I built myself an FM transmitter with an enable option so I can interface it with a micro controller. The Enable line is set through the micro via a 1K resistor (not shown). Once set to enable, the FM transmitter starts sending whatever sound is injected to the input on the left (in this case sound from the computer earphone terminal via earphone extension plug).

With this design (excluding the circled parts and using test point 1 as a 6-inch wire antenna) I'm able to achieve a transmission of up to about 50 meters.

I then researched buffer stages and adopted an idea from http://talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-2.html by adding the circled components and using TP2 as an antenna instead of TP1. After testing again with the parts added, I get no audio signal at all, even if I have the transmitter and receiver within 10 centimeters of each other.

I'm trying to run this transmitter with a carrier between 300 and 500Mhz and I'd like to achieve a few hundred feet distance.

Rather than place an order for trimmer capacitors and waiting weeks for them to come in so I can ultra-fine-tune the band (which I didn't have to do when testing with the red components excluded from my design), is there anything else I can do to increase the range of my transmitter and still receive an audio signal?


Is capacitor or transistor wrong?

Two things that no one pointed out yet were the 10pF capacitor value (capacitor circled in red) and the add-on transistor (also circled in red).

I'm just wondering if a certain reactance amount in ohms between the first and second amplifier stage needs to be met for both stages to function.

Also, could I get away with a Pn2222 instead of a Pn3563, or maybe even use a set of diodes instead? I'm asking because 99% of the time when I make a circuit with Pn3563, the transistor internally blows up and I have to replace it (no I didn't short the circuit board). Also the maximum current limit for Pn3563 is 50mA and I think with the part values I use and the frequency of interest, I'm approaching and/or exceeding that limit.

  • \$\begingroup\$ The wires from the left connect to the audio source only. the enable wire that is marked "ENABLE" in the diagram will be connected to the micro to control whether the transmitter is enabled or not (carrier on/off) \$\endgroup\$
    – user143136
    Apr 30, 2017 at 3:01
  • \$\begingroup\$ Now I see it, never mind. \$\endgroup\$
    – The Photon
    Apr 30, 2017 at 4:12
  • \$\begingroup\$ I think you should use a faster transistor than the 2N3904 for a carrier of 300-500MHz. Tf of this is only 250MHz (but has an Ic of 150mA.) The PN3563 is 600MHz but 50mA. Could try 3 in parallel for 150mA rating, but the input capacitance will be 3x as much. \$\endgroup\$
    – rdtsc
    Apr 30, 2017 at 20:05
  • \$\begingroup\$ I'm starting to think the problem is I'm burning the insides of transistors without knowing it because I connected the leads directly together and soldered them without a breadboard or circuit board. Now I'll try a circuit board with track lengths no greater than 5mm and with 70mil width and see what happens \$\endgroup\$
    – user143136
    May 1, 2017 at 20:43

1 Answer 1


I think that your buffer is killing the osc meaning that you will get no signal or it is capacitively detuning it downwards making your frequency way lower than it is supposed to be .Remember that the CE BJT has a low input impedance at your collector currents .Your osc has a relatively high impedance on the collector where you are taking the base drive power from .There are lots of ways to cure this .You are spoilt with options so I will state some fixes.Take the drive from the emmitter of the osc BJT adjusting CX for feedback if needed.Consider an emmitter follower for the buffer .Consider a JFET buffer .

  • \$\begingroup\$ I'm curious. if I take the signal from the emitter, wouldn't gain/distance be reduced? and I guess you're saying the coupling capacitor at the collector needs to be included with the tank capacitor and feedback capacitor (1pf and xpf) when calculating frequency? I wonder if I could replace the cap with a reverse-biased diode and get better results. hmm.. \$\endgroup\$
    – user143136
    Apr 30, 2017 at 5:43
  • \$\begingroup\$ So I attempted to take the signal from the emitter instead of collector and I get no signal again \$\endgroup\$
    – user143136
    Apr 30, 2017 at 6:21
  • \$\begingroup\$ So now tune up the buffer output collector circuit \$\endgroup\$
    – Autistic
    Aug 23, 2023 at 1:23

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