Antenna Theory Parabolic Reflector - Antenna Theory

What is Antenna Theory Parabolic Reflector?

Parabolic Reflectors are Microwave antennas. For superior accepting of these antennas, the idea of parabolic reflector has to be talk over.

Frequency Range

The frequency range used for the application of parabolic reflector antennas is above 1MHz. These antennas are generally used for radio and wireless applications.

Principle of Operation

The standard definition of a parabola is - Locus of a point, which changes in such a way that its distance from the fixed point (called focus) plus its distance from a straight line (called directrix) is constant.

The resulting figure displays the geometry of parabolic reflector. The point F is the focus (feed is given) and V is the vertex. The line joining F and V is the axis of symmetry. PQ is the reflected rays where L signifies the line directrix on which the replicated points lie (to say that they are being collinear). Therefore, as per the above meaning, the distance among F and L lie continuous with respect to the waves being focused.

parabolic_reflector

The reflected wave procedures a collimated wave front, out of the parabolic shape. The ratio of focal length to aperture size (ie., f/D) known as “f over D ratio”is an significant parameter of parabolic reflector. Its value alters from 0.25 to 0.50.

The law of reflection states that the angle of occurrence and the angle of reflection are equal. This law when used along with a parabola helps the beam focus. The shape of the

Parabola when used for the purpose of reflection of waves, displays some things of the parabola, which are helpful for building an antenna, using the waves reflected.

Properties of Parabola

All the waves creating from focus, replicates back to the parabolic axis. Therefore, all the waves attainment the aperture are in phase.
As the waves are in phase, the beam of radiation along the parabolic axis will be strong and concentrated.

Resulting these points, the parabolic reflectors help in producing high directivity with narrower beam width.

Construction & Working of a Parabolic Reflector

If a Parabolic Reflector antenna is used for conveying a signal, the signal from the feed, derives out of a dipole or a horn antenna, to focus the wave on to the parabola. It means that, the waves come out of the focal point and strike the Paraboloidal reflector. This wave now gets replicated as collimated wave front, as discussed earlier, to get transmitted.

The same antenna is used as a receiver. When the electromagnetic wave hits the shape of the parabola, the wave gets reflected onto the feed point. The dipole or the horn antenna, which acts as the receiver antenna at its feed, receives this signal, to convert it into electric signal and forwards it to the receiver circuitry.

The resulting image displays a Parabolic Reflector Antenna.

The gain of the paraboloid is a function of aperture ratio (D/λ). The Effective Radiated Power (ERP) of an antenna is the multiplication of the input power fed to the antenna and its power gain.

Typically a wave guide horn antenna is used as a feed radiator for the paraboloid reflector antenna. Alongside with this method, we have additional type of feed given to the paraboloid reflector antenna, called as Cassegrain feed.

parabolic_reflector_antenna (1)

Cassegrain Feed

Casse grain is added type of feed specified to the reflector antenna. In this type, the feed is located at the vertex of the paraboloid, unlike in the parabolic reflector. A convex shaped reflector, which acts as a hyperboloid is located opposite to the feed of the antenna. It is also known as secondary hyperboloid reflector or sub-reflector. It is placed such that its one of the foci coincides with the focus of the paraboloid. Thus, the wave gets reflected twice.

sub_reflector


The beyond figure spectacles the working model of cassegrain feed.

Working of a Cassegrain Antenna

When the antenna acts as a transmitting antenna, the energy from the feed emits over a horn antenna onto the hyperboloid concave reflector, which once more replicates back on to the parabolic reflector. The signal gets imitated into the space from there. Therefore, wastage of power is measured and the directivity grows better.

When the similar antenna is used for reception, the electromagnetic waves strike the reflector, gets replicated on to the concave hyperboloid and from there, it reaches to the feed. A wave guide horn antenna offers there to receive this signal and sends to the receiver circuitry for amplification.

Take a look at the resulting image. It shows a paraboloid reflector with cassegrain feed.

Advantages

The resulting are the advantages of Parabolic reflector antenna −

  • Reduction of minor lobes
  • Wastage of power is reduced
  • Equivalent focal length is achieved
  • Feed can be placed in any location, according to our convenience
  • Adjustment of beam (narrowing or widening) is done by adjusting the reflecting surfaces

Disadvantage

The resulting is the disadvantage of a Parabolic reflector antenna −

  • Some of the power that gets reflected from the parabolic reflector is obstructed. This becomes a problem with small dimension paraboloid.

Applications

The following are the applications of Parabolic reflector antenna −

  • The cassegrain feed parabolic reflector is mainly used in satellite communications.
  • Also used in wireless telecommunication systems.
  • Let us look at the other type of feed called as Gregorian feed for the parabolic reflectors.

Gregorian Feed

This is one more type of feed used. A pair of certain outlines is there, where the feed beam width is gradually improved while antenna dimensions are held fixed. Such a type of feed is known as Gregorian feed. Now, the convex shaped hyperboloid of casssegrain is substituted with a concave shaped paraboloid reflector, which is of course, smaller in size


These Gregorian feed type reflectors can be used in four ways −

  • Gregorian systems using reflector ellipsoidal sub-reflector at foci F1.
  • Gregorian systems using reflector ellipsoidal sub-reflector at foci F2.
  • Cassegrain systems using hyperboloid sub-reflector (convex).
  • Cassegrain systems using hyperboloid sub-reflector (concave but the feed being very near to it.)

These are all just to mention since they are not general and are not extensively used. They have got their boundaries.


gregorin_feed

The figure obviously shows the working pattern of all the types of reflectors. There are other types of paraboloid Reflectors such as −

  • Cut- paraboloid
  • Parabolic cylinder
  • Pill-box paraboloid

On the other hand, all of them are rarely used for the reason that of the limitations and difficulties they have in their working conditions.
Therefore, of all the kinds of reflector antennas, the simple parabolic indicators and the cassegrain feed parabolic reflectors are the most usually used ones.

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