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I recently built this circuit, and could hear a nice full quieting signal on my short-wave radio with a 7.2 megahertz crystal for the 40 meter band.

What is the simplest way I could Amplitude Modulate this signal with a microphone or computer output port so my Short wave radio could hear this signal and I could potentially make some HAM contacts?

And by the way I do have a license. It is KDØWHB and I have general privileges.

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  • \$\begingroup\$ Very few Ham operators use amplitude modulation so no one may hear you. Also, you do need a license to transmit on amateur radio frequencies. \$\endgroup\$
    – Barry
    Nov 19 '13 at 0:00
  • \$\begingroup\$ I have a general class license, KDØWHB , and I will be building 2 units for testing. I also found a frequency that is commonly used with AM, so once I get this to work, I will buy a crystal for that. \$\endgroup\$
    – skyler
    Nov 19 '13 at 0:27
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    \$\begingroup\$ Just for something experimental if you have a small mic pre-amp or signal generator try attaching it to the circuit via say a 4.7uF cap to the base of the 2N3904 and see how it goes. \$\endgroup\$
    – PeterJ
    Nov 19 '13 at 3:22
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    \$\begingroup\$ Notice: this question is cross-posted. \$\endgroup\$
    – Kevin Reid
    Nov 19 '13 at 3:23
  • \$\begingroup\$ Congratulations on getting your ham licence, btw :) \$\endgroup\$
    – pjc50
    Nov 19 '13 at 13:09
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What you have here is a straightforward minimal component crystal oscillator.

Your best bet is going to be to follow it up with an amplitude-modulated amplifier stage, along with the necessary microphone amplifier and signal conditioning.

Best resource will be OLD copies of the ARRL Handbook. Hit your libraries.

If you can find a copy of DeMaw & Hayward's "Solid State Design for the Radio Amateur", it will give you a lot of help, although not much on AM. Copies OCCASIONALLY show up on Amazon for reasonable prices.

--de John WB5YOO (Amateur Extra)

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I am sure it is not the most elegant way to do it, but a straightforward way to do it is to put an audio amp on the microphone (microphone signal is very low) and then feed both the output of your oscillator and the amped mic signal into a mixer.

Please note that the mixer produces all the harmonics, so if you don't have a narrow band antenna for filtering, you may want to have a simple filter following the mixer.

The mixer is also a passive lossy device, so you may need to have another power amp to boost the signal strength

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  • \$\begingroup\$ With a properly-adjusted mixer double-balanced mixer (NE/SA602/612) or Mini-Circuits SBL-1 (or any of a host of other parts they make) and proper drive, that will give you double sideband suppressed carrier. SOME mixers will allow you to put in DC bias, to produce the carrier, but not all. You will probably have an easier time doing "plate" modulation on an RF power amp stage. Old Sams CB Photofacts are a good resource for general ideas in this area. \$\endgroup\$ Dec 6 '13 at 22:49
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A simple unlicensed circuit like this is OK for experimenting in a small location, perhaps as far as a friend's house in the same neighborhood.

However, don't even try to scale this circuit to the power level you'd need for meaningful ham communication. Even the neighborhood-scale circuit is technically violating the law, but if you keep the power low and stay away from used frequencies, nobody will notice and therefore won't care. That is not true for a ham-level circuit.

If you want to do ham radio, get the right equipment and a license. Even with the right equipment and no license you'll get into trouble. You will have to regularly ID yourself. If you don't, that will be suspicious. If you do, you would have to use a fradulent callsign. Somebody is going to look up whatever callsign you use and when they find it not listed, you're in trouble.

In addition to all that, this circuit is inherently low power. This topology doesn't scale well to higher power levels. Making a RF power stage that produces significant power but is very pure (few unwanted harmonics), is not anywhere as easy as you seem to think it is.

As Barry said, most ham communication is not AM anyway. There is more single sideband, which is a lot trickier to design a circuit for.

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  • \$\begingroup\$ I have a general license \$\endgroup\$
    – skyler
    Nov 19 '13 at 0:40
  • \$\begingroup\$ @skyler: OK, that's a good start. You might have mentioned this up front. However, that still doesn't address the other issues. \$\endgroup\$ Nov 19 '13 at 0:44
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    \$\begingroup\$ And this 'answer' doesn't address the question. \$\endgroup\$
    – Samuel
    Nov 19 '13 at 2:19
  • \$\begingroup\$ +1 - The answer is less useful than some given the extra information provided but in the wider arena that the question chose to play in the answer provided useful guidance. The issues of need for scalability, or not, and power level are not well addressed by the questioner. Adding a second stage to a circuit like this adds minimal cost and complexity and makes amplitude modulation FAR easier. \$\endgroup\$
    – Russell McMahon
    Nov 19 '13 at 7:24

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