Page 7 - Complete Wireless Design

Wireless Essentials

6 Chapter One

d diameter, in inches, of the inside of the coil (the same diameter as
the form used to wind the coil)
l length, in inches, of the coil (if this length is not met after winding
the turns, then spread the individual coils outward until this value
is reached)

But this should be kept in mind: The formula is only accurate for coils with a
length that is at least half the coil’s diameter or longer, while accuracy also
suffers as the frequency is increased into the very high frequency (VHF) region
and above. This is a result of the excessive growth of conductor thickness with
coil diameter. Only varnished (“magnet”) wire should be used in coil construc-
tion to prevent turn-to-turn shorts.

Toroids. Inductors that are constructed from doughnut-shaped powdered iron
or ferrite cores are called toroids (Fig. 1.4). Ferrite toroidal cores can function
from as low as 1 kHz all the way up to 1 GHz, but the maximum frequency
attainable with a particular toroid will depend on the kind of ferrite material
employed in its construction. Toroids are mainly found in low- to medium-pow-
er, lower-frequency designs.
Toroidal inductors are valuable components because they will exhibit only
small amounts of flux leakage and are thus far less sensitive to coupling
effects between other coils and the toroid inductor itself. This circular con-
struction keeps the toroid from radiating RF into the surrounding circuits,
unlike air-core inductors (and transformers), which may require some type of
shielding and/or an alteration in their physical positioning on the printed cir-
cuit board (PCB). And since almost every magnetic field line that is created by
the primary makes it to the secondary, toroids are also very efficient. Air-core
transformers do not share these abilities.
At low frequencies, toroids are also used to prevent hum from reaching the
receiver from the mains and any transmitter-generated interference from
entering the power lines. This is accomplished by placing toroid inductors in
series with the supply power, choking out most of the undesired “hash.”
Toroids are identified by their outer diameter and their core material. For
instance, an FT-23-61 core designation would indicate that the core is a ferrite
toroid (FT) with an outer diameter of 0.23 inches and composed of a 61-mix

Figure 1.4 A toroid core
inductor.

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