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IARU  : International Amateur Radio Union. An international society that works for and with national organisations to promote the interests of hams.

IF : Intermediate Frequency, at which signals are amplified and processed internally within a receiver.

IFK : Incremental Frequency Keying a variant of MFSK

Image frequency : A frequency separated from that of the desired received signal by twice the receiver's intermediate frequency.

Ionosphere : a region in the atmosphere that contains ionised gasses that refract (bend) radio waves and direct them back to earth.

Inductance : A measure of how well a coil stores energy in the form of a magnetic field.

Inductor : a coil of wire. Inductors are often used with capacitors in tuned circuits such as filters and oscillators. Some inductors are wound around a core of metallic material. This affects their inductance.

Interface: usually refers to a circuit used to connect the sound card of a computer to a transceiver to allow digital modes to be used.

Interference : The interaction between a desired frequency and an undesired frequency.

Intermodulation : The undesired mixing of two or more frequencies, producing sometimes undesirable additional frequencies.

Insulator : A substance through which electricity does not pass easily - known as a dielectric in capacitors

IRLP Internet Radio Linking Project. A method of cross-world linking of repeaters so that very basic equipment can be used to communicate with hams in other countries.

Inverter : a device that converts a DC source to and AC output.

ITU : International telecommunication Union. The international organisation set up to standardise and regulate radio communications.