Simple RF amp for iphone FM transmitter?

Discussion in 'Wireless & RF Design' started by rudyauction8, Feb 10, 2014.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I recently purchased an fm transmitter of ebay for my iPhone and it works fine but has a short range of like 3 feet so I was thinking of building a mini amp to get the range up to about 10 feet or so. How simple could I make it? All I could find on google were transmitters and 10 watt amps with transmitters.
     
  2. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    ...anyone?...
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It would be nice to know WICH transmitter you have.
    That way we can see if it is possiible and what is needed.

    Bertus
     
  4. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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  5. nerdegutta

    Moderator

    Dec 15, 2009
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  6. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I've thought of that but the output power is so low (I think like .01mw) that a simple amp would help a lot more than just adding an amp. Could just using a single transistor amplifying the original signal to a new antenna help or is it more complicated than that?
     
  7. Jibby

    New Member

    Sep 12, 2013
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    It is not easy.
     
  8. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    Please continue, I have plenty of breadboard space and hundreds of transistors and other components, just never worked on RF circuits before. Just looking for a complete answer to the question or a link to instructions for a relatively simple FM amplifier without all of the other stuff. If that's not possible, I'll start messing around and hopefully come up with something without ruining my transmitter.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    fm amps are simple, more simple than other modulation scemes. it still would be a good idea to open up the transmitter you have, your going to have to anyway to get the output for the amplifier. a lot of those "transmitters" have an attenuator in the output to reuce the output to legal part 15 levels.
    to amplify fm, class c operation is fine, no complicated bias necessary.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    The problem is a difficult one which may explain the lack of a definitive response. You have a number of problems with such a design including low drive level, power source, breadboarding a VHF circuit and so on. By all means -- mess around and let us know what you come up with.
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    an mmic should do it, like an mar1 from minicircuits lab. two capacitors and a resistor are all it takes, and the minicircuits web page shows you how to make up the simple board and calculate the resistor value.
     
  12. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    The datasheet shows that the device is made up of 2 transistors and some resistors. Is it possible to recreate the device using the ones I have? I'm not looking for insane range, just an extra 10-20 feet. I tried the antenna last night and it got me an extra 4-5 feet which helped but still isn't enough. If I can't build the amp from scratch, I'll grab one of those amp chips and put it on a circuit board for use with this and any other RF projects.
     
  13. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I've had it open a couple times so far, I don't see an attenuator, just a couple chips and a short (2in) antenna (I lengthened it to 8 in last night without much difference), so I think the output power is what's cutting down my range. If the chip listed above is as easy to use as the datasheet says, I think I may try it.
     
  14. nerdegutta

    Moderator

    Dec 15, 2009
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    Can you take a picture of the circuit? Both sides?
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Could you take sharp pictures of the modeule?
    Both upper and lower side please.
    That way we can see if there is an attenuator and antenna.

    Bertus
     
  16. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Here's pics, they're the best I could get.
     
  17. bertus

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    Apr 5, 2008
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  18. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    The camera doesn't have a flash, that's from my flashlight, without it the pics were blurry. I don't have any other cameras (my iphone won't connect to my computer or I'd use it) or working scanners.
     
  19. davidGG

    Member

    Dec 22, 2012
    51
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    If you have the components, just make this. It goes a distance of 15 feet easily, even further if you match your antenna properly.
    http://www.talkingelectronics.com/projects/BeetleMklll/BeetleMkIII.html

    By the way (Sorry don't want to hijack your thread), in the above link, what purpose does the 22nF capacitor in parallel with the source play? the FM transmitter won't work without it.

    EDIT: instead of the guitar + potentiometer, just put your headphones.
     
  20. mattepps717

    Member

    Jan 20, 2014
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    I agree with nerdegutta and bertus, lengthen the antenna to increase the range. These are intentionally a limited range transmitter. Find the antenna point (red wire?) and coiling 30 awg copper wire to add range, You may only get a few extra feet of range.

    So the car next to you doesn't get to listen in on you playing stairway to heaven. :-0

    I am not sure what your application is, automotive to add range in your car. Or if you are using it in your home. If it's from a Bluetooth enabled device, you could pick up a Bluetooth receiver to connect to your audio device. 30 feet of BT range, and they work pretty well.

    http://www.ebay.com/itm/Bluetooth-R...381?pt=LH_DefaultDomain_0&hash=item5406b2226d
     
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