The
coil
, in conjunction with the capacitor, forms the tuning circuit for the radio. The coil is designated as “
L1
” in
the schematic diagram. Both the coil and capacitor, working together can separate the various stations. The
“perfect” coil that yields the best all-around performance is a “square” coil, one in which the diameter of the coil is
approximately each to the length of the windings. Good performance can be obtained with coils of many differing
dimensions. The ideal value of inductance of the coil for the AM broadcast band is 240
µ
h. Inductance values from
160
µ
h to 350
µ
h will also work but part of the tuning range of the radio may be outside the AM broadcast band.
Winding the coil –
generalized instructions for a toilet paper form and 28 to 22 gauge enamel coated wire
:
1.
Punch two holes at the end of the paper toilet
form as shown and thread 10” wire through
the holes.
2.
Wind 90 turns of wire around the form and
tap as follows: 5 turns, 10 turns, 15 turns, 30
turns, 45 turns, and 60 turns. Make the taps
by twisting ½ inch of the wire to leave a loop
at the end.
3.
Punch two holes at the other end of the paper
toilet form and thread the wire through to
anchor the turns. Leave 10” and cut the wire.
4.
Use the sandpaper or knife to remove
insulation from the wiretaps. Remove an inch
of insulation from the wire ends. The coil is
complete. The coil to the right follows the
above instructions.
5.
Adjust the steps above to suit different size
coil forms.
The coil below uses 60 turns of #18 plastic coated stranded wire wound on a mailing tube with a 4.2” outside
diameter. The taps are at turns number 5, 10, 15, 30, and 45. The plastic-coated wire is not as stiff as the enamel
wire so a wood dowel is used as an aid for tap formation. Get the formulas for calculating the inductance of the coil
from your math instructor. Use this number for inductance in addition to the capacitance to solve for the tuning
range of your radio.
