2-10 (A) Solenoid wound inductor with transformer coupling liitk; (B) inductor used as a uink circuit in radio transmitter; (C) tapped inductor: (D) tapped inductor on mobile antenna (E) roller inductor.
The air-core coil shown in Fig. 2-IOE is a rotary inductor of the type found in some HF transmitters, antenna-tuning units, and other applications where continuous control of frequency is required. The inductor coil is mounted on a ceramic form that can be rotated using a shaft protruding from one end. As the form rotates, a movable shorting element rides along the turns of the coil to select the required inductance. Notice in Fig. 2-IOE that the pitch (number of turns per unit length) is not constant along the length of the coil. This "pitch-winding" method is used to provide a nearly constant change of inductance for each revolution of the adjustment shaft.
There are several practical problems with the preceding standard fixed coil. For one thing, the inductance cannot easily be adjusted-—either to time the radio nv to trim the tuning circuits to account for the tolerances in the circuit.
Air-core coils are difficult to adjust. They can be lengthened or shortened, (he number of turns can be changed, or a tap or series of taps can be established on the coil in order to allow art external switch to select the number of turns that are allowed to be effective. None of these methods is terribly elegant—even though all have been used in one application or another.
The solution to the adjustable inductor problem that was developed relatively early in the history of mass-produced radios, and is still used today, is to insert a powdered iron or ferrite core (or "slug") inside the coil form (Fig. 2-11). The permeability of the core will increase or decrease The inductance according to how much of the core is inside the coil. If the core is made with either a hexagonal hole or screwdriver slot, then the inductance of the coil can be adjusted by moving the core in or out of the coil. These coils are called slug-tuned inductors.
Capacitors find capacitance 33
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