Solenoidal and toroidal inductors are both used in amateur radio equipment. A solenoidal inductor is a coil of wire wound around a cylindrical core, while a toroidal inductor is a coil of wire wound around a circular or toroidal core. Solenoidal inductors often have just an air core, while toroidal inductors are wound around a ferrite or powdered-iron core.
The primary advantage of using a toroidal core inductor is that the core confines the magnetic file within the core material, meaning that there’s less chance of unwanted mutual induction. Another advantage is that they are more compact than solenoidal inductors with the same inductance. The characteristic of a toroid core that determines the inductance of a toroidal inductor is its permeability of the core.
QUESTION: What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor? (E6D10)
ANSWER: Toroidal cores confine most of the magnetic field within the core material
QUESTION: What core material property determines the inductance of an inductor? (E6D06)
ANSWER: Permeability
There are two types of toroidal cores commonly used in amateur radio equipment: powdered-iron cores and ferrite cores. One reason for using powdered-iron toroids rather than ferrite toroids in an inductor is that powdered-iron toroids generally maintain their characteristics at higher currents. One reason for using ferrite toroids rather than powdered-iron toroids in an inductor is that ferrite toroids generally require fewer turns to produce a given inductance value. This characteristic makes ferrite beads a good choice for use as VHF and UHF parasitic suppressors at the input and output terminals of transistorized HF amplifiers.
QUESTION: What is one reason for using powdered-iron cores rather than ferrite cores in an inductor? (E6D08)
ANSWER: Powdered-iron cores generally maintain their characteristics at higher currents
QUESTION: What is one reason for using ferrite cores rather than powdered iron in an inductor? (E6D05)
ANSWER: Ferrite toroids generally require fewer turns to produce a given inductance value
QUESTION: What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier? (E6D09)
ANSWERS: Ferrite beads
When designing circuits with ferrite-core inductors, you have to be careful not to saturate the core. Saturating the core means that you have exceeded the ability of the core to store magnetic energy. Saturating the core of a toroidal inductor can cause it to change inductance and increase losses.
QUESTION: What is inductor saturation? (E6D12)
ANSWER: The ability of the inductor’s core to store magnetic energy has been exceeded
One problem that may occur in a circuit with inductors is self-resonance. At some frequency, also called the “self-resonant frequency,” inter-turn capacitance forms a parallel resonant circuit with the inductor.
QUESTION: What is the primary cause of inductor self-resonance? (E6D13)
ANSWER: Inter-turn capacitance
Variable inductors are made by inserting a slug, generally either ferrite or brass, into an air-core inductor. By varying the position of the slug, you vary the inductance. A brass slug decreases inductance when inserted into a coil.
QUESTION: Which materials are commonly used as a core in an inductor? (E6D04)
ANSWER: Ferrite and brass
QUESTION: Which type of core material decreases inductance when inserted into a coil? (E6D11)
ANSWER: Brass
Transformers
A transformer consists of two inductors that are closely coupled. Connecting an AC voltage across one of the inductors, called the primary winding, causes a current to flow in the primary, which then generates a magnetic field. As the lines of this field cross the turns of the secondary winding, it induces a current to flow in the secondary, and the voltage across the secondary is equal to the voltage across the primary winding times the number of turns in the secondary winding divided by the number of turns in the primary winding. The current in the primary winding of a transformer is called the magnetizing current if no load is attached to the secondary.
Transformers are often used to match the output impedance of one circuit to the input impedance of another. In this application, it’s important not to saturate the core of the transformer. Saturating the core of a conventional impedance matching transformer should be avoided because doing so could create harmonics and distort the signal..
QUESTION: What is current in the primary winding of a transformer called if no load is attached to the secondary? (E6D07)
ANSWER: Magnetizing current
QUESTION: Why should core saturation of an impedance matching transformer be avoided? (E6D01)
ANSWER: Harmonics and distortion could result
Piezoelectric devices
Piezoelectric crystals are used in many amateur radio applications, including oscillators and filters. They are called piezoelectric crystals because they rely on the piezoelectric effect, which is the physical deformation of a crystal by the application of a voltage. The equivalent circuit of a quartz crystal is a capacitance, inductance, and resistance in series, all in parallel with a shunt capacitor representing electrode and stray capacitance.
QUESTION: Which of the following is an aspect of the piezoelectric effect? (E6D03)
ANSWER: Mechanical deformation of material by the application of a voltage
QUESTION: What is the equivalent circuit of a quartz crystal? (E6D02)
ANSWER: Motional capacitance, motional inductance, and loss resistance in series, all in parallel with a shunt capacitor representing electrode and stray capacitance
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