Permeability tuning
Solenoidal, or cylindrical, coils can be made to have variable inductance by sliding ferromagnetic
cores in and out of them. This is a common practice in radio communications.
The frequency of a radio circuit can be adjusted in this way, as you’ll learn later in
this book.
Because moving the core in and out changes the effective permeability within a coil
of wire, this method of tuning is called permeability tuning. The in/out motion can be
precisely controlled by attaching the core to a screw shaft, and anchoring a nut at one
end of the coil (Fig. 10-7). As the screw shaft is rotated clockwise, the core enters the
coil, so that the inductance increases. As the screw shaft is rotated counterclockwise,
the core moves out of the coil and the inductance decreases.
Toroids
Inductor coils do not have to be wound on cylindrical forms, or on cylindrical ferromagnetic
cores. In recent years, a new form of coil has become increasingly common. This
is the toroid. It gets its name from the donut shape of the ferromagnetic core. The coil
is wound over a core having this shape (Fig. 10-8).
There are several advantages to toroidal coils over solenoidal, or cylindrical, ones.
First, fewer turns of wire are needed to get a certain inductance with a toroid, as compared
with a solenoid. Second, a toroid can be physically smaller for a given inductance
and current-carrying capacity. Third, and perhaps most important, essentially all of the
flux in a toroidal inductor is contained within the core material. This reduces unwanted
mutual inductances with components near the toroid.
There are some disadvantages, or limitations in the flexibifity, of toroidal coils. It is
more difficult to permeability-tune a toroidal coil than it is to tune a solenoidal one. It’s
been done, but the hardware is cumbersome. Toroidal coils are harder to wind than solenoidal
ones.
Sometimes, mutual inductance between or among physically separate coils is
wanted; with a toroid, the coils have to be wound on the same form for this to be possible.
