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I've made this simple AM transmitter:

AM transmitter

http://makerf.com/posts/fun_with_crystal_oscillators_part_3

The author of this circuit, claims that would be possible to extend the range of this transmitter by attaching a longer antenna and a better plane ground: i'm using a 10 feet of wire as antenna and a plane ground, but this transmitter is still barely able to transmit to a max distance of few inches. Why? Maybe there would be a way to amplify a little bit more the output signal, maybe using a small transistor?

Thank you.

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  • \$\begingroup\$ Here's a test of your crystal oscillator to try: just power it from its normal voltage, no op-amp, and see how far it transmits now. There won't be any sound, but the radio will go quiet when tuned to its frequency. To extend the range, try another 10 feet of wire, lying spread out in the other direction connected to the ground terminal / battery negative. \$\endgroup\$
    – tomnexus
    Commented Mar 12, 2015 at 18:40
  • \$\begingroup\$ @tomnexus Are you saying that would be a good idea to change the op-amp? Using a better one could also increase the range of the transmitter? \$\endgroup\$
    – Giov
    Commented Mar 12, 2015 at 18:41
  • \$\begingroup\$ @tomnexus if i power the crystal oscillator without the op-amp, with its normal voltage, is even more weak: it should stay attacched to the radio receiver, to notice any effect. \$\endgroup\$
    – Giov
    Commented Mar 12, 2015 at 18:43
  • \$\begingroup\$ @tomnexus the crystal oscillator is rated to work at 5 volt. There is a better schematic? \$\endgroup\$
    – Giov
    Commented Mar 12, 2015 at 18:43
  • \$\begingroup\$ @tomnexus (and others) The LM386 is an audio power amplifier, not an op-amp. See this datasheet. Negative feedback is not required. \$\endgroup\$
    – tcrosley
    Commented Mar 12, 2015 at 18:47

2 Answers 2

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The antenna needs a live wire and a ground wire. Try grounding the battery negative, or the earth of the oscillator. Either connect it to a water main or window frame or something, or just use another equal length of wire, laid out in the opposite direction.

Use of this circuit to transmit without a license is might break the law, depending on where you are.

It is also likely that the crystal oscillator is radiating at 3 MHz, 5 MHz, 7 MHz, etc, and probably more effectively than it radiates at 1 MHz, which isn't so nice. Keep the antenna short, and turn the circuit off when you've finished the experiment!

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  • \$\begingroup\$ Obviously is not in my intention to make an illegal transmitter: would be enough to transmit across some rooms in my home: I live in Italy and this kind of transmitters are allowed. Anyway I've tried any solution as you suggested, but the transmitter is still very weak. Would be a wrong idea to use a transistor to improve the signal? And I could implement this solution? (if is possible) \$\endgroup\$
    – Giov
    Commented Mar 12, 2015 at 19:30
  • \$\begingroup\$ Hmmm. If it's an oscillator from an old motherboard, it will probably have a 5 V square wave output. This should be quite strong. Try making a roughly circular or square loop of wire, just bring the other end of the 10 foot wire around and ground it through a resistor. The resistor is to protect the oscillator, you could use 100 ohms or so. This might work better than the dipole. \$\endgroup\$
    – tomnexus
    Commented Mar 12, 2015 at 19:44
  • \$\begingroup\$ Which kind of object could I use to make a circular loop of wire and what about the circumference? Are you meaning to use something like a pringles tube? \$\endgroup\$
    – Giov
    Commented Mar 12, 2015 at 19:52
  • \$\begingroup\$ No, much bigger. (also, a pringles tube is metallic, so won't work at all). I guess you could wrap it one or more times around the table. Ideally you should make the loop vertical, so go across the table, down under the legs, and up again. Probably best if it's a wooden table. Shape is not important, just the area of the loop. I think just use the 10 foot antenna wire, curled up. It's just an idea, I'm surprised that the dipole antenna isn't working already, but the loop might work. \$\endgroup\$
    – tomnexus
    Commented Mar 12, 2015 at 19:56
  • \$\begingroup\$ @Giov precisely what radio are you using to receive the signal and on what band? Also, what signal are you feeding the input to the 386 and how? \$\endgroup\$
    – Andy aka
    Commented Mar 12, 2015 at 21:07
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Try winding wire around a toilet roll, and join a variable capacitor in parallel with the coil. connect the output of the transmitter to one end, and the earth output to the other. Connect your antenna and ground, and tune till you get a strong signal on your radio. Experiment with different numbers of turns if you can not peak the signal. You might be surprised by the result. Stumbled on this when I used to play with little circuits when I was about 9 years old.

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