Puzzled About The Antenna Of An FM Transmitter In A Mobile Phone

The specs for the latest and greatest Nokia N78 lists a new feature which so far has only been available as an add-on for various products: An FM-transmitter to stream music and podcasts wirelessly to a Hifi set or the car stereo. What puzzles me is where they put the antenna inside the mobile or how it looks like!? At 100 MHz, the wavelength is around 300 cm. Even at a quarter of the wavelength the antenna still needs to have a length of 75 centimeters. How does that fit into a mobile phone? Can anyone shed some light on how this works in practice?

6 thoughts on “Puzzled About The Antenna Of An FM Transmitter In A Mobile Phone”

  1. The antenna just needs to be somehow “resonating” at the required frequency. It doesn’t need to be a full dipole, it can be shorter. And you can just add a coil to make it electrically longer. It’s really not an issue.

  2. Martin, the headphone cable is the antenna – they assume you will be listening to radio with headphones so the long wire of the headphone cables is used as an antenna and coupled via the headphone jack.


  3. Hi Andrew,

    yes, for the FM receiver they use the headset cable as the antenna. But for the FM transmitter I don’t think they use an external cable.


  4. I assume it’s similar to the iTrip transmit antenna; at short range it doesn’t need to be that efficient. Where pirate radio is rife (eg London) these FM devices are prone to random interference dependant on location or frequency chosen.

  5. Hi Martin,

    I have seen such kind of built in antenna for FM radio, developed by Laird Technologies (formerly Centurion in Sweden).

    A quarter wavelength antenna placed within dielectric material with special properties allows it to be excited by longer wavelegths.

    The other option is to use a patch antenna.

    They don’t do it on a large scale comercially since the cost is high and the place within the phone is highly valuable and also it is much easier as we know to use the headphone cable.

    It is interesting to know that at Centurion they’ve developed a mini AM radio antenna for the India market.

    Best Regards, Bogdan

  6. FWIW: There are other phones out there with this ability. I know of the LG “Muziq”, which has been out since last summer.

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