I need to make some noise

Discussion in 'Wireless & RF Design' started by kc9cra, Feb 28, 2013.

  1. kc9cra

    Thread Starter New Member

    Feb 28, 2013
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    I have never built a transmitter before in my life. I've wanted to get in to building them recently, and was hoping for a very basic explanation of how to put one together, what parts do what, that sort of thing.

    I was hoping to build something using the 13.56 mhz ism band at low power No need for a mic or audio in jack, but I'm hoping to make the signal standout amung all the rfid's on that frequency. Just some noise to hear on my receiver.

    Somebody on EBay is selling some transmitter circuits that operate at 28.2. Might it be possible to modify these to operate on different frequencies. I am not licensed for 10 meters. This makes the ism bands conveniant. If this is possible, I'd love to know how to do it, because it would make my job a whole lot easier by removing the necesity to build the whole unit.
     
  2. w2aew

    Member

    Jan 3, 2012
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    You'd probably be better off studying one of the MANY kits available for 20m CW QRP operation. These can likely be more easily modified for 13.56MHz. Be careful though, even though this is an unlicensed band, there are still restrictions on the allowable power output, etc.
     
  3. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Trying to block RFID readers?

    I am sure you are not allowed to emit in that band. Much more in the way you say.
     
  4. kc9cra

    Thread Starter New Member

    Feb 28, 2013
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    I know rfid readers use the band, but they use it because it is an unlicensed band. I am sure that I am allowed to transmit, but as the previous poster said, there are restrictions.

    I will say though that if I break some very exact specification by the fcc, they won't come charging after me. In most cases, they only enforce those rules when someone complains. If I used 200mw instead of 100mw of output power, they'll never know anyway. It's just one of those things that is impossible to enforce except on people who really start trouble, or those who step on the toes of certain people/groups. If I did happen to jam an rfid reader, that might do the trick. I guess all I can do is be careful.
     
  5. James Bryant

    New Member

    Mar 1, 2013
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    You can build a 13.65 MHz ISM band transmitter using a CMOS 4093 quad schmitt input NAND gate, three resistors, three capacitors, a 13.650 MHz quartz crystal (which you can get on eBay for a couple of dollars - see item 140778284710 or several similar ones - since they come from the Far East they will take 10-15 days to arrive), and two switches. The transmitter is modulated at about 1 kHz, if you do not wish to turn the modulation on and off you can remove S2 and R3 and join pins 8 & 9 of the 4093.

    [​IMG]

    If you do not require modulation you can also eliminate R2 and C2 and must join pins 12, 13 & 14 of the 4093 together (in this case pins 8 & 9 must also be joined together, but not to anything else, as well).

    The RF oscillator uses an RC relaxation oscillator comprising R1, C1 and the capacitance of the crystal and the 4093 inputs - its frequency is locked to that of the crystal. The quartz crystal may be intended to work with 20 pf or 30 pf of parallel capacitance. The input capacitance of the 4093 is about 5 pF per input so an additional capacitance, C1, will be needed. C1 should be 10 pF if the crystal works with 20 pF, or 20 pF if the crystal needs 30 pF. It is possible to replace a fixed C1 with a 8-30 pF variable and adjust for better accuracy.

    If C1 is 10 pF R1 will probably need to be somewhere between 1.5 and 2.2kΩ, perhaps a little more or less, if it is 20 pF you will need it between 1 and 1.5kΩ - experiment to determine the best value.

    The modulation oscillator frequency is set by R2 and C2. Try values of R2 around 200kΩ. The reason that the values of R1 and R2 are uncertain is that the 4093 is a digital schmitt trigger circuit and its threshold voltages are rather inaccurate. This does not matter in digital systems but it makes adjustment of relaxation oscillators a little harder.

    For low frequency applications 4000 Series logic will work with 3V supplies, but to oscillate at 13.65 MHz it is best to use a higher voltage. An inexpensive 9V battery, or a wall wart (low voltage DC mains power supply) with an output somewhere between +9 and +15V, are suitable. S1 is the on/off switch.

    James Bryant - G4CLF
     
  6. James Bryant

    New Member

    Mar 1, 2013
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    That should be "8, 9 & 11 joined together and not to anything else" in paragraph 2.
    Sorry - James
     
  7. w2aew

    Member

    Jan 3, 2012
    219
    64
    Making a transmitter with a digital circuit like this is a bad idea. The square wave output will have the right frequency, but will also generate energy at many odd harmonics of the main frequency also. If you decide to use the digital circuit, please but a low pass filter on the output before the antenna to filter off the odd harmonics.

    Just because you can get away with doing something illegal, doesn't give you the right to do it.
     
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