What are Parabolic Antennas?

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- everything RF

Jun 4, 2024

A parabolic antenna (or a dish antenna) is a type of directional, high-gain antenna characterized by a parabolic reflector. It consists of two main parts: a parabolic dish/reflector and the feed antenna located in the middle of the parabolic dish. The parabolic reflector typically has very high gain and low cross-polarization and is used to direct the radio waves in a narrow beam or receive them from a specific direction. The dish’s parabolic shape ensures that signals are focused, resulting in enhanced signal strength and range. Due to the high directivity and gain, the parabolic antenna is used with high-frequency signals, usually above 1 MHz are made of metal or other reflective materials. The most common metal is aluminium due to its lightweight and reflective properties. Other metals, such as steel or copper, may be used, but they’re more expensive and heavier in weight. The reflector is coated with a layer of paint to extend its service life.


Working Principle of a Parabolic Antenna

Parabolic antennas are based on the geometrical property of the paraboloid that parallel rays of electromagnetic waves that are incident upon the parabolic surface are reflected to converge at a single point, known as the focal point. In a transmitting antenna, radio frequency current from a transmitter is supplied through a transmission line cable to the feed antenna, which converts it into radio waves. The radio waves are emitted back towards the dish by the feed antenna and reflect off the dish into a parallel beam, whereas in a receiving antenna, the incoming radio waves bounce off the dish and are focused to a point at the feed antenna, which converts them into electric current which travels through the transmission line to the radio receiver. 


Classification of Parabolic Antennas 

Parabolic antennas are classified on the basis of several criteria, such as the position and the type of the feed system, the shape of the reflector, etc.  

Based on the shape of the parabolic antenna are classified as Paraboloidal, Cylindrical, and Shaped-beam antennas. In paraboloidal, the reflector is shaped like a paraboloid and radiates a narrow pencil-shaped beam along the axis of the dish. In cylindrical, the reflector is curved in only one direction and flat in the other. The radio waves come to a focus not at a point but along a line. The shaped-beam antennas produce a beam or beams of a particular shape, rather than just the narrow “pencil” or “fan” beams. 

Based on the type of feed, the parabolic antenna is classified as Axial, Off-axis, Cassegrain, and Gregorian. The axial is the most common type of feed, with the feed antenna located in front of the dish at the focus on the beam axis, pointed back toward the dish. In the Off-axis feed, the reflector is an asymmetrical segment of a paraboloid, so the focus and the feed antenna are located on one side of the dish. In a Cassegrain antenna, the feed is located on or behind the dish and radiates forward, illuminating a convex hyperboloidal secondary reflector at the focus of the dish. The Gregorian is similar to the Cassegrain design except that the secondary reflector is concave in shape. 


Advantages of Parabolic Antennas

Parabolic antennas offer several advantages, which make them a popular choice for a wide range of applications. They can achieve high gain due to their ability to focus electromagnetic energy into a narrow beam, which makes them ideal for long-distance communication. They have a highly directional beam, which minimizes the interference from unwanted sources. Also, it can be used both as a transmitting antenna and a receiving antenna due to the principle of reciprocity. 

Limitations of Parabolic Antennas

Parabolic antennas can be quite large and heavy, especially for higher frequencies, making them difficult to install and transport. Due to their high directivity, they become susceptible to interference from objects or other signals. Also, they require careful installation and alignment to ensure optimal performance. The weather conditions can also impact the performance of parabolic antennas, especially at high frequencies.

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