Page 36 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

5. Jameco Electronics at www.jameco.com

Chapter 3

Components and Hacking/Modifying Parts for

Radio Circuits

This chapter will present some of the basic components or parts needed for
building radios. These components include variable capacitors, antenna coils, and
transformers. Other parts that will be used in the projects include transistors,
diodes, capacitors, and inductors.

Antenna Coils

Basically, the antenna coils that will be used in this book are the ferrite rod or
ferrite bar types (Figure 3-1). These types of antenna coils are used commonly in
all portable amplitude-modulated (AM) broadcast radios. They are small in size but
receive radio-frequency (RF) signals equivalently in strength to the older, large
air-core-Ioop antennas.
The antenna coil at the top of the figure is much longer than the other two, which

allows for more sensitivity. That is, given the same RF signal, the longer rod
antenna coil will yield more signal at its coil winding. This coil also has a secondary
winding, which is "stepped" down by 10- to 20-fold to load into low-impedance
transistor amplifiers. The primary winding of this antenna coil is normally connected
to a tuning capacitor (variable capacitor). The primary winding inductance was

measured at 430 IJH, which matches with a variable capaCitor of about 180 pF to
200 pF.
In the center of the figure is an antenna coil that is more miniaturized and will have
less sensitivity to the antenna coil at the top of the figure. However, its primary
winding inductance is actually higher at about 640 IJH, which matches to a (mlore
commonly available) 140-pF variable capacitor. This antenna coil also has a

secondary winding that is stepped down.
Finally, the bar antenna coil at the bottom of the figure has an inductance of about
740 pH. At 740 IJH of inductance, this is a bit higher than needed, and some
portion of the winding will have to be removed for use with standard 140-pF,
180-pF, 270-pF, or 365-pF variable capaCitors.

It should be noted that all three antenna coils in Figure 3-1 allow changing the
inductance further by sliding the coil to different locations on the ferrite rod or bar.
For example, to increase inductance, slide the coil to the middle, and to decrease
inductance, slide the coil toward either end of the rod or bar.