Practical_Antenna_Handbook_0071639586

C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 543
A
B
Figure 24.9 Winding a toroidal transformer. (A)
Parallel wound. (B) Twist wound.
being wound on the core. With one end of the three
wires secured in the drill chuck, anchor their far
ends in something that will hold them taut. Some
people use a bench vise for this purpose. Run the
drill at slow speed and allow the wires to twist together
until the desired pitch is achieved. Remember
to wear eye protection in case a wire breaks or
gets loose from its mooring at the far end.
To actually wind the bundle of wires on the
toroid, pass the bundle through the doughnut
hole until the toroid is about in the middle of the length of wire. Loop the wire over and
around the outside surface of the toroid and pass it through the hole again. Repeat this
process until the correct number of turns is wound onto the core. Be sure to press the
wire against the toroid form, and keep it taut as you wind the coils.
To minimize the chance of a chipped core breaking through the wire insulation,
wrap the core with a slightly overlapped layer of fiberglass packing tape before starting
to wind. To prevent windings (especially those made with very fine wire) from unwinding,
secure the ends of the wires with a tiny dab of rubber cement or RTV silicone sealer.
Mounting Toroid Cores
To mount a properly wound toroidal inductor or transformer, consider these options:
• If the wire is heavy enough, just use the wire connections to the circuit board or
terminal strip to support the component—especially if the connections are
directly above or below the toroid.
• Smaller, lighter toroids can be laid flat on the circuit board and cemented in
place with silicone seal or rubber cement.
• Drill a hole in the wiring board and use a screw and nut to secure the toroid. Do
not use metallic hardware for mounting the toroid! Metallic fasteners will alter
the inductance of the component and possibly render it unuseable. Use nylon
hardware for mounting the toroidal component.
How Many Turns?
Three factors must be taken into consideration when making toroid transformers or inductors:
toroid size, core material, and number of turns of wire. Toroid size is a function of
power-Âhandling capability and handling/installation convenience. Core material depends
on the frequency range of the circuit and the application (balun, common mode
choke, etc.).
Equation (24.23) provides a rough rule of thumb for the turns count. Based on the
availability of a parameter called the A L factor, the number of turns needed to obtain a
specified inductance is given by
N = 100
L
A
L
(24.23)
where N = number of turns on winding
L = inductance, in microhenrys
A L = core factor, in microhenrys per 100 turns