Homemade Simple Radio

For me, the invention of the radio is quite miraculous. Through wireless transmission, sound can propagate to such distant places and be played back through a radio by restoring the sound. This article attempts to build a simple radio circuit to understand the underlying principles. After simplifying the components of the radio, it can mainly be composed of three parts:

Resonance circuit
Filtering
Amplification circuit

Therefore, this article will also introduce these three parts. But before that, let's talk about how sound propagates.

Transmitter: Amplitude Modulation (AM) and Frequency Modulation (FM)

The current passing through a conductor generates an electromagnetic field around it. By adjusting the strength of the current, different amplitudes and frequencies of radio waves can be produced. Through this characteristic, we can put information on the radio waves and transmit them to further places through wireless transmission. The process of mixing the original signal (data) into the radio waves is called modulation¹, which can generally be divided into Amplitude Modulation (AM) and Frequency Modulation (FM). AM does not change the frequency of the original radio wave but changes the amplitude, while FM changes the frequency to carry the original signal.

Animation of audio, amplitude modulation, and frequency modulation carriers.

Image Source: Wikipedia (Berserkerus, CC BY-SA 2.5)

Through this method, we can "embed" sound into the radio waves and transmit them through a base station. For example, Fukuoka Tower is a radio tower located in Fukuoka.

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Receiver: Resonance Circuit

So how do we receive signals on the receiving end? In basic electrical theory, we can use a resonance circuit to receive specific frequencies of radio waves. Radio waves can be thought of as small signals of alternating current surrounding us.

If there is capacitance and inductance in an AC circuit, the voltage and current are constantly changing. When the current changes, the inductance generates an electromotive force to resist the change in current. The generated electromotive force causes the capacitance to produce capacitive reactance, and they continuously absorb and release energy from each other. At a fixed frequency, the reactance of the inductance and the reactance of the capacitance can cancel each other out, resulting in the maximum current at a specific frequency. The antenna is essentially a conductor that can generate electromagnetic induction inside when receiving radio signals. This, in turn, generates current, and the resonance circuit receives signals of specific frequencies.

The resonance frequency can be calculated using the following formula:

f = \frac{1}{2\pi\sqrt{LC}}

where f is the frequency, L is the inductance value, and C is the capacitance value.

Filtering Circuit

In radio applications, the voltage of the sound will always be positive. Therefore, a diode is added for rectification, as a diode has a one-way conduction effect.

Amplification Circuit

The signal received from the radio is very weak and difficult to hear. It needs to be amplified before we can hear the sound clearly. Signal amplification can be achieved through operational amplifiers or transistors. In practice, specialized operational amplifier ICs designed for audio are usually chosen.

Implementation

Screenshot 2021-12-01 3.04.36 PM

The circuit is very simple. The front part is a resonance circuit, using a variable capacitor to conveniently adjust the frequency. The diode used is a Schottky diode, which has a lower forward voltage than a regular diode, ensuring that small signals can still pass through. The amplification circuit uses the TA7368 audio amplifier from Toshiba² instead of a transistor. Finally, connect a speaker, and the project is completed.

During the experiment, I found that the range of the variable capacitor was too small, resulting in very vague sound at the beginning of the test. Later, I found that when I placed my hand on the metal part, I could receive the sound properly. This is because the hand also has some capacitance, which happened to be within the frequency range of the radio broadcast.

Afterword

In modern times, it is not common to see such a homemade radio circuit. Usually, there are pre-designed ICs available for use, or frequency selection and volume adjustment are done through digital signals. However, it is still quite fun to personally build and understand the underlying principles.

There is also a device called the Fox Radio³. During World War II, when there was no internet and soldiers needed to avoid being detected by enemies, some soldiers invented a radio that did not require any power source. Its principle is to use copper wire as an antenna and inductor, and use an oxidized razor blade and pencil lead as a diode for rectification, completely relying on the electrical power of radio waves to operate. For more details, you can refer to this video:

Homemade Simple Radio