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FM transmitter wanted with more power

Discussion in 'Electronic Design' started by Larry Brasfield, Jun 16, 2005.

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  1. You might be surprised to learn that ignorance of the law
    is not considered a defense against charges of violating it,
    at least not in court. You might get some sympathy from
    such a defense, but you will get no break from a judge.

    You should also be aware that the emission limits set by
    the FCC for unlicensed stations are not dependent on
    how the equipment was procurred.
    Good luck finding somebody equally willing to break
    the law. By the way, this post may well serve to make
    even the weak "I was ignorant" defense untenable.
  2. John - KD5YI

    John - KD5YI Guest

    he direction of the knowledge to do it myself, or buy one from someone on this list. I have Googled around, but the quality of online web searches can only be verified by people who have experience with these things -- for instance -- you!
    You would do well to heed Mr. Brasfield's comments.

    Here is a link to the FCC's rules so you can verify it for yourself. Find
    and read about modifications to approved equipment. What you want to do is
    strictly illegal.

  3. Mac

    Mac Guest

    What kind of antenna does your transmitter use? Is there any provision for
    connecting an external antenna?

  4. JustMe

    JustMe Guest

    Perhaps you can use the existing transmitting device by directing the output
    into a "leaky feeder" placed where you require reception. This should not
    put you outside the law. Google came back with 4100 hits. There should be
    some help there.
  5. I am looking to buy/modify/build an FM transmitter that will work on normal USA radio frequency range of 88-108 MHz. I have purchased an in-store product that limits the distance of effective trasmission to ~15 feet -- most likely due to FCC requirements for car FM transmitting devices. However, I have a somewhat larger vehicle and would like to transmit a more powerful signal so that interference is not a problem. To my knowledge, the FCC restrictions apply to consumer products, and not to educational experimentation/projects. This would be powered by the car's cigarette lighter, so I am sure that it can provide for more power -- given that the transmitter's circuitry would allow it. I currently am using two different devices, neither of which have worked well. The product that worked best was the iRiver AFT-100. I would like to either modify this device (or send it to you for modification if it requires more knowledge/tools than I have), have someone point me in the direction of the knowledge to do it myself, or buy one from someone on this list. I have Googled around, but the quality of online web searches can only be verified by people who have experience with these things -- for instance -- you!

    I am not a total dummy, and even took some courses in Electrical Engineering before majoring in Computer Science. Any help would be appreciated though. If I decide to have one of you build it for me at a fair price, there is also a future project I am working on that would require a more powerful FM transmitter -- about a mile or so of directed transmission. Thanks, and hope to hear from you soon! You can email me off list if you like as well -- just remove the "NOSPAM" portion of my address...

    Kristian Hermansen

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  6. Mebart

    Mebart Guest

    You should be able to get much more than 15 feet range and still be
    within the law.

    Ramsey used to make a very high quality part 15 transmitter, but it is

    With 100 milliwatts and a 3 meter antenna, you should be able to do
    300 to 500 feet with a decent receiver.

    I'm assuming you want a real FM transmitter, capable of doing stereo
    with good frequency stability (quartz based frequency).

    If you just want someting to make noise with, there are many
    schematics and products you can buy and operate legally. But, you will
    have to chase the signal up and down the FM band unless you get one
    that uses a quartz crystal to determine it's transmit frequency.

  7. John - KD5YI

    John - KD5YI Guest

    Actually, the rules permit 100 mW and a 3 meter antenna (combined with
    ground and feeder) on the AM broadcast band (525 - 1705 kHz), but not in the
    FM broadcast band (88 - 108 MHz). For the latter, the rules state:

    [Code of Federal Regulations]
    [Title 47, Volume 1]
    [Revised as of October 1, 2004]
    From the U.S. Government Printing Office via GPO Access
    [CITE: 47CFR15.239]

    [Page 816-817]



    PART 15_RADIO FREQUENCY DEVICES--Table of Contents

    Subpart C_Intentional Radiators

    Sec. 15.239 Operation in the band 88-108 MHz.

    (a) Emissions from the intentional radiator shall be confined within
    a band 200 kHz wide centered on the operating frequency. The 200 kHz
    band shall lie wholly within the frequency range of 88-108 MHz.
    (b) The field strength of any emissions within the permitted 200 kHz
    band shall not exceed 250 microvolts/meter at 3 meters. The emission
    limit in this paragraph is based on measurement instrumentation
    employing an average detector. The provisions in Sec. 15.35 for
    limiting peak emissions apply.

    It is paragraph (b) above that is the stickler. It takes only about 20
    nanowatts into an efficient antenna (say, a quarter or half wave) to get
    that 250 microvolts/meter at 3 meters. Of course, you could use a kilowatt
    into a really bad antenna so long as you don't produce a field greater than
    250 microvolts/meter at 3 meters.

    Sorry to be the bearer of bad news.

  8. Mac

    Mac Guest

    In the AM case, wouldn't it be pretty hard to push those 3mW into a 3
    meter antenna at those frequencies? I'm not saying it couldn't be done,
    mind you.

  9. John - KD5YI

    John - KD5YI Guest

    Hi, Mac -

    Did you mean 100 mW? Yes. It's very difficult when the FCC limits your
    entire antenna installation (ground, feeder, and antenna) to 3 meters:

    "Sec. 15.219 Operation in the band 510-1705 kHz.

    (a) The total input power to the final radio frequency stage
    (exclusive of filament or heater power) shall not exceed 100 milliwatts.
    (b) The total length of the transmission line, antenna and ground
    lead (if used) shall not exceed 3 meters."

    This makes the radiating system very low in efficiency. The whole idea (as
    far as the FCC is concerned) is to limit the field strength thus limiting
    the ability to interfere with licensed stations. It makes sense to me.

    Nevertheless, I've heard that the range achievable on the AM band far
    exceeds the range achievable on the FM band when both meet the FCC rules.

  10. Mac

    Mac Guest

    Yes. But it would probably be difficult either way. ;-)
    Limiting interference is one of the main mandates of the FCC. So I'm not
    complaining about that. It's just that it would be a lot easier to radiate
    100 mW if you could build a larger antenna! Maybe you can buy antennas
    designed for this application. I guess some kind of resonant matching
    network would help quite a bit. I'm not really an antenna expert. Or maybe
    a magnetic dipole.
    Makes sense. An awful lot more power, if you can just get the little
    antenna to radiate!

  11. John - KD5YI

    John - KD5YI Guest

    Hi, Mac -

    A good place to start is Low Band DXing by ON4UN. Although the book
    addresses the frequencies below the AM band (160 to 190 kHz, I think), the
    inherent problems of short antennas are discussed, I am told. On that band,
    an input power of one Watt and an antenna length (including ground and
    transmission line) of 15 meters is allowed. Even so, if I recall correctly,
    radiated power is in the low milliwatts due to low antenna and network

    It is a challenge.

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