I've recently built this circuit: http://makerf.com/posts/mighty_simple_shortwave_transmitter
It works on the desired frequency as you can see from this video : https://www.youtube.com/watch?v=t6nFi7fdoK0 .
Now I'm trying to understand why and how it works, I would appreciate if you can guide me through the circuit in order to understand what each single components do.

This is what I think I've understood:

  1. The oscillation begins in the crystal (xtal) for the reason explained here What makes a crystal oscillation to begin
  2. There is a feedback loop which goes through the transistor Q1. Basically the RF is injected into the base and it gets amplified (and shifted by 180 deg?) by the transistor exiting from the collector.
  3. The L1 - variable capacitor is a tuned circuit but I don't understand why the collector is tapped there and in general what is its purpose.
  4. The L1 - L2 transformer is the way through the RF goest to the antenna and the transformer is required to adapt the impedance of the circuit to the 50 ohm required from the antenna

Finally, I'm not sure about the two resistor and the 0.05uF capacior.

Also, which oscillator circuit did inspired this design? Is it an Hartley Oscillator?
Thanks for your patience and if you have any good resource (book or website) which could help me in understanding oscillators, please let me know.

  • \$\begingroup\$ You obviously do not have a suitable license, and operation of such a transmitter is illegal. \$\endgroup\$ Feb 15, 2015 at 10:39
  • 1
    \$\begingroup\$ Actually I have. My Ham license call is IZ1KSW (for operation in Italy) or EA7JUG (when I operate from Spain).And, btw, I'm not connecting any antenna to the circuit, just a 50 ohm dummy load (as you can see in the video) \$\endgroup\$
    – gabric
    Feb 15, 2015 at 10:41
  • \$\begingroup\$ How can you have a ham license and not understand how a simple crystal oscillator works? \$\endgroup\$
    – Barry
    Feb 15, 2015 at 14:50
  • 7
    \$\begingroup\$ Barry, I don't think the original question was about my knowledge and my technical background. If I ask something about a crystal oscillator is because I admit that my knowledge is limited and I would like to improve it. I'm glad that you, with your BSEE and MSEE plus 42 years experience in electrical engineer, find the question so trivial that doesn't deserve an answer. Having a radio ham license doesn't automatically make me an engineer. Sorry to have wasted 10 seconds of your precious time \$\endgroup\$
    – gabric
    Feb 15, 2015 at 15:05
  • 1
    \$\begingroup\$ It's people like Barry who give ham operators a bad name. We're supposed to help each other out not point out someone's shortcomings. \$\endgroup\$ Oct 6, 2015 at 2:09

2 Answers 2


Gabric - here are my answers to the 4 questions:

1.) The oscillation starts as soon as you close the switch. This closing causes a sharp inrush transient which contains (among others) also the frequency component that fulfills Barkhausen`s oscillation condition (see point 2).

2.) The oscillation condition requires a loop gain (slighly) larger than unity and a loop phase of -360 deg (0 deg). The transistor in common emitter configuration contributes -180 deg phase shift between base and collector. The remaining phase shift is provided by the feedback loop.

This loop again consists of a third-order lowpass which is able to provide these -180 deg at one single frequency only. The lowpass consists of a second-order lowpass (L1-C) working upon a first-order lowpass (Lq-r,in). Here, Lq is the crystal working as an high-quality inductor and r,in is the finite input resistance of the transistor at the base node.

3.) The primary inductor is tapped to provide a certain kind of ac voltage division in order to limit the gain of the amplifier stage (because it should not be too large, just sufficient to allow self-excitement without severe non-linearities).

4.) The 10k resistor is necessary to provide the correct DC biasing of the transistor; the small emitter resistor provides dc feedback for stabilizing the dc bias point. The 0.05µF capacitor establishes signal ground potential at the pos. supply pin. This is necessary because the capacitor of the first L-C lowpass block is connected to the pos. supply but must be referenced to ac ground.


I post this very useful answer which comes from a fellow ham who still believes that helping people to understand is part of the noble art of amateur radio. Someone who posted comments above should think about it.

Start by thinking of this circuit as an amplifier. The 27 ohm resistor from the emitter to ground (negative terminal) puts a limit on how much current will flow.

The 10K resistor from the base to the positive terminal puts a positive voltage on the base and biases it so that current will flow through the transistor.

Now the fun begins! It is an amplifier, but it has no input signal! The input signal is the output signal -- it is like a dog chasing his tail!

The crystal is very important. It is the main frequency determining element, and it is the conduit for the feedback that gets this thing oscillating. It is a piece of quartz. If you put a voltage across it, it will begin vibrating (physically) at a specific frequency. As it physically vibrates, it also creates electrical vibrations.

So, when you turn this thing on, noise in the circuit will put a bit of charge on the crystal. It will begin to ring, much like a musical tuning fork. The electrical vibrations from the crystal will go to the base. They will be amplified by the transistor and will emerge (stronger) from the collector. From the collector, they go to the tuned circuit formed by the big coil and the variable capacitor.

The coil wound on the film box serves several purposes. The portion of the coil between the positive terminal and the collector carries the 12V DC to the collector of the transistor. It also carries the amplified RF signal coming from the collector. This signal goes through the lower portion of the coil and causes the coil and the capacitor to resonate. The signal at the top of the tuned circuit peaks when the tuned circuit is tuned to the resonating frequency.

The capacitor/coil tuned circuit (with the tapped coil) are set up so that the right amount of energy is fed back from the output to the input, and that this energy is fed back in the proper phase relationship to the signal at the input. Think of a child's swing at a park: To keep the swing oscillating, you have to push at the right moment (frequency and phase) and with the right amount of energy.

The little capacitor across the battery is to prevent "key clicks." The output coil on the main coil takes some of the energy and sends it to the antenna while converting the impedance of the antenna to a suitable "load" for the transistor.


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