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Related to radio horizon: optical horizon
radio horizon[′rād·ē·ō hə′rīz·ən]
The locus of points at which direct rays from a transmitter become tangential to the surface of the earth; the distance to the radio horizon is affected by atmospheric refraction.
The locus of points at which rays from a radio transmitter become tangential to the earth's surface; it depends on the refractive index of the atmosphere. See radar horizon.