This is an FM transmitter that I built. I wanted to interface it with a microcontroller.

The enable connection seems to work, but the range is extremely weak (about one inch), even when running off of a 9V battery.

I know I didn't show it in the circuit, but I connected a 22uF capacitor across the 9V battery.

The antenna length I used is 3 inches.

I selected a low value for the grounded base capacitor (470pF) because I wanted to filter out the mains frequency and allow the tone from the 555 to go through.

I do apologize but the bottom transistor is actually 2N3904 and the top one is PN3563, and I used PN3563 for the transmitter because I'm transmitting at a high frequency.

Is there any way I can increase the range without requiring the battery to drain quickly and without requiring a higher voltage source? Do I need different transistors?

What am I doing wrong?


  • \$\begingroup\$ What is your carrier frequency? \$\endgroup\$ Commented Sep 12, 2016 at 21:24
  • \$\begingroup\$ For the FM broadcast band (88 - 108 MHz) a 1/4 wave whip antenna should be about 28 inches long. Using a correct-size antenna should increase the range. \$\endgroup\$ Commented Sep 12, 2016 at 21:33
  • \$\begingroup\$ My target frequency is roughly 350 Mhz. So I guess I'll need about a 6 inch antenna? \$\endgroup\$
    – user116345
    Commented Sep 12, 2016 at 21:49
  • \$\begingroup\$ c / 350MHz / 4 ≈ 8.43in \$\endgroup\$ Commented Sep 12, 2016 at 23:03
  • \$\begingroup\$ What data receiver did you use? \$\endgroup\$
    – Andy aka
    Commented Sep 13, 2016 at 8:16

2 Answers 2


When you're working at VHF frequencies (like 100 MHz), parts layout is very important - all components around the oscillating transistor should have short leads, and be placed in a compact space.
-It is also easy to miss the fundamental oscillating frequency when tuning your receiver...although the FM broadcast band is wide, it is still easy to miss. You may be tuning to a harmonic or your receiver may be so overloaded by a strong out-of-band oscillator that you're mistaking a spurious signal for the real output. If you tune around the band, and hear a similar-strength signal in various spots, this is very likely. The best way to find the oscillator's fundamental frequency is by coupling in a frequency counter loosely to the coil.
-Your 470 pf base bypass capacitor might be better placed between base-to-Vcc. Connect its Vcc end quite close to the point where 0.1uH attaches to Vcc. And connect its base end very close to T1's base.
-Be aware that attaching an antenna will likely move the oscillator's frequency, or possibly make the oscillator quit altogether.
-Try attaching your antenna to T1's emitter, rather than to collector. It is difficult to tell what impedance your antenna is (your 3-inch antenna has a small capacitive impedance). Matching antenna coupling to the oscillator can improve signal strength, but you likely have no impedance-matching instrumentation or tools. Experiment with different antenna lengths.
-Your 2N3563 for T1 is quite appropriate for this application and your substitution of 2N3904 is fine as well for T2.

  • \$\begingroup\$ I managed to have all my components to the transmitter in a 2cm square space of PCB and each track is 1/2 mm wide and spacing between any two components is about 1/2 mm at most, so I don't think distance is an issue. I also thought maybe connecting my 470pf to vcc instead of ground, but I don't understand why (other than the reason of it might work). and the only professional electronic tool I have is a digital multimeter. It doesn't measure frequency. \$\endgroup\$
    – user116345
    Commented Sep 12, 2016 at 22:41
  • \$\begingroup\$ @Mike - The usual target is 100MHz, trying for 350 MHz is a considerably more difficult target, where stray circuit inductances and capacitances influence frequency greatly. Perhaps you can hunt frequency by using for the inductor an air-coil of 5 turns, about one-quarter inch dia, same length. Move turns together to lower frequency, spread turns apart to raise frequency. Hunting for the right frequency is tedious, but that's what you get for having no instrumentation ;-) \$\endgroup\$
    – glen_geek
    Commented Sep 12, 2016 at 23:00
  • \$\begingroup\$ in my circuit I use pre-made axial-lead 0.12uH inductors. I didn't wind the inductors myself. Could I somehow avoid such stray capacitances and inductances? I'm thinking I could redo my circuit but make the tracks a different distance apart or a different width? \$\endgroup\$
    – user116345
    Commented Sep 13, 2016 at 0:14
  • \$\begingroup\$ @Mike - I see that the capacitor in parallel with the 0.1uH inductor has a range 2.5 - 7.5 pf. This is meant to be a variable tuning capacitor that varies the frequency. With a substitution of 0.12uH for the circuit value 0.1uH, your frequency may be too low, requiring the variable capacitor to be set at its low-capacitance end. Stray capacitance/inductance is never avoidable. \$\endgroup\$
    – glen_geek
    Commented Sep 13, 2016 at 1:11

Antenna must have a ground plane if just a 1/4 wave.

Square law Friis losses dictate you must have 4x the power to get twice the distance. antenna gain is the best way with directional Yagi. Rotor control improves range with directional error.



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