Tuning Capacitor

A tuning capacitor is a variable capacitor used in an electronic circuit of a radio, and usually connects in parallel to a loop antenna to form a parallel tuned circuit. There are many uses and applications for a variable capacitor; the most typical use being in an AM/FM radio circuit. If you were building a crystal radio, then you will most definitely need this component, and a typical capacitor will look like the photograph shown above. It will usually have six pins at the back, and sometimes an additional pin at the front-side, which connects to the shaft.

Inside, they typically consist of four individually ganged variable capacitors, where two gangs of variable capacitance are for the AM side, and two gangs of for the FM side. The first priority is to determine which is the FM side and the AM side. There is normally some writing at the back to provide a clue to the orientation of the component. The three pins on the lower side are usually for the FM Radio. If you are building an AM Crystal Radio, then you ignore those pins. The three pins on the top side are for the AM radio. For a crystal radio, you need to connect the centre pin to the earthy side of your coil, and only one of the side pins to the antenna side of your coil.

Schematic Symbol

The schematic symbol shows four individual variable capacitors in a ganged arrangement. There are two variable capacitors for the FM section and two for the AM section, and the centre pin is common to both variable capacitors.

In some designs, C1 and C2 are of equal value of around 20 pF, whilst C3 and C4 are of equal value of around 160 pF. For a Crystal Radio, you use the AM radio common pin, which is the centre pin on the AM side of the capacitor, and the AM antenna pin C4. The centre pin, on the AM side, usually connects to the earthy side of the loop antenna. If you had a digital meter that measured capacitance, then you could double check to see which terminals exhibit the greatest capacitance.

Pinout Connections

The shaft rotates four individually separate variable capacitors. The reason for having four is because modern radios usually have an FM band and an AM band. In addition, each band requires two dedicated variable capacitors, where one is for station tuning and the other is for the local oscillator circuit. This means that the FM band has an antenna variable capacitor, and an oscillator variable capacitor, and the AM band has its antenna variable capacitor, and its oscillator variable capacitor.

Obviously, all of these variable capacitors within one package mean that this component will have many terminals. Many hobbyists are perturbed from using it and often end up scratching their heads trying to figure out how to connect it. As a result, they end up buying the old-fashioned open plate capacitors, which have a simpler pinout.

You will not find much documentation of a tuning capacitor; however, the pinout tends to be similar due to the geometry and mechanics of the component. If you look closely at the back of the variable capacitor, you will see that the spaces between the terminals are marked with C1, C2, C3, and C4. Even if they are not marked, I have found that many manufacturers follow the same convention. As you can see from the animated pinout schematic above, there are three pins on one side and three on the opposite. C1 and C2 mark the pins for the FM circuitry, whilst C3 and C4 mark pins for the AM circuitry.

Connecting an AM Crystal Radio

If you are building an AM Crystal Radio, and you do not have a digital meter that measures capacitance, you will be wondering which pins to connect to the loop antenna. Wiring this component is very easy, because the lower three pins are usually for the FM band, which leaves just three pins at the top. The pin marked by C3 is usually for the AM oscillator circuit, and since your crystal radio does not have an AM oscillator, you should ignore that as well. This then leaves two pins marked by C4, which are the ones you should try for your Crystal Radio. A simple trick that takes the guessing out is to remember that when capacitors are connected in parallel, the capacitance increases. Hence, if you were building a crystal radio without any measuring equipment, connect the centre pins together to the earthy side, and connect all the corner pins together to the antenna side. This way, all the plates are used giving you the largest possible capacitance.

Typical Capacitance Range

For the MW band, the frequency range is approximately 520 kHz to 1650 kHz. Therefore, the typical capacitance range tends to be from 0 pF to 160 pF, and for the FM band, the typical capacitance range is approximately 0 pF to 20 pF.

If you were to (incorrectly) connect the pins for the FM band to your AM Crystal radio, then the variable capacitor will only go up to 20 pF, therefore you will receive approximately 1/8 th of the AM band. If you find that you are not getting enough stations, then this is the most common problem. The solution is to use the pins on the opposite side of the variable capacitor.

For UK coverage of the AM band, 140 pF is the absolute minimum for heterodyne receivers. Although many manufacturers might shift the values slightly, these tend to be the typical values I seem to remember.

Station Clumping

Although these tuning capacitors work with a crystal radio, they are for heterodyne receivers; hence, the additional ganged plates for the local oscillator. If you find that your crystal radio has a clump of stations at the extreme end, then it means that your ferrite coil is good and picking up those stations, however you are unable to tune into them because the variable capacitor does not have the necessary capacitance range. Increasing the capacitance usually helps, and one trick (mentioned above) is to connect all the gangs in parallel. The other alternative is to have taps in the ferrite coil such as in my Project Ultra, The Little Whippersnappers 8 Band Crystal Radio.

Trimmer Screws

The manufacturers have also provided two trimmer screws for each band. For the FM side, there is a trimmer for the FM antenna and another for the FM oscillator. Similarly, there is also a trimmer for the AM antenna, and another for the AM oscillator. The trimmers are useful if your radio had a dial and you needed to calibrate the dial. Otherwise, if you are building an AM Crystal Radio, you ignore the trimmer capacitors, as they will be of very little use. Keep them on the maximum value, when their plates are not overlapping.

Buying Variable Capacitors Online

Now that you know how to connect a modern variable capacitor, you have a greater selection choice, and you can look for the best deal. The cheapest ones I found were on eBay where sellers are offering ganged ones very cheaply. One can expect to pay approximately two pounds including postage.