Also known as: dish antenna, parabolic reflector, satellite dish, reflector antenna
A parabolic antenna is a reflector antenna that uses a metal dish shaped as a paraboloid to collect an incoming plane wave and focus all of its energy onto a small feed placed at the reflector’s focus.1 Because a paraboloid turns a parallel beam into a single point (and vice versa on transmit), the dish behaves as a large collecting aperture, so it delivers very high gain and a narrow beam — the two defining traits that make it the workhorse of microwave links, radar, and satellite ground stations.
How it works
The reflecting surface is a section of a paraboloid of revolution. A parabola has the geometric property that every ray arriving parallel to its axis reflects to a common focal point, and all such paths have equal length. So a plane wave sweeping in from a distant transmitter arrives at the feed in phase and adds coherently, while noise and signals from other directions do not. On transmit the process reverses: the feed illuminates the dish and the reflector collimates the energy into a tight pencil beam.
The dish is an aperture antenna — its performance is set by physical size, not by resonant length. Its gain follows the aperture relation
G ≈ (4π / λ²) · A · η
where A is the physical area, λ the wavelength, and η the aperture efficiency (typically 0.5–0.7 after feed spillover, blockage, and illumination taper). Equivalently the dish presents an effective aperture of ηA to the incoming field. Two consequences follow directly: gain rises with the square of diameter (double the dish, gain up ~6 dB), and rises with frequency, which is why dishes dominate at microwave bands where a modest physical size is many wavelengths across. The half-power beamwidth shrinks roughly as θ ≈ 70·λ/D degrees, so a large high-frequency dish can have a beam under a degree wide — demanding accurate pointing.
Variants
- Prime-focus (front-fed): the feed sits at the focus on the dish axis. Simple, but the feed and its support struts block part of the aperture and add spillover noise.
- Offset-fed: the reflector is a slice of the paraboloid cut away from the axis, so the feed sits below the beam and causes no blockage. Common on home satellite-TV and VSAT dishes.
- Cassegrain / Gregorian: a small convex (Cassegrain) or concave (Gregorian) sub-reflector near the focus folds the path back to a feed at the dish vertex. This shortens feed lines and lets the feed “look” at cold sky rather than warm ground, lowering noise — standard on large radio-astronomy and deep-space antennas.
The feed itself is usually a horn antenna or a dipole with a small reflector, chosen so its pattern illuminates the dish evenly without spilling energy past the rim.
Relevance to SDR
Parabolic dishes appear wherever a link needs high gain and a narrow beam: satellite downlinks (Ku/Ka-band TV, VSAT data, Inmarsat and other L-band terminals), terrestrial microwave backhaul, weather-satellite and deep-space reception, and radar. For the SDR hobbyist, a dish (or its cheaper mesh/grid cousin) is the usual front end for receiving weak signals from geostationary and low-earth-orbit satellites, where the extra tens of dB of gain is what lifts the carrier above the noise floor. Because the dish is only a passive collector, an SDR sees exactly what lands at the feed — the receiver’s job is unchanged, but the link budget improves dramatically.
GopherTrunk is a land-mobile trunking decoder (P25, DMR, NXDN, TETRA and similar), and those systems use omnidirectional or modest gain vertical antennas, not dishes. So a parabolic antenna is not part of a typical GopherTrunk install; it is relevant here as the canonical high-gain aperture antenna and the reference point for understanding effective aperture and gain in the wider RF world.
Sources
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Parabolic antenna — Wikipedia, for reflector geometry, feed variants, and the aperture gain relation. ↩