Page 10 - Complete Wireless Design

Wireless Essentials

Wireless Essentials 9

of operation. As an example, if a 1:1 transformer’s primary had a 50-ohm
amplifier attached to its input, and the secondary had a 50-ohm antenna at its
output, then the primary winding’s reactance (X ) should be at least 200 ohms,
P
while the secondary winding’s reactance (X ) should also be 200 ohms at its
S
lowest frequency of operation.
To design a toroidal transformer, follow these steps:
1. Calculate the required reactances of both the primary and the secondary of
the transformer at its lowest frequency:
X 4 Z and X 4 Z
P OUT S IN
where X required primary reactance at the lowest frequency of
P
transformer operation
Z output impedance of the prior stage
OUT
X required secondary reactance at its lowest frequency
S
Z input impedance of the next stage
IN
2. Now, calculate the inductance of the primary and secondary windings:

X X
P
S
L and L
P 2 f S 2 f
LOW LOW
3. Choose a core that can operate at the desired frequency, with a high per-
meability and as small a size as practical, and then calculate the number of
primary and secondary turns required*

L
L
S
S
N 100 or N 1000
S A S A
L L

L
P
N N
P S L
S
4. Now wind the primary as a single layer around the entire toroid. Wind the
secondary over the top of the primary winding at one end (Fig. 1.8). Reverse
the windings for a step-up transformer.
1.2 Semiconductors

1.2.1 Introduction
Semiconductors, as opposed to the vacuum tubes of the past, are small, depend-
able, rugged, and need only low bias voltages. These devices are utilized not

*The formula for N will depend on how A is given in data sheet: 100 for H, 1000 for mH.
S L

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