Overview

Inductors and capacitors are the principal components used in RF tuning circuits. The resonant frequency of a tank circuit is the frequency to which the inductor capacitor combination is tuned and is found from:

(4-1)

or, if either the inductance ( L) or capacitance ( C) is known or preselected then the other can be found by solving Eq. (4-1) for the unknown, or:

(4-2)

and

(4-3)

In all three equations, L is in henrys, C is in farads, and F is in hertz (don t forget to convert values to microhenrys and picofarads after calculations are made).

Capacitors are easily obtained in a wide variety of values. But tuning inductors are either unavailable or are available in other people s ideas of what you need. As a result, it is often difficult to find the kinds of parts that you need. This chapter will look at how to make your own slug-tuned adjustable inductors, RF transformers, and IF transformers (yes, you can build your own IF transformers).

Tuning inductors can be either air-, ferrite-, or powdered-iron-core coils. The air-core coils are not adjustable unless either an expensive roller-contact mechanism or clumsy taps on the winding of the coil are provided. However, the ferrite- and powdered-iron slug-tuned core coils are adjustable.

Figure 4-1 shows one form of slug-tuned adjustable coil. The form is made of plastic, phenolic, fiberglass, nylon, or ceramic materials and is internally threaded. The windings of the coil (or coils in the case...