A tuning capacitor is a variable capacitor used in an electronic circuit of a radio. It usually connects in parallel to a loop antenna. There are many uses and applications for a variable capacitor; the most typical use is in an AM Radio Circuit. If you were building a crystal radio, then you will most definitely need this component.
A typical capacitor will look like this. It will often have six pins at the back, and sometimes an additional pin at the front-side, which connects to the shaft. It is normally used in a radio capable of receiving AM and FM bands.
They typically consist of four individually ganged variable capacitors inside. There are two gangs of equal capacitance for the AM side, and two gangs of equal capacitance for the FM side.
The first priority is to figure which is the top side and the lower 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 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.
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. The centre pin is common to both variable capacitors.
Typically, 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.
The shaft rotates four individually separate variable capacitors. The reason for having four is because modern radios often have an FM band and an AM band, and each band requires two dedicated capacitors.
In addition to the loop antenna capacitor, modern circuits also require a variable capacitor for the local oscillator; therefore, the manufacturers have cleverly incorporated both of these for each band. This means that the FM band has an antenna variable capacitor, and an oscillator variable capacitor, whilst the AM band has its antenna variable capacitor, and an 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 the same 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, you will be wondering which pins to connect to the loop antenna.
Wiring this component is very easy, and you should not use the lower three pins, because they are for the FM radio circuit, which leaves three pins at the top. The two pins marked with C3 between them are for the AM oscillator circuit, and since your crystal radio does not have an AM oscillator, you should ignore that as well.
The two pins marked between C4 are the only ones you should use for your Crystal Radio.
Typical Capacitance Range
For the MW band, the frequency range is typically 520 kHz to 1650 kHz. Therefore, the typical capacitance range tends to be from 0 pF to 160 pF. For the FM band, the typical capacitance range is from 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.
For the 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.
Although they work with a crystal radio, these tuning capacitors are for heterodyne receivers; hence, the additional ganged plates for the local oscillator. These are for a signal coming in on a vector, but with a crystal radio, you will not have a local oscillator, and the signal will be all over the place.
Although these capacitors work with a crystal radio with around 55 turns of the coil, ideally you want one with a greater range. Those old-fashioned ones with open blades, with a value close to 300 pF is probably the best, but expensive. You could even get ones with a value of up to 500 pF, but they will be expensive...
If you find that your crystal radio normally 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 range. Increasing the value normally helps. The other alternative is to have taps in the ferrite coil such as in my Project Ultra, The Little Whippersnappers 8 Band Crystal Radio.
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.
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 can look for the best deal.
The best deal I found was on eBay. There are many sellers selling ganged ones very cheaply. The cheapest ones I saw were around two pounds including postage.
Radio Shack has these as well. I did not find any at Maplin though. For some reason they do not do these yet. I would make a great director to lead them... I have some great ideas.