First CW Transmitter

Note

(I initially wrote this over a year ago, some things are not representative of my current understanding of theory lol)

Introduction

In the fall of 2023, I joined Northeastern Wireless Club and got acquainted with amateur radio. Soon enough, I got an amateur radio license and began to enjoy talking on HF and contesting. However, I also became curious about how radio actually works and wanted to build a radio transmitter. With some friends in the club, we began experimenting and building it with whatever parts we could find around.

What is CW?

In order to transmit over radio, the transmitter generates an alternating current which is sent to an antenna. The antenna is a conductor, which will radiate the signal outwards creating a radio broadcast. In order to broadcast meaningul information over radio, we create a carrier wave, a sine wave at a certain frequency and change the signal to carry data.

A simple way to send information is to send pulses using just the carrier wave. Morse Code is used to encode information into a series of short and long pulses called dots and dashes. In amateur radio circles, Morse Code is also referred to as CW (continuous wave).

International Morse Code defines Latin letters, arabic numbers, as well as some special symbols

Oscillators

RF oscillators generally fall in two categories

• LC oscillator - inductor and capacitor connected in parallel (tank circuit) create resonance at their natural frequency
• Crystal oscillator - a piezoelectric crystal vibrates at a certain frequency; crystal oscillators are generally better.

When doing research on transmitter desing, we found this guide that used a crystal in its design. Unfortunatelly, the crystal broke and we couldn’t find any more. For this project, I used an LC oscillator. The advantage of using a tank circuit is that it was very easy to change the frequency by swapping out different parts.

With LC oscillator, either a Colpitts or Hartley oscillators are used. Since this was my first one, I decided to keep it simple and build a basic LC tank.

Schematic

After a signal is produced by the oscillator, it needs to be amplified. For this project, I chose to use a common emitter amplifier. I found a 2N3391 transistor, but any NPN BJT transistor should work fine (as long as it can handle the current and voltage). Watch out for the pinout on the transistor: The 3391 has the base and collecter pins flipped and the schematic might not match your transistor!

• L1 and C1 are the tank circuit from above; I didn’t fill these values because I changed these components out a lot.
• C2, C3 and C4 are decoupling capacitors. They block DC, but allow AC to pass through.
• SW1 is the Morse Code key. When pressed, the circuit will complete and the transmitter will send out radio waves.
• R1 and R2 set the gain of the amplifier.
• R3 and R4 create bias at the base of the transistor.

The voltage gain of the amplifier is roughly equal to the ratio of the collector resistance to the emitter resistance; for this circuit the gain is about R1/R2 = 4.5. Without R2, the amplifier would have a high gain, but adding the resistor makes the circuit more stable. Also, the sum of R1 and R2 regulate the current flowing through the transistor, so it doesn’t exceed this transistor’s 300 mA current limit. $$I_c = \frac{V}{R} = \frac{12}{100 + 22} = 98 \thickspace \mathrm{mA}$$

The resonant frequency of an LC circuit is given by the formula: $$f = \frac{1}{2\pi \sqrt{LC}}$$ Decreasing the values of L and C will increase the frequency of the transmitter.

Biasing

When I was working on this, I went in without knowing much about amplifiers so I did not pay much care to biasing my amplifier properly! I just picked some that I saw online without much thought. Even though I can’t fix that now, I will calculate what I actually should have used and we can see if it’s close.

We already know the current flowing through the transistor, but we need to find the current that we need in the base. The two values are related by a transistor’s β or h_fe value, which can be found in the datasheet for the component. For my transistor β is between 250 and 500.

The current on the base can be found by dividing the current on the collector by β. In my case, the base current is $[200, 400] \thickspace \mathrm{\mu A}$. The voltage on the base is 0.65 V (BJT voltage drop) added to voltage on the emitter, which is the same as the voltage drop across the 22 ohm resistor, which comes out to be: $$V_b = 12 \times \frac{22}{100 + 22} = 2.8 \thickspace \mathrm V$$

Using Ohm’s Law, we calculate the resitance for R4: $$R_4 = \frac{V_b}{I_b} = [7, 14] \thickspace \mathrm{k\Omega}$$ The resistance for R3 is: $$R_3 = \frac{12 - V_b}{I_b} = [23, 47] \thickspace \mathrm{k\Omega}$$

Clearly, the values I chose were not quite in range, but they weren’t horribly off, which is why the amplifier did work.

Building

I started off by making the cirucit on a breadboard. For my LC tank, I used an two 22 pF capacitors in series (eq. 11 pF) and a 4.7 uH inductor. The oscilloscope shows a frequency of 9.75 MHz.

Using the LC circuit formula, the expected frequency was 22 MHz. What is going on? There must be some parastic elements. To test this out, I removed the LC circuit and used the breadboard as a capacitor and a jumper as the inductor. The oscilloscope showed a 46 MHz output!

This was a good start, but I really hoped I could reach the 2m amateur band, which is just below 150 MHz. Clearly building the circuit on the broadboard was not good enough. I’ve never used perfboards before, but I decided to use one in hopes that it would eliminate the annoying capacitance from the breadboard. I’ve also changed up the LC circuit: it now has 3 capacitors for an equivalent 7 pF and two tiny tiny wires in parallel for the inductor.

This change almost doubled the frequency, up to 100 MHz. Unfortunately, it was still outside of the amateur band. Although I technically shouldn’t transmit at this frequency, the output power of the transmitter was very low. I couldn’t catch the signal from across the room, so it certainly met the under 200 feet requirement for Part 15 devices (please don’t be mad at me FCC).

Testing

Our transmitter is missing an antenna. I opted to use a random wire dipole to save time. I connected two wires to the output and layed them out on the table opposite from each other. The receive the signal, I used an AirSpy SDR and program called SDR++.

I tuned it to the apparent frequency of the signal, but instead saw this!

Turns out that my LC had really bad frequency stability and I figured it was because of the inductor that I used. Looking at the scope, I saw the frequency dancing up and down as I continued transmitting.

I decided to replace the inductor with something more suitable for the job.

That meant that my frequency went down, but it seems to have remove the wide bandwith emissions, but the frequency was still not stable sadly. Smallest mechanical vibrations would cause it to change frequency, so I wasn’t able to properly send a CW wave sadly. Here’s my best attempt.

Even with the underwhelming results, this project was a lot of fun. I decided to write this post, because I didn’t see many beginner-friendly guides on the subject online, so I hope you learned something from my little adventure. Like in everything in life, we need to make mistakes to get better at something, so don’t be ashamed of your failed projects.

Notes

• Common Emitter Amplifier Explanation - Youtube
• Voltage Divider Biasing - Youtube