Boost FM Transmitter Power?

Discussion in 'Wireless & RF Design' started by jptrsn, Jun 30, 2008.

  1. jptrsn

    Thread Starter Member

    May 3, 2008

    I've got an FM transmitter in my car for use with my iPod. I got it from The Source (RadioShack for those from the States) for cheap, and it is barely satisfactory.

    The problem is that the transmitter power is very low, so I get interference no matter what frequency I choose to use. I can't place it any closer to my vehicle's antenna.

    How hard would it be to boost the power output of the transmitter? Would it be as simple as replacing a resistor, adding a transistor, or some other simple operation, or would it require much more difficult work? I'm fairly comfortable working on circuit boards and creating circuits, so I don't think that a simple change would be beyond my abilities.

    I really don't know much about how these things work. I'm familiar with electronics, but not with any sort of transmitter or receiver stuff. At this point, I'm looking for the theoretical way of doing it, and if it's worth it, I'll put more time, research, and effort into finding out exactly how to go about doing it.

    If it would help, I can get the model and make of the transmitter (it's still in the car right now). I'd just like to know if this is something worth putting some effort into, or if I'd be better off living with it or buying something more powerful.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    With no knowledge of the transmitter, it's pretty hard to give an answer about increasing power out.

    However, the FCC has very stringent limitations on transmitters operating unlicensed in a broadcast band. Upping power could result in problems like a whopping fine. You might just want to experiment with placing it closer to the antenna for better reception.
  3. Wendy


    Mar 24, 2008
    One possible method is to run a wire from the transmitter to the antenna, though this can be tricky. There is a 10 foot limit in the states for this kind of setup, because the wire is also an antenna. I'm not sure about the legalities of using a coupling wire that isn't connected to either end (coil the wire around the car antenna is a slow spiral).
  4. jptrsn

    Thread Starter Member

    May 3, 2008
    I've really got no way of getting the transmitter closer to the antenna, so I'm looking at alternatives. I'm not too worried about the FCC, because 1) I'm in Canada (so it'd be the CRTC I'd be concerned with), and 2) I'll be on the move, so any interference on anyone else would be temporary. I'm not looking at having a commercial-sized transmitter; just something powerful enough to block out interference from stations that are near its frequency.

    Would adding a longer antenna help with the clarity of the signal? And how close would it have to be to work with the stereo? Right now I'm using the transmitter mostly in the car, but sometimes in my house as well. If it's strong enough, I'd like to use it to play music over the radio in my house, but as it stands it's not powerful enough to reach more than one radio at a time. I've seen kits to build a home transmitter from but I don't want to spend 50 bucks on something I already have.
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    Did you consider the device might be faulty?

    How close can you get to the antenna?

    Those things tune the whole band, surely there is a quiet spot you can use.
  6. crash1555

    New Member

    Nov 19, 2008
    i don't know if this will help you, but i always get better reception on lower frequency's like 88 - 92 mhz when using an fm transmitter..
  7. Audioguru

    New Member

    Dec 20, 2007
    The Source sells cheap junk at very high prices.
    A good FM stereo transmitter works very well. They have an attenuator feeding their antenna so they don't break the law about low power.

    Cana-Kits are 50 years old circuits that operate poorly.
    Last edited by a moderator: Nov 20, 2008
  8. DrGabriel

    New Member

    Jul 27, 2009
    Because of FCC and state laws manufacturers are forced to severely limit the output on these devices which is why they all sound terrible. If you do not care about violating FCC rules then this is how you substantially increase the power on any fm transmitter.

    There are various youtube videos that detail this so in short what you'll have to do is open the device and find the antenna. attached to the antenna you'll find a resistor (usually a black cube. you want to bypass this resistor by taking out the resistor and replacing it with a short wire, or connect a new wire (essentially a new antenna) to the lead before the resistor. Good luck. Oh and this probably will void the warranty, but who cares right?
  9. yourownfree

    Active Member

    Jul 16, 2008
    I think you need to move on and get another transmitter, and just plug your audio in it. They are cheap, or make your own. I made one, one time, I know it was illegal, it used one transistor, a 2.5 foot antenna (a piece of wire) and it went 3 miles. I estimated 1/16 of a watt, as i recall. best today if you don't do those naughty things. Go buy an FM transmitter.
  10. Audioguru

    New Member

    Dec 20, 2007
    Maybe it is the poor quality of your FM radio that is causing interference.
    FM has a "capture ratio". It means that one signal can capture a radio if it is slightly stronger than another signal that is on the same frequency. The difference in signals strength for a good radio is small so the signals do not interfere with each other. The radio plays only the stongest one.

    But a cheap radio needs to have one signal much stronger than the other or there is interference of both playing at the same time.

    Cheap radios are also overloaded by many strong signals. They appear at many spots on the dial and are distorted or mixed together.

    A good radio is very sensitive so that it plays the signal well or plays nothing. A cheap radio plays signals that sound noisy and weak.

    Senior Member

    May 26, 2009
    It could be intermodulation. Not sure though.
  12. Damo666


    Aug 25, 2009
    Just bin it and get the 30 watt model from here, like i did . I get 8 miles out of it with an half wave vertical.
  13. bertus


    Apr 5, 2008

    Why can I get 20 miles with a rubber ducky on my portable?
    It is a Yaesu FT-209RH 2 meter portable.
    The power is less than 3 Watts.