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Crystal radio

Computing coil parameters:

If you haven't installed the interpreter software yet, go back to the main page and do so. Unzip all the files to a directory of your choice on your hard drive.

Run the FLIP.EXE program (double click on its icon). When the program comes up and gives you the little triangle prompt, enter LOAD COILCALC and press enter. Follow the instructions on the screen.

Using the program, verify the following calculations:

The lowest frequency in the AM broadcast band is 530 KHz. If you have a dual section variable capacitor of 270 pF and 180 pF (pF = picoFarad, which is the same as a micromicroFarad or uuF) for a total of 450 pF, then what inductance in microHenries do you need your antenna coil to be? Answer: 200 uH.
If your coil form is 1.25 inches in diameter and the winding length is to be 2.5 inches, how many turns are needed? Answer: 125.

Since you will have 125 turns of closely spaced wire in 2.5 inches length, the wire diameter must be 0.020 inches. Looking up in a table of wire sizes, #24 is the correct solid magnet wire size to use. 40/44 Litz wire is also close to this diameter, which is what I used.

Here is a table of wire diameters:

            Magnet                            
 Litz     Wire Size  O.D.Inches  Turns/Inch   Enameled solid
--------  ---------  ----------  ----------   --------------
  15/44       29       .0132          76           28
  30/44       25       .0182          55           25
  40/44       24       .0213          47           24
 165/46       22       .0286          35           21
 330/46       18       .0374          26.5         19
 660/46       16       .0555          18           18

The equations used in the program are as follows:

Resonant frequency of a coil and capacitor:
f = 1 / ( SQRT(2*pi)*L*C )
Inductance required for a given frequency and capacitance:
L = 1 / ( (2*pi*f)^2*C )
Inductance of a coil given diameter, length, and number of turns:
L ~ r^2 * N^2 / ( 9*r + 10*l ) where r = d/2
Number of turns required for an inductance L given coil dimensions of diameter and length:
n ~ SQRT( L * (9*r + 10*l) ) / r where r = d/2

where F is frequency in Hertz, L is inductance in Henrys, C is capacitance in Farads, n is the number of wire turns on the coil, d is the diameter of the coil in inches, and l is the length of the winding in inches.

Click here for equations in math format.

The program actually accepts frequency in kiloHertz, inductance in microHenrys, and capacitance in picoFarads, so constants are placed in the formulas to account for the scaling. If you list the program you can see the formulas I used. (Hit the ESC key and then type LIST. Each time you press the space bar, the listing will scroll one page.)

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Crystal radio