Crystal Radio Tuning Capacitor

A tuning capacitor, also known as a variable capacitor, is a very critical component within a radio and forms a part of the tuned circuit when connected to the ferrite coil. There are two main types of capacitor construction; air spaced, and polyester film spaced which refers to the filling between the plates. All the modern capacitors are of polyester type and they are usually very low capacitance due to their small physical size.

When buying a capacitor, for example on eBay, the sellers usually mention the range of the capacitor in picofarads (pF). For example, a typical polyester type to be found on the Internet has a range of 3 pF to 200 pF. As a general rule of thumb, a capacitor with the largest capacitance is better, as it provides a greater tuning range. However, these types of capacitors are usually air spaced and they do not make them anymore. The air spaced ones can have a capacitance of up to 300 pF, 500 pF and even 800 pF. These types of capacitors are usually in vintage radios and eBay is often the place where people sell them.

If you can find an old radio that someone is throwing away, then it is usually a good source of components such as the tuning capacitor, and ferrite rod. This tuning capacitor came out of an old Ferranti radio, which had those rotten germanium transistors (AF116) that had died. It was beyond repair as the circuit tracks had perished, and the power supply had seen better days.

This variable capacitor has two identical sections each with seven plates. You can increase the capacitance by joining both sections in parallel, effectively making a 14-plate capacitor. Hence, this tuning capacitor has a total capacitance of 425 pF.

The alternative is to use the modern polyester type, which will not provide the complete tuning range to cover the entire medium wave (MW) band. I tested a 200 pF one, which allowed me to tune into only half of the MW band. I also tested another one that came out of a modern heterodyne pocket radio. It was 120 pF in value and provided only a quarter of the range on the MW band.

Since the 200 pF variable capacitor is cheaply available, one alternative would be to use it together with a 200 pF fixed capacitor connected in parallel. This connection could have a switch so that once you have tuned through the first half of the MW band, connecting the fixed capacitor, using the switch, then allows you to tune through the second half of the band, but it is not ideal.

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