Page 38 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

FIGURE 3-2 AM band loop antenna.

The loop antenna in this figure has insufficient inductance to work with any of the
standard variable capacitors (e.g., 140 pF to 365 pF). Therefore, this antenna is
connected to a step-up RF transformer, and the RF transformer is matched with a
standard variable capacitor. In this book, oscillator coils and/or hacked
intermediate-frequency (IF) transformers (see lower right-hand corner of Figure
3-2) will be used as the RF transformer for these types of loop antennas. It should

be noted that these types of loop antennas are commonly available at MCM
Electronics as replacement antennas for stereo receivers.
Variable Capacitors

These days, choosing variable capacitors for AM radios is limited to roughly two
types of poly-varicon variable capacitors. Poly-varicon variable capacitors use
polyester sheets between the plates as opposed to air-dielectric variable capacitors
(Figure 3-3). A multiple gang variable capacitor such as a two, three, or four gang

variable capacitor refers to the number of sections it has and all sections share a
common tuning shaft. In general, a multiple gang variable capacitor is equivalent to
a multiple section variable capacitor. However, some multiple section variable
capacitors such as dual trimmer variable capacitors have two independent
adjustments for varying the capacitance of each section. For this book, the main