Build an 1 MHz Morse-Code Radio-Telegraph Transmitter

In this project you will build a pocket‑size AM transmitter that lets you send Morse code two meters through the air. The transmitter core is a Colpitts oscillator tuned to roughly 1 MHz (the lower edge of the standard AM broadcast band) supplied by DC-current.

While the circuit is tiny, it combines a number of engineering disciplines: RF simulation, analogue electronics, coil‑winding, electromagnetic shielding, and even a touch of mechanical design. This step‑by‑step guide takes you from first LTspice simulation to a working breadboard prototype placed inside a small Faraday cage so only the antenna radiates. Along the way you will also create your own inductor.

Regulatory note!

Any equipment that deliberately emits RF energy is subject to national regulations. Keep the output power low, test in an isolated lab if possible, and never connect a large outdoor antenna.

Bill of materials

Electronics & components

1. BC547C NPN transistor (Q1)
2. Resistors: 68 kΩ (R1), 470 Ω (R2), 22 Ω (R3)
3. Capacitors: 2× 3,200 pF (C1, C2), 4.7 nF (C3)
4. Custom-wound coil: 0.40 mm copper wire, 52 turns, approximately 15.5 µH (L1)
5. Breadboard & jumper wires
6. 9 V battery + connector

Coil assembly

1. 2.45m of 0.40 mm enameled copper wire
2. 2 cut-off arduino pins
3. anti-enamel flux
4. Tape to insulate coil

Housing

1. Sheet metal mesh for Faraday cage
2. Pliers, soldering iron, cutter

Tools

Lathe (or power drill) for coil winding, ruler or callipers, side cutters, multimeter, tape

For a radiowave to be sent, an antenna needs to receive a oscillating signal. A DC-current however doesn't oscillate, so to remedy this you need to build an oscillator . A Colpitts oscillator feeds a fraction of the LC-tank voltage back to the transistor base through the C1/C2 divider. You can see the formulas in the picture, so for the circuit we can calculate the frequency with:

f=1/(2*pi*√(0.5*LC))

With C = 3200 pico Farad and L=15.5 micro Henry, we can predict f≈1.01MHz.

Copy the schematic in step 1 in LT-Spice, swap the transistor for BC547C, set L=15.5 µH and C1=C2=3 200 pF. Before you run the circuit select the following settings: .tran 0 0.1ms 0 50n startup. The "startup" part gives some start noise for the circuit which will start the oscillation (as is the case in real life). The graph you should get should look like the one in the picture. You have to measure the output side of the circuit for the right graph.

If you want to use different parts or values, this is the time to test if they will work on your circuit. If the resulting circuit in LT-spice works, we can start building!

Grab a 4 cm long, 1.5 cm diameter plastic tube and a 0.40 mm diameter enamelled copper wire.

For a long solenoid:

N=√((L*ℓ​​) / (μA))

We want to create a solenoid with inductance of 15 micro Henry. With L = 15 micro Henry, ℓ = 4 cm, A = 0.25 * (0.015)^2 * pi, μ = 4*pi*10^(-7) (permeability in a vacuum).

This gives N = 52

Wind the copper wire tightly on a lathe or hand-drill, then solder hookup wires for the breadboard and wrap the coil in tape so it never shorts to the cage. On the lathe, start with a slow turning speed. It will reach the end of the tube before it has done the 52 windings, but that is okay as it gave you a feel for how fast you should put the speed of the Lathe. Remove the copper from the tube and start again. If the 52 windings are still not reached, remove the wire, increase the speed and repeat till you reach 52 windings at the end of the tube.

For the building of this circuit we used a simple breadboard and some wire bridges to connect al the components. We used the circuit from the picture to build it. Above is also shown how our breadboard circuit looked like in the end. Note that we connected a push button in the circuit. We use this to send the Morse-code signal. The button is not incorporated in the LTspice circuit.

Cut and fold stainless steel mesh into a 12cm x 12cm x 4cm, big enough for the breadboard. Here we used some simple pliers to cut and fold the mesh if necessary. Make sure all the wires are inside the cage, to prevent the circuit becoming the antenna. The Faraday cage blocks the possible electromagnetic radiation send out by the oscillator.

At a frequency of 1 MHz, the corresponding wavelength is approximately 300 meters. This is impractically long for amateur applications. To address this, we constructed a long-wire antenna using insulated copper wire. The wire is fixed to a wooden rod using tape. The base of the rod is securely mounted inside the Faraday cage, as shown in one of the images above. The wire is cut and stripped at the far end of the antenna to prepare it for connection.

If you have correctly build your circuit you can check it!

Put the oscilloscope probe on the antenna, ground clip on the cage. Press the button on the oscillator, now you should see a sinusoide at ~1 MHz, ≈ 0.8–1 Vpp.

If the circuit oscillates you can test it on a radio!

Use a pocket AM-radio or SDR-dongle a few meters away; If you press the button and listen to the radio you should hear a silent noise instead of the usual noise on the radio.

Open the cage lid: the signal strength should spike, proving the shield works. Close it again to continue legal & clean testing.

Now you have made a functioning radio transmitter!

The oscilator we use is from the following source:

https://www.youtube.com/watch?v=mcD3uBld4Y4&t=134s

I can higly recommend to make his full circuit, witch is way more powerfull! (again, check your local regulations of course)