Parabolic Antenna
Understanding Parabolic Antennas
Parabolic reflector antennas are the workhorse of satellite communications, radio astronomy, radar, and terrestrial microwave links. Their ability to focus electromagnetic energy into extremely narrow beams provides the gain needed for long-distance communication links.
Types of Parabolic Antennas
- Prime-focus: Feed at the focal point. Simple but feed and supports block the aperture.
- Cassegrain: Subreflector at focal point redirects energy to a feed behind the main reflector. Better aperture efficiency, easier feed access.
- Offset-fed: Feed below the reflector edge. No blockage. Best aperture efficiency. Standard for satellite TV dishes.
- Gregorian: Ellipsoidal subreflector. Better cross-polarization than Cassegrain.
G = eta x (pi D / lambda)^2
G (dBi) = 10 log(eta) + 20 log(D/lambda) + 9.94
Example: 3m dish at 12 GHz (eta=0.6):
G = 0.6 x (pi x 3 / 0.025)^2 = 85,271 = 49.3 dBi
3 dB beamwidth:
theta = 70 lambda / D (degrees, approximately)
= 70 x 0.025 / 3 = 0.58 degrees
Frequently Asked Questions
What is a parabolic antenna?
A parabolic antenna uses a curved reflector to focus electromagnetic waves at a feed point, creating a narrow, high-gain beam. Gains of 30-60 dBi are typical. They are used for satellite, radar, microwave links, and radio astronomy.
How much gain does a parabolic antenna have?
Gain = eta x (pi D/lambda)^2. A 1m dish at 12 GHz achieves about 40 dBi. Doubling the diameter adds 6 dBi. Doubling the frequency also adds 6 dBi (at the same physical size). Aperture efficiency eta is typically 55-65%.
What determines the beamwidth?
Beamwidth is approximately 70 x lambda/D degrees. A 3m dish at 12 GHz has a 0.58-degree beam. Larger dish or higher frequency produces a narrower beam. Very narrow beams require precise pointing systems.