Page 8 - Complete Wireless Design

Wireless Essentials

Wireless Essentials 7

type of ferrite material. A T designation (instead of FT) would indicate a pow-
dered iron core as opposed to a ferrite core.
Toroid coil design. As mentioned above, powdered-iron toroidal inductor cores
are available up to 1 GHz. To design and wind an iron toroidal inductor or
choke the A must be found on the core’s data sheet. A symbolizes the value
L L
of the inductance in microhenrys ( H) when the core is wrapped with 100
turns of single-layer wire. All the inductor designer is required to do in order
to design a powdered-iron toroidal coil is to choose the core size that is just
large enough to hold the number of turns:

L
N 100
A
L
where N number of single-layer turns for the desired value of L
L inductance desired for the coil, H
A value, as read on the core’s data sheet, of the chosen size and
L
powdered-iron mix of the core, H per 100 turns
Alternatively, if designing a ferrite toroidal core, the designer would use the
formula

L
N 1000
A
L
where N number of single-layer turns
L inductance desired, mH
A value, as read on the core’s data sheet, of the chosen core size
L
and ferrite mix, mH per 1000 turns

Notes
A values have a tolerance of typically ±20 percent.
L
The core material must never become saturated by excess power levels,
either DC or AC.
Wind a single-layer toroid inductor or transformer with a 30-degree spacing
between ends 1 and 2, as shown in the inductor of Fig. 1.5, to minimize dis-
tributed capacitance, and thus to maximize inductor Q.
The chosen mix for the core determines the core’s maximum operating
frequency.

1.1.5 Transformers
RF transformers are typically purchased as a complete component, but can
also be constructed in toroidal form (Fig. 1.6). Toroids have replaced most air
cores as interstage transformers in low-frequency radio designs (Fig. 1.7).

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