The ferrite antenna comes in a wide variety of shapes and sizes. You can get some from Antique Electronic Supply. After you get there don't select anything from the drop down menu next to "Product Line" and leave the box next to "Product #" blank. Type the word "ferrite" (without the quotes) in the "keyword" box. Buy an antenna with a 250 or 230 Microhenry coil. If your feeling more ambitious you can buy the empty rods and wind your own coils.
Some of these antennas are found in pocket radios and some in larger portable radios. I bought this assortment from an electronics surplus house years ago. We are not actually going to use it as an antenna but as a hi Q coil. (If you don't know yet what Q is A high Q means a narrow bandwidth. That's good for discriminating between stations, a high Q means a high selectivity). All of these antennas have two windings which means four wires coming out. One winding has many more turns than the other one. The one with the most turns we will call the resonating winding and the other one is the base (as in a transistor) winding. We will add a third winding we will call the antenna winding.
Look at your ferrite antenna. Some have the resonating winding covering almost the whole rod. In others the factory installed windings are mostly at one end. If the latter case is true wind your antenna winding at the end away from the other windings. Use number 22 or 24 gage insulated or enameled wire and wind about six turns around the ferrite rod. If the big winding covers almost all of the rod you will see that the base winding is at one end of the resonating winding, either wound between the turns or over the top of the other winding. Wind your antenna winding at this end of the rod not over the other end of the windings. This end of the coils will be called the ground or cold end of the resonating and base windings.
The variable capacitor from a pocket radio is small and enclosed in a plastic case (not always clear) to keep out dust. The plates of the capacitor are separated by a thin plastic film. This gives more capacitance in a smaller space. Another type of variable capacitor is the air variable. You can find a nice assortment at Antique Electronic Supply. After you get there select "Capacitors" from the drop down menu and type the word "variable" (without the quotes) in the "keyword" box. For the first few projects the single 365 pf capacitor will do nicely but eventually we will end up building a truly deluxe AM radio and you will need the three section 365 variable capacitor.
If you have one of these capacitors by all means use it. You might not have to buy a radio just to disassemble for parts. If you have a multi section capacitor use only one section. The frame of the capacitor is the common or ground connection. This connects to the rotor. the other connection is to a lug connecting to the stationary part of the capacitor called the stator. If you have one with one rotor section smaller than the other one use the larger section.
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This site begun March 14, 2001
This page last updated May 9, 2002