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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.

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.

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.

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