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Broadband fm transmitter

Discussion in 'General Electronics Discussion' started by Marcoiv, Apr 9, 2011.

  1. Marcoiv

    Marcoiv

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    Apr 9, 2011
    I'd like to know if anyone has some experience building an FM transmitter that could be simultaneously received across the entire typical FM spectrum found in motor vehicles. Does anyone make these? Would it be difficult to make one? The use has to do with a number of public safety applications.
    THANKS.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What you ask for is not possible. Transmitters transmit on a single frequency, and radios can be tuned across multiple frequencies (to get multiple stations).

    Whilst a strong station can swamp out weaker stations that have frequencies very close to it, it *should* be impractical to do this for the entire FM band. In any case, the signal quality would almost certainly drop to the point at which what you were receiving was more akin to interference (as it indeed is) than usable signal.
     
  3. Resqueline

    Resqueline

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    Jul 31, 2009
    That amounts to making a noise transmitter (pretty much what the old spark transmitter was). No easy task for high power/ long range, & accurate harmonics.
    You'll need one harmonic frequency for every 200kHz across the entire FM band (20MHz), making 100 frequencies in all, and making sure nothing gets into other bands.
    And that's for the US channel allocations only, for european channel allocations you'll have to double that.
    The Germans have a Traffic Announcement system already btw.. It interrupts even if you're playing tape/CD. A system adressing the individual FM stations is better.
    But it could perhaps be achived the hard way with a carefully tuned duty cycle square wave, or a sawtooth wave.
    Or perhaps it could be done by targeting the Intermediate Frequency used in most FM radios; 10.7Mhz, hoping the IF circuit is not too well shielded.
    You'd have to do some serious lobbying to get such a brute-force transmitting system approved for use though.
    A point is that it could actually be considered useless since many are not listening to a radio station to begin with.
     
    Last edited: Apr 9, 2011
  4. Marcoiv

    Marcoiv

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    Apr 9, 2011
    Proof of Concept

    Thanks very much. In another thread somewhere on the site and in the above note it seems some R&D could come up with something that worked. At this point I'm just looking for plausibility sufficient to put forward a research project for funding. And yes, not everyone has the radio on in their car; but again, that would be part of the research project. I think the system in Europe relies on car manufacturers installing something in vehicles to allow for the emergency intrusion, isn't that so? I honestly don't know. I'm interested in developing something that would work with what's likely to already be present; one or more of the three obvious: cellphones, radio, stereo.
     
  5. Resqueline

    Resqueline

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    Yes, the German system relies on German radios (most of which have the TA system built-in). It was employed already in the 80's I think.
    The RDS system also has provisions for this, as does the DAB system. The US (your place?) already employed such TMC systems one year ago (see Wikipedia).
    I find it unlikely that the world needs a new, expensive, disruptive, old-fashioned, brute-force system. Better to try to unite on one standard.
     
  6. Marcoiv

    Marcoiv

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    Apr 9, 2011
    Thanks. That's what the research and the pre-research would want to find out. I'd just like to explore the technologies to see if indeed they would in fact be expensive; and just how disruptive they'd be and what brute force might mean in this context. It may just be a non-starter. I don't know that yet. I'm in New Zealand, btw.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Oh, I got the wrong end of the stick...

    I thought you wanted a transmitter *in* a car that would be picked up by an FM radio without needing to tune the radio to it.

    I'm not having an impressive couple of days.

    It seems to me that the best approach would be one where commercial transmitters were required to monitor some sort of emergency signal and to cede their transmitters to it when it was activated. It's easier to deal with a small number of commercial operators than to deal with *all* cars or have to set up your own transmitting apparatus.

    If this were indeed for emergency broadcast purposes it would be cheaper to provide these sites with emergency power (gratis) than set up your own transmitters that would be used very rarely.

    Of course, a system such as that which Resqueline refers to that can break in to playing CD would require appropriate radios, and that is something which would not happen fast unless legislated (and even then, you'd have to wait for older cars to be phased out).

    We have a tunnel here that can break in on many radio stations for emergency information, but they do that because they re-broadcast the local radio in the tunnel and can thus take it over at will.
     
  8. Marcoiv

    Marcoiv

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    Apr 9, 2011
    I've heard about this tunnel... Maybe there's something there to think about. There must be something in there that broadcasts on all the FM frequencies, yes? Or have I got it wrong? You'd think if they could do it in tunnel they could do it in ambulance say.
    Thanks.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The advantage of a tunnel is that it blocks out the reception of radio signals.

    The advantage of an ambulance is that they have their own radio system.
     
  10. Marcoiv

    Marcoiv

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    Apr 9, 2011
    Just to isolate the point about the tunnel... it appears that there is some transmitter that can transmit a single broadcast across all or most of the frequencies picked up in typical radio in a car.
     
  11. davenn

    davenn Moderator

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    I havent figured out how they do that yet. Havent come across anyone that can tell me ;) there's 4 road tunnels around Sydney that have that system. I do know that the antenna system is nothing more than a long wire (leaky coax) extending for the length of the tunnel. So there is no particularly hi TX power involved.

    cheers
    Dave
     
  12. Resqueline

    Resqueline

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    Yes, I understand tunnel antennas are "just" large coaxial "cables" that have slots cut in their shield.
    The repeater setup is probably (or most often) tuned amplifiers forwarding a few selected channels.
    It won't require much power per channel & mile to "fill" the tunnel, especially since the tunnel itself is shielded from the surroundings..
    A broadband amplifier setup however would most likely be riddled with intermodulation and hence hopelessly poor reception quality.
    It's a whole different matter to fill the open sky with 100 channels, each channel having to overpower any broadcast station channel in the area.
    Intermodulation would probably make reception/understanding unpredictable too.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I do note that the level of modulation in the tunnel is significantly lower than outside.

    From that, I assume they are actually receiving, then regenerating a new signal that is broadcast through their leaky coax.

    And that makes sense because it makes it easier to swap the audio over.

    I'm not sure how many stations they retransmit and I haven't thought to scan through the stations whilst inside the tunnel to find out.
     
  14. poor mystic

    poor mystic

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    Apr 8, 2011
    I have heard that the New Zealand Civil Defence can send simultaneous text messages to all cellphones in a vicinity.
     
  15. davenn

    davenn Moderator

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    thats a whole different ball game and requires linking between the different cell phone companies
    A lot of organisations are getting into that now around the world

    Dave
     
  16. MattyMatt

    MattyMatt

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    Mar 24, 2011
    Quite a nifty technology, I've heard it called several things but most generically, Cell Broadcast.... Last I heard they were trying to get something like that going in the US for amber alerts and such of the like.... I think it would be great, but it seems that no one wants to cooperate or has the technology to cooperate... annoying... Europe has been able to do it for quite a while.
     
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