1. What Part of the Radio Does the Dial Change? Overlapping Metal Plates in a Capacitor
A capacitor is a component that stores electricity by facing two metal plates toward each other. The amount of electricity it can store is called "capacitance," and it changes depending on the distance between the plates and the area where they overlap [4].
A variable capacitor (often shortened to "varicon") is a capacitor designed to change its capacitance by rotating an axis. This rotation changes the area where fixed plates and moving plates face each other [3]. A common design features several semi-circular plates arranged on the same axis, which is used in radio tuning dials [4].
2. How Electricity Moves Back and Forth Between a Coil and a Capacitor
In a circuit connecting a capacitor and a coil (an LC circuit), energy is exchanged back and forth. The capacitor stores electricity as an electric field and then releases it, while the coil stores it as a magnetic field and returns it [1]. This continuous exchange creates an oscillation where electricity moves back and forth.
This oscillation has a specific frequency determined by the coil's property (inductance) and the capacitor's capacitance [1]. Just as a swing goes higher when you push it at the right timing, the circuit reacts strongly when a radio wave arrives at this matching frequency. In radio tuning, the circuit's frequency is matched to the carrier wave frequency of the station you want to hear [1].
3. Why Only the Station That Matches the Circuit Becomes Strong
An antenna receives radio waves from many stations at the same time. Among these, the voltage across the coil becomes large only when the radio wave matches the circuit's frequency [2].
When you turn the variable capacitor, the capacitance changes, and the frequency at which the circuit reacts shifts [2]. The dial's scale shows "which station is heard at which frequency" based on this movement. Since specific station frequencies sometimes change, it is best to check with your actual radio guide or broadcaster information.
4. How Much the Capacitor Must Change to Cover the AM Radio Range
Medium wave (AM) broadcasting in Japan takes place in the range of 531 kHz to 1602 kHz [5]. Dividing the highest frequency by the lowest frequency (1602 ÷ 531) gives a ratio of about 3.0 times.
The resonant frequency is inversely proportional to the square root of the capacitance [1]. Therefore, to change the frequency by 3 times, the capacitance must change by about 9 times (since 3.0² is approximately 9.1) [1]. However, this calculation applies to an ideal circuit with only a coil and capacitor. In actual radios, the necessary ratio changes because of capacitance inherent in the coil and circuit itself.
Variable capacitors for actual radios are often built to move from several hundred pF (picofarads) down to much smaller values [8]. Because the capacitance of other components is added to the circuit, the ratio of the variable capacitor alone may be smaller than the theoretical calculation [8].
5. Lodge's Experiment and Marconi's Patent on Tuning Radio Circuits
Oliver Lodge, a British physicist, is said to have conducted an experiment around 1889 using two circuits arranged side by side, each with a Leyden jar (an early capacitor) and a variable coil [6]. He adjusted the coils to match the frequencies of the two circuits. Only when the frequencies matched did the spark in one circuit cause a spark in the other [6].
He called this act of "matching" by the musical term "syntony" and reportedly obtained a patent for it in 1897 [6]. Around 1900, Marconi outlined a method to separate signals by combining tuned circuits on both the transmitting and receiving sides in British Patent No. 7777 (known as the "Four Sevens") [7]. The background to this was the problem of interference [7].
6. Things You Can Try or Look Up Yourself About Radio Tuning
If you have an AM radio, try turning the dial slowly and listening to how the static changes between stations. You can also write down the movement of the dial needle and the order of the stations you hear. Please listen at a low volume.
If you want to learn more, searching for terms like "LC circuit," "resonant frequency," and "variable capacitor" can help you find the meaning of the formulas and photos of the components. You can also check how broadcast bands are used in frequency materials from the Ministry of Internal Affairs and Communications [5].