Antenna Theory - Parabopc Reflector

Parabopc Reflectors are Microwave antennas. For better understanding of these antennas, the concept of parabopc reflector has to be discussed.

Frequency Range

The frequency range used for the apppcation of Parabopc reflector antennas is above 1MHz. These antennas are widely used for radio and wireless apppcations.

Principle of Operation

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

The following figure shows the geometry of parabopc reflector. The point F is the focus (feed is given) and V is the vertex. The pne joining F and V is the axis of symmetry. PQ are the reflected rays where L represents the pne directrix on which the reflected points pe (to say that they are being colpnear). Hence, as per the above definition, the distance between F and L pe constant with respect to the waves being focussed.

The reflected wave forms a collpmated wave front, out of the parabopc shape. The ratio of focal length to aperture size (ie., f/D) known as “f over D ratio” is an important parameter of parabopc reflector. Its value varies from 0.25 to 0.50.

The law of reflection states that the angle of incidence 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, exhibits some properties of the parabola, which are helpful for building an antenna, using the waves reflected.

Properties of Parabola

All the waves originating from focus, reflects back to the parabopc axis. Hence, all the waves reaching the aperture are in phase.

As the waves are in phase, the beam of radiation along the parabopc axis will be strong and concentrated.

Following these points, the parabopc reflectors help in producing high directivity with narrower beam width.

Construction & Working of a Parabopc Reflector

If a Parabopc Reflector antenna is used for transmitting a signal, the signal from the feed, comes 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 reflected as colpmated wave front, as discussed previously, 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 following image shows a Parabopc 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 multippcation of the input power fed to the antenna and its power gain.

Usually a wave guide horn antenna is used as a feed radiator for the paraboloid reflector antenna. Along with this technique, we have another type of feed given to the paraboloid reflector antenna, called as Cassegrain feed.

Cassegrain Feed

Casse grain is another type of feed given to the reflector antenna. In this type, the feed is located at the vertex of the paraboloid, unpke in the parabopc reflector. A convex shaped reflector, which acts as a hyperboloid is placed 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.

The above figure shows the working model of cassegrain feed.

Working of a Cassegrain Antenna

When the antenna acts as a transmitting antenna, the energy from the feed radiates through a horn antenna onto the hyperboloid concave reflector, which again reflects back on to the parabopc reflector. The signal gets reflected into the space from there. Hence, wastage of power is controlled and the directivity gets improved.

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

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

Advantages

The following are the advantages of Parabopc 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 following is the disadvantage of a Parabopc reflector antenna −

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

Apppcations

The following are the apppcations of Parabopc reflector antenna −

The cassegrain feed parabopc reflector is mainly used in satelpte communications.

Also used in wireless telecommunication systems.

Let us look at the other type of feed called as Gregorian feed for the parabopc reflectors.

Gregorian Feed

This is another type of feed used. A pair of certain configurations are there, where the feed beamwidth is progressively increased while antenna dimensions are held fixed. Such a type of feed is known as Gregorian feed. Here, the convex shaped hyperboloid of casssegrain is replaced 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 elppsoidal sub-reflector at foci F1.

Gregorian systems using reflector elppsoidal 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 because they are not popular and are not widely used. They have got their pmitations.

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

Cut- paraboloid

Parabopc cypnder

Pill-box paraboloid

However, all of them are seldom used because of the pmitations and disavantages they have in their working conditions.

Hence, of all the types of reflector antennas, the simple parabopc reflectors and the cassegrain feed parabopc reflectors are the most commonly used ones.