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Thread: Extending the range of a personal FM transmitter

  1. #1

    Default Extending the range of a personal FM transmitter

    I know this isn't a flashlight related topic, but I know there are quite a few engineers on here and maybe someone can help me with this....

    I am a police officer and for the next three months I will be stuck stuck on the night shift at work. While we are usually pretty busy up until around 1, past 3am it is pretty much dead. Leaving me with nothing to do except drive around and listen to the radio which, of course, sucks at night.

    So I purchased one of those FM transmitters (belkin) that hook up to your MP3 player and plays through your car radio. You plug the transmitter into the headphone jack of the mp3 player. Since I mostly listen to talk shows and podcasts, I wasn't looking for good quality. The problem is I can't get it to listenable quality on any station.

    So does anyone have any tips on making this baby work better? Today I went to radio shack and purchased a headphone extender. My theory is that it will give the transmitter an 8 foot longer "antennae" (maybe?). So I'm going to try that tonight. Any other ideas?

  2. #2

    Default Re: Extending the range of a personal FM transmitter

    Open up the plastic shell and find the antenna. Sometimes they use the headphone lead, but often there's a small coil or something similar that you can extend. Be sure to extend in multiples of your desired wavelength for best performance.
    EDC E1B, Tigerlight w/LED dropin for when I need more light. Love my new Zebralight H30 for floody tasks and my Quark Mini123 for being awesomely small and bright.

  3. #3
    Flashaholic* Databyter's Avatar
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    Default Re: Extending the range of a personal FM transmitter

    You are going to be losing fidelity regardless of how far it transmitts. Your taking an already compressed data stream, and then converting the data into an even more diluted version for FM transmission. My advice is use a personal MP3 player, don't go thru the FM radio at all.

    Also, they are designed to be a weak signal to comply with FCC regulations. If you pump them up illegally make sure you are on a wavelength that no other station gets close to or you will ruin somebody elses reception.

    there are many high quality MP3 players that will fit into the change pocket of a pair of bluejeans, and I use lightweight quality audio earplugs all night on patrol and hide the wires in my shirt. In my case they are hooked to my cellphone which also has an Mp3 player built in.

    You won't ever get high fidelity in a portable FM converter. Depending on how you listen to music, and what type of music it is, that might be fine for you. personally I used my FM converter once, and it's been on my shelf ever since, and it was top of the line.

    If your radio has an aux in port, then that is the best solution. I've gone through three types of patrol cars in the last few years, and without exception they have all had an aux in for me to hook up an MP3 player to, and that DOES sound excellent, especially when you have a great collection of mp3's sampled at 320KBS. Just hook the headphone out on your MP3 player, to the AUX in port on the radio.

    Every once in a while I'll forget to turn it down when I do a focus patrol of some area, and I quickly turn down my blast-mobile to transformer-ize it into my patrol vehicle again.

    Mostly I just listen to talk radio at night. There is alot going on these days for us to talk about, and it keeps me alert.
    Last edited by Databyter; 03-19-2010 at 06:01 PM.
    Databyter (Mike)
    databyter@cox.net

  4. #4

    Default Re: Extending the range of a personal FM transmitter


  5. #5
    Flashaholic* Databyter's Avatar
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    Default Re: Extending the range of a personal FM transmitter

    Quote Originally Posted by curtispdx View Post
    Regarding this, Your better off replacing the resister with a resister of a lower value than just hot-wiring it with a copper wire. At least if you plan on it working all night. Chances are a hot-wire will make it get hot and it will work great until it fries the transmitter.

    Just a guess, but a good one I think.
    Databyter (Mike)
    databyter@cox.net

  6. #6
    Flashaholic
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    Default Re: Extending the range of a personal FM transmitter

    Quote Originally Posted by csa View Post
    Be sure to extend in multiples of your desired wavelength for best performance.
    You'd think that's the way to go, but it's not. Most of these devices use an antenna that isn't close resonant so the RF output isn't effected by proximity. Basically it's intentionally detuned for uniform performance. Adding an antenna that is resonant actually makes the output very quirky and probably won't work well in a mobile environment.

    If you must use one of these devices try to place it so that the car's antenna is in direct sight of the transmitter. Putting it on the floor is the absolute worst place. The dashboard is ideal if you have a way to secure the player and transmitter. Velcro squares are one idea.

    I agree that the best solution is to use an MP3 player directly.

  7. #7

    Default Re: Extending the range of a personal FM transmitter

    If you don't have an auxiliary input, and your stereo is old enough, the second best option is a cassette adapter. If you have the know-how, replace the cable with a shielded one. It's much more reliable than a typical FM transmitter even with the stock cable, but with the shielded cable, it's more reliable than even a good one running near the legal limit.

    Sound quality is at least as good as a proper FM box. Almost all the boxes you can buy are chopper-type stereo, with major losses to sound quality -- I have a kit-built MPX-96 with proper synthesized stereo, and it's a lot better. My car was always too loud to hear the difference, if any, between the MPX-96 and the cassette adapter, but the cassette adapter should be even better.


    If you don't have either a tape deck or a line-in jack...

    As others have suggested, the way to get the most out of an off-the-shelf FM transmitter is mainly to improve placement (a long headphone cable will be much better used to improve placement than from any long-antenna effect it may provide), and also to lengthen the antenna.

    As for the "they put resistors in to reduce the power, short it out!" view described in the linked article. No, really, the resistors are there for appropriate matching a short antenna with minimal radiation impedance to whatever the output stage is designed for. Removing or altering these resistances (or changing the antenna) can simply decrease radiated output, burn up the output stage's transistors (although this is quite unlikely, considering the typical power levels involved), or increase output as hoped, but it's a safe bet the factory configuration is near optimum for the factory antenna. Actually calculating what modifications should be made to the matching network when you extend the antenna is something of an art even to most EEs and hams, and a one-size-fits all answer from some random webpage is not the way to go.

    Don't worry about going over the legal limit and/or interfering with stations on adjacent channels unless you add amplification or build your own, most of these are so far under part 15 specs that there's no risk there. What you probably should be concerned about is interference with your patrol car and/or belt radio's IF -- if you wind up modding it, you'll want to make sure the EMI shielding is intact when you reassemble it, and if by chance it didn't have EMI shielding (bad!), add aluminum foil.

  8. #8
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    Evilgrin07 Re: Extending the range of a personal FM transmitter

    As a owner of a personal fm transmitter i know about how lousy the sound quality is and advise you to avoid them like the plauge. They may also make a buzzing sound as well.

  9. #9
    Flashaholic* LEDAdd1ct's Avatar
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    Default Re: Extending the range of a personal FM transmitter

    I know the perfect product for you.

    Excellent range, and excellent sound quality.

    Drop me an email and I'll point you in the right direction.

    My email is underneath this post in my signature.
    "...and the diode multiplied and grew in brightness. And God saw that it was good."

  10. #10
    Flashaholic* EZO's Avatar
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    Default Re: Extending the range of a personal FM transmitter

    I've owned two Belkin FM transmitters and they've both had abysmal performance. I ended up buying a cheap Chinese FM transmitter from DX, ....SKU: 625 and much to my surprise it totally exceeded my expectations by blowing the doors off any other device like it I've found and it cost all of $7.19 delivered! The main thing about it is that it puts out a pretty hot signal. I now use it plugged into the back of my computer in my office to broadcast internet radio over to my bedroom or downstairs to the kitchen. Granted, I live in a small wooden house but the Belkins couldn't transmit a decent signal 5 feet. The unit comes with a cigarette lighter power plug but works on two AAA batteries that seem to last forever. It also can run from a USB plug. A nice feature is that you can set it to any frequency and it has ten pre-sets. It also has a built in Celsius thermometer which I find is totally useless and it has a blue light-up dial which is also sort of useless. It seems like a device that would break any minute but I've had mine over a year and it's still going strong. Some people complain (on DX's reviews) that the power plug sends engine noise through to the radio so you may need to use batteries, but like I said, it just seems to sip on them. In fact it says, "Ultra low power consumption" on the package. So, for the risk of 7 bucks you might end up with a simple solution to your problem, or whomever is following this thread nowadays.

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