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Impedance
The opposition to electric current in a circuit. Impedance includes factors other than resistance, and applies to alternating currents. Ideally, the characteristic impedance of a feed line is the same as the transmitter output impedance and the antenna input impedance.

Impedance-matching device
A device that matches one impedance level to another. For example, it may match the impedance of an antenna system to the impedance of a transmitter or receiver. Amateurs also call such devices a Transmatch, impedance-matching network or antenna tuner.

Inductance
A measure of the ability of a coil to store energy in a magnetic field.

Inductor
An electrical component usually composed of a coil of wire wound on a central core. An inductor stores energy in a magnetic field.

Input frequency
A repeater's receiving frequency. To use a repeater, transmit on the input frequency and receive on the output frequency.

Insulator
A material that maintains a tight grip on its electrons, so that an electric current cannot pass through it (within voltage limits).

Intermediate frequency (IF)
The output frequency of a mixing stage in a superheterodyne receiver. The subsequent stages in the receiver are tuned for maximum efficiency at the IF.

Ionizing radiation
Electromagnetic radiation that has sufficient energy to knock electrons free from their atoms, producing positive and negative ions. X-rays, gamma rays and ultraviolet radiation are examples of ionizing radiation.

Ionosphere
A region of electrically charged (ionized) gases high in the atmosphere. The ionosphere bends radio waves as they travel through it, returning them to Earth. Also see sky-wave propagation.

Jumper
A small piece of wire used to connect two parts of a circuit. In computers and other devices, a jumper may take the form of a smaller plastic piece with an internal conductor that fits over two circuit board posts.