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How many turns for an inductor?

I'm building a crystal radio for my first time. Every example I've seen uses a tube of radius 2 inches around which the wire is wound. My goal is to build something which fits in an altoid container. I'm planning on using magnet wire and a section of a disposable bic pen for my inductor. I'm waiting on which gauge of wire to purchase because I assume the formula takes that into account as wellany suggestions?


Inductance is a function of cross sectional area so if you reduce the cross section you have to increase the number of turns. The approximate formula for an air inductor is L(uH) d^2*n^2/(9N+10L) where d is the diameter, n is the number of turns and L is the length of the winding. So if you decrease the the diameter you'll have to add a corresponding number of turns. A narrower coil will reduce the Q (goodness factor) of the coil as well. If you want to use an Altoid container I'd recommend you use a plastic one and wind the coil around the outside and varnish it, alternatively wind the coil around a roll of card or paper on a ferrite rod. About 88 turns on a ferrite rod (or a toilet roll inner approx 1.5 if you don't use the Altoid container) is good for the AM band, probably need an extra 10+ turns or so if you wind over a bic pen. Try winding 110 turns and tap every ten turns from the 80th turn. Use 24-28 (awg/swg) gauge wire. You can use up to 34 gauge but it's more fragile and the Q of a winding is lower. Other tips, use a decent germanium diode (OA81, IN34 etc.) for your detector and an earphone from an old telephone handset. Crystal sets are completely reliant on a good antenna and a good ground. The ground from a power outlet will work although a ground spike in the earth outside the house is better. The antenna should be the longest wire you can manage, at least 20ft, preferably double that. Before you start coil winding or construction there are a number of crystal radio (sometimes called foxhole radios) kits on Amazon and the like which are quite reasonably priced. It's not by accident they all use large inductors as they give better performance.
Jan 26, 2018

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