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FM stereo transmitter?

Discussion in 'Electronic Basics' started by Bob Masta, Dec 5, 2006.

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  1. Bob Masta

    Bob Masta Guest

    Here's the scenario: I'd like to be able to
    play an audiobook on my living room stereo
    where my wife would be listening as she
    works on her projects, but I want to be able
    to listen at the same time on a Walkman
    while I'm working in the basement, etc.
    Rather than string wires, this seems like a
    perfect application for a little FM transmitter.

    But when I Google for such a beast, I am
    overwhelmed by the number of products,
    from under $20 to over $200.

    It looks like many of the cheaper products
    are intended just to get sound from some
    portable device into a car radio, so I would
    guess that the range may be intentionally
    limited. Or can I just run a long antenna
    on the transmitter?

    Anyone have any experience with such

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  2. MRW

    MRW Guest

    Bob Masta wrote:
    I suppose you can, but you have to worry about matching the antenna
    with the transmitter output to ensure that you minimize the reflection
    coefficient. From my experience, the output has an impedance of 50
    ohms. Running a simple dipole antenna at about half or a quarter of a
    wavelength of the FM frequency can probably work, but some FM
    transmitters have a fold-back scheme. This means that if you don't have
    proper matching (high VSWR - voltage standing wave ratio), then the
    output power is cut back (folded back) to minimize the reflected power
    to the transmitter.
    My friend had an iPod FM transmitter that was able to interfere with
    cars on the next lane. I guess he tweaked its power output. I wouldn't
    be surprised that most of the cheapo FM transmitter (iPod included)
    uses Maxim's MAX2605-2609 FM IC. You can request samples from Maxim and
    try building your own. The datasheet includes a basic FM application. I
    think with this one you can get away with running a long wire as your
    antenna and still receive decent field strength in your basement.
  3. MRW

    MRW Guest

    By the way, thanks for DaqGen. I'm using it as my signal generator for
    my audio experiments. :)
  4. Beware of the ones with LC oscillators as they drift like crazy and are
  5. Joerg

    Joerg Guest

    It's usually not intentionally limited but the FCC limits what they can
    do. If the basement is almost underneath the living room and you don't
    have concrete floors above like we do it might work. A month ago I saw a
    modulator for $19.95 at Target. Thing is, these are limited to four
    frequencies at the low end of the FM band. So first you have to make
    sure that at least one of them isn't occupied by a strong station.
  6. Rich Webb

    Rich Webb Guest

    Ramsey is the go-to place for these small FM transmitters. Most
    available as kits, some also as assembled.
  7. Bob Masta

    Bob Masta Guest

    You are most welcome! As it turns out, I am
    putting the finishing touches on Daqarta for Windows.
    (Probably a couple more weeks until release.)
    I won't be developing DaqGen any more as a stand-alone
    app, since Daqarta includes all DaqGen's features
    plus a whole lot more. But even though Daqarta is
    shareware (not freeware), you will still be able to use
    it as a freeware generator since when the trial period
    expires all that happens is it no longer allows you to
    see inputs from the outside world... which DaqGen
    can't do anyway. You will be left with all the DaqGen
    features, plus all the new features from Daqarta
    (except inputs)... for free!

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  8. Eric R Snow

    Eric R Snow Guest

    I have one of their FM kits. It's the one that is digitally tuned,
    about $100.00 I think. It works very well, no drift at all. The sound
    quality is great.
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