Why are the Satellite TV Dishes and other Communication Dishes Parabolic?

Continuing on the shapes of things we see every day. Today, we will be stopping at the satellite tv dish. It is something we have a lot of in most homes. They are those bowl-like metal contraption that graces most outside walls of our homes

^{Wikipedia Creative Commons Attribution-ShareAlike 3.0: TV Satellite Dishes}
The satellite dish is a parabolic antenna which can transmit or receive radio waves (microwave signals) from a communication satellite. The smaller size dish supports the high frequencies, while the bigger size dishes are for low-frequency signals applications.

They are concave-shaped dishes. I know at the mention of concave it may remind you a little of optics in high school physics if you are a science student.

They are also known as the parabolic antenna since its paraboloid in nature.

As you can see from the image, the parabola has the property of reflecting all incident rays coming as a parallel beam to a particular spot known as the focus depicted by F.

Irrespective of the angle of incident, the rays all converge at a point, F. The rays can be reflected in this unusual method due to the curvature of the concave dish.

The communication satellite transmits the radio wave signals from a great distance, on getting to the dish, the concave nature of the parabolic dish focuses all of the beams at the point F, which is the position of the feedhorn, the radio receiver. The focal point is the best place for the radio receiver as it is the point where the maximum signal strength of the signal concentrates.

The focus of every parabolic antenna is not usually at the centre of the dish. In some application, like in the satellite tv antenna, the feedhorn is placed at a position that is a bit off from the centre of the dish. Hence it is named as off-axis or off-set antennas.

The reflector is said to have an asymmetric segment of a paraboloid since its focus is located to a side. The design concept is to make the antenna feedhorn not obstruct the path of the incoming radio waves.

The centre placed antennas (axial or front feed) sometimes produce what is known as "shadow" which attenuates (reduces) incoming radio waves.

It is important to note that most household tv satellite dishes are receivers, they only receive signals sent from the transmitting satellite which is in its orbit in space.

The concave or parabolic antenna is the best concept for use in satellite communication.

Satelite tv plus other communication satellites and weather satellites are known as geostationary Earth orbit. They follow the earth rotation and thus appear fixed to an observer or satellite on earth, hence no need for the satellite to move to track them.

They usually orbit at an altitude of 35,786 kilometres or 22,300 miles above.

This orbit is a lot of distance to earth, but with a concave antenna dish, the incoming attenuated signals are consolidated, and the combined energy of the individual signals build up the signals on reception. The radio waves can now be better focused on the receiver.

But that is not all; the parabolic nature ensures that the individual signals do not interfere with each other, which will lead to loss and bad reception, but are maintained "in phase".

In conclusion, the transmitting satellites have a large footprint, a large area of coverage. For instance, the Astra 2F communication satellites from the global satellite operator in the eastern part of the country of Luxembourg, has a footprint that covers Europe and Africa with dish sizes ranging from 40cm to 1m featuring more than 100 tv channels.

The parabola is the best shape to capture a signal that is sent from thousands of kilometres away.

The plain surfaces may be ideal for scattering and reflection of incoming waves, but for a convergence of the received waves, nothing beats a concave surface for the job.

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^REFERENCES

• ^{Geostationary Orbit}
• ^{Offset Antenna}
• ^{Why are satellite dishes parabolic?}
• ^{Parabolic Reflector}

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