I am doing a circuit for an AM transmitter and receiver. I extracted it from http://www.pyroelectro.com/projects/pyro_rf_receiver_27mhz/index.html.



I have already done the transmitter, and the antenna seems to irradiate as there is some interference with other devices. However, my question in this part of the circuit is if the indcutor L3 (the one right before the antenna) is needed, and why? can I change its value?

On the other hand, I have also done the receiver; however, it does not seem to receive anything. It is important to say that I tailored the design so that I could use a 20MHz carrier. In the transmitter I used a crystal of 20MHz, and in the receiver (tank circuit), I used an inductor of 10uH and a combination of ceramic capacitors of around 6.66 pF. I suppose I do not detect anything because I used a choke inductor (those that are fabricated and come in a green package), and I did not made my own (I did not made my own as I have no way of meassuring it). Also, I do not see any envelope detector on the receiver... which part in this circuit acts as the envelope detector. And finally, why does it not work? I really appreciate your answers

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    \$\begingroup\$ 1) you should not be transmitting at 20 MHz, in the 27 MHz there are "free" frequencies, not so in the 20 MHz band. It is possible that you're breaking the law. 2) You did not do exactly what they did in the example so no wonder it does not work. It's a very basic and crappy design as well and very sensitive to component values. There is no L3 in the schematic. If you have to ask why certain components are needed, why are you leaving them out ? \$\endgroup\$ Commented Jun 27, 2017 at 15:34
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    \$\begingroup\$ Regenerative receivers are extremely picky with the inductors. L1 and L2 are inductively coupled to each other. Too weak coupling and this circuit won't pick up the signal, too high coupling and you've got an oscillator instead. I would recommend two things: trimmer capacitor for C2 so you can fine tune the frequency, and someway to adjust the positive feedback. \$\endgroup\$
    – Oskar Skog
    Commented Jun 27, 2017 at 16:33
  • \$\begingroup\$ There aren't any frequencies that you can transmit on legally without some form licence. And it's interfering with other equipment. Wait for the knock on the door. \$\endgroup\$
    – Chu
    Commented Jun 27, 2017 at 17:08
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    \$\begingroup\$ There are lots of frequencies in the US that can be utilized without a license subject to power and other limits. If the OP isn't in the US, any speculation is likely just that. \$\endgroup\$
    – Glenn W9IQ
    Commented Jun 27, 2017 at 21:38

1 Answer 1


In your text you describe this as a 20 MHz transmitter but the original design is for a 27 MHz transmitter and receiver. If you have attempted to alter the design, it would be helpful for you to describe your design changes.

The transmitter is a very marginal design that will have significant harmonic content in its output. Take care that you are meeting any local regulations as this could cause interference to other services. On the other hand, you can use this harmonic content to confirm that your transmitter is working if you have a receiver cable of detecting any of the harmonics.

L3 in the transmitter helps to match the antenna impedance to the final amplifier of the transmitter. Removing it without lengthening the antenna will reduce the ERP and therefore shorten the effective range of the transmitter.

On the receiver side, the detection is performed by the regenerative design. The values of L2 and C2 are critical for correct functioning of the receiver.


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