For a given propagation mode of an electromagnetic wave, the ratio of the field strength at a specified height to the field strength at the surface of the Earth.

An antenna that has the form of a helix.
Note: When the helix circumference is much smaller than one wavelength, the antenna radiates at right angles to the axis of the helix. When the helix circumference is one wavelength, maximum radiation is along the helix axis.

A method of recording video information diagonally on a tape, used in home and professional video cassette recorders (VCRs).
Note: High-speed rotating video heads scan these diagonal video tracks, giving an effective tape speed much higher than the actual tape speed, allowing more information to be recorded on a given length of magnetic tape.

The organization that administers the 800 Service Management System (SMS/800) for the centralized management of Toll-Free Numbers. The Help Desk is the organization within the TFNA that handles customer-facing transactions and other operations of the SMS/800 TFN Registry.

Abbreviation for high-altitude electromagnetic pulse.

Abbreviation for hazards of electromagnetic radiation to fuel.

Abbreviation for hazards of electromagnetic radiation to ordnance.

Abbreviation for hazards of electromagnetic radiation to personnel.

1. The SI unit of frequency, equal to one cycle per second.
Note: A periodic phenomenon that has a period of one second has a frequency of one hertz.
2. A unit of frequency which is equivalent to one cycle per second. [NTIA]

Synonym radio wave.