouth, and taped the wire down flat. It looks quite neat and tidy.

    My question is in relation to the FM radio. The device has a little wire a bit less than an inch long, sticking out of it. This, as i understand, is the antenna for the FM radio.
    The radio struggles with reception a lot of the time. What I wanted to know is, is their likely to be much of an improvment if I extend this wire, and if so, should i use any particular sort/thickness etc of wire?

    The length isnt an issue, (within reason) the wire can be easily tucked inside the Windjammer thingy (google it) that wraps around the bottom of the helmet.

  2. Probably not. The RF section of your radio is probably tuned to work best with that length antenna.

    An ideal straight-wire antenna (called a monopole) is 1/4 the wavelength. FM radio is around 100MHz so a 1/4 wavelength would be 0.75m. Any multiple of that will work but with less sensitivity as you go shorter. Best case here is a 0.75m wire going straight up from your radio. If you're self-conscious that is probably not for you.

    Then you have polarisation. FM radio in Australia is usually vertically polarised, so you want the antenna as vertical as possible.

    Curling up a monopole antenna actually makes it less sensitive. For best effect you want the antenna straight and vertical. Making it horizontal reduces the effectiveness to almost zero and an x^2 relationship for angles in between. If part of the antenna goes one way, and part goes another like in a circle, you're receiving a signal on one bit that cancels out signal received on another bit, reducing the effectiveness.

    To work with a shorter antenna, you can use a tank circuit that adjusts the antenna's "Q" to match but this is a more advanced topic that would require liberal application of beer.

    Your real problem is that your radio isn't sensitive enough. You're after a high signal-to-noise ratio. To get better reception, you either improve the signal (better antenna, Rf amplifier) or reduce the noise (better shielding, diversity, filtering, noise reduction by choking etc.)

    TL-DR: Make the antenna as straight and vertical as possible but your radio is cheap and therefore not very sensitive.

    Disclaimer: I only got a pass grade in RF comms at Uni. No credit or distinction.
  3. thanks for the detailed answer, much appreciated