Radar Systems - Radar Antennas
In this chapter, let's learn about antennas, which are useful in radar communication. We can classify radar antennas into the following two types based on the physical structure.
- Parabolic reflector antennas
- Objective antennas
In our following sections we will discuss in detail the two types of antennas.
Parabolic reflector antennas
Parabolic reflector antennas are microwave antennas. Knowledge of the parabolic reflector is essential to understand how antennas work in depth.
Principle of Operation
Parabola is nothing other than the Locus of points, which move in such a way that its distance by compared to the fixed point (called focus) plus its distance from a straight line (called directrix) is constant.
The figure followednte shows the geometry of the parabolic reflector . Points F and V are respectively the focus (power is given) and the vertex. The line joining F and V is the axis of symmetry. $ P_1Q_1, P_2Q_2 $ and $ P_3Q_3 $ are the reflected rays. The line L represents the directrix on which the reflected points rest (to say that they are collinear).
As shown in the figure, the distance between F and L is constant with respect to the focused waves. The reflected wave forms a collimated wavefront, out of the parabolic shape. The ratio of focal length to aperture size (i.e. $ f / D $) is called "ratio f on D ". This is an important parameter of the parabolic reflector and 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 with a parabola, helps in focusing the faisceau. The shape of the dish when used for the purpose of reflecting waves, exhibits certain properties of the dish, which are useful for constructing an antenna, using the reflected waves.
Properties of the parabola
Here are the different properties of the parabola -
All waves coming from of the focus are reflected on the parabolic axis. Therefore, all waves reaching the aperture are in phase.
As the waves are in phase, the beam of radiation along the parabolic axis will be strong and focused.
By following these points, parabolic reflectors help produce high directivity with a narrower beamwidth.
Construction and operation of a parabolic reflector
If a parabolic reflector antenna is used to transmit a signal , the signal from the power supply goes out. '' a dipole antenna orof a horn antenna, to focus the wave on the parabola. This means that the waves come out of the focal point and hit the paraboloid reflector. This wave is now reflected as a collimated wavefront, as previously indicated, to be transmitted.
The same antenna is used as receiver . When the electromagnetic wave reaches the shape of the parabola, the wave is reflected on the feed point. The dipole antenna or horn antenna, which acts as the antenna of the receiver to its feed receives this signal, to convert it into an electrical signal and transmits it to the receiver circuit.
The gain of the paraboloid is a function of the opening ratio $ D / lambda $. The effective radiated power (ERP) of an antenna is the multiplication of the input power supplied to the antenna and its power gain.
Usually a waveguide horn antenna is used as a power radiator for the 'Paraboloid reflector antenna. Along with this technique, we have the following two types of power given to the paraboloid reflector antenna.
- Cassegrain Feed
- Gregorian Feed
In this Typically, the feed is located at the top of the para boloid, unlike the parabolic reflector. A convex shaped reflector, which acts like a hyperboloid, is placed opposite to the antenna feed. It is also called a secondary hyperboloid reflector or sub-reflector. It is placed in such a way that one of its focal points coincides with the focal point of the paraboloid. Thus, the wave is reflected twice.
The figure above shows the operating model of the cassegrain feed.
The type of feeding where a pair of certain configurations are present and the width of the feeding bundle is progressiveincreased while the dimensions of the antenna are kept fixed is known as Gregorian feed . Here, Cassegrain's convex shaped hyperboloid is replaced by a concave shaped paraboloid reflector, which is of course smaller in size.
These Gregorian type reflectors can be used in the following four ways -
Gregorian systems using ellipsoidal reflector sub-reflector at focal points F1.
Gregorian systems using ellipsoidal reflector sub-reflector at focal points F2.
Cassegrain systems using a hyperboloid (convex) sub-reflector.
Cassegrain systems using a hyperboloid reflector sub-reflector (concave but the feed being very close).
Among the different types of reflector antennas, simple parabolic reflectors and Cass feed parabolic reflectorsgrain are the most common
Lens antennas use the curved surface for transmitting and receiving signals. These antennas are made of glass, where the converging and frequency range of lens antenna usage starts at 1 GHz but its usage is greatest at 3 GHz and above .
A knowledge of Lens is necessary to understand in depth how Lens Antenna works. Remember that a no rmal glass lens works on the principle of refraction .
Construction and Function of the Lens Antenna
If a light source is assumed to be present at a focal point of a lens, which is at a focal length of the lens lens, then the rays pass through the lens as collimated or parallel rays on the plane wavefront.
There are two phenomena that occur when rays fall from different sides of a lens. They are given here -
Rays that pass through the center of the lens are less refracted than rays that pass through the edges of the lens. All rays are sent parallel to the plane wavefront. This Lens phenomenon is called Divergence .
The same procedure is reversed if a light beam is sent from the right side to the left side of the same goal. Then the beam is refracted and meets at a point called the focal point, at a focal length of the lens. This phenomenon is called Convergence.
The following diagram will help us better understand the phenomenon.
The ray diagram represents the focal point and focal length from source to lens. The parallel rays obtained are also called collima raysyour.
In the figure above, the source at the focal point, at a focal length of the lens, is collimated in the plane wave front. This phenomenon can be reversed, which means that the light, if sent from the left side, converges to the right side of the lens.
It is because of this reciprocity that the lens can be used as an antenna, since the same phenomenon helps to use the same antenna for transmission and reception.
To obtain the focusing properties at higher frequencies, the refractive index must be less than unity. Regardless of the refractive index, the goal of Lens is to straighten out the waveform. On this basis, E-plan and H-plane objectives are developed, which also delay or accelerate the wavefront.