Physics · Ch 15 — Communication Systems
Space wave
Space wave
Space Wave
A third propagation mode is the space wave, which travels in a straight line directly from the transmitting antenna to the receiving antenna. Space waves are what make line-of-sight (LOS) communication and satellite links possible. Above about 40 MHz, communication is essentially restricted to line-of-sight paths; at these higher frequencies, antennas can be made comparatively small and are typically mounted well above ground level.
Because the space wave travels in a straight line, the curvature of the earth eventually blocks it - so a receiver located beyond the horizon needs a sufficiently tall receiving antenna to intercept the line-of-sight wave (Figure 15.5). For a transmitting antenna of height , the distance to its radio horizon (the point where the line of sight just grazes the earth's surface) is:
where is the radius of the earth (about 6400 km). More generally, for a transmitting antenna of height and a receiving antenna of height , the maximum line-of-sight distance between them is:
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What this figure shows. Simple geometric diagram over a grey curved 'earth surface' mound: two lattice towers stand on the curved earth, one on the left of height h_T and one on the right of height h_R (both labelled with vertical double-headed arrows). A dashed light-blue curve arcs from the top of the left tower to the top of the right tower, representing the line-of-sight radio path following the earth's curvature. Horizontal double-headed arrows above the towers mark distances d_T (from the left tower to a point beneath the arc's midpoint) and d_M (the tot …
What this figure shows. Composite line drawing over a grey curved 'Earth' surface. At the top centre, a schematic 'Communication satellite' (small circle with hatched solar panels on both sides) sits above a horizontal elliptical band labelled 'Ionosphere'. Two ground-based lattice towers stand on the earth's curved surface, one left and one right, each drawn inside a small hatched box. A cartoon airliner is shown in the middle of the sky between the towers. Curved/straight arrows show four propagation paths: 'Ground wave' (hugging the earth's surface directly between the two towers), 'Sky wave' (from the right tower up to the ionosphere and back down, reflecting), 'Space wave' (from the satellite down to a tower), plus two paths marked 'Los' between a tower and the airplane, and between the airplane and the other tower, showing line-of-sigh …