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Antennas
An antenna is a device which transmits and receives electromagnetic energy. The energy, in the form of waves with combined electric and magnetic fields is propagating in space. In order to have a successful communication, two antennas, an antenna to transmit and an antenna to receive, are needed. At the transmission side, the electric energy (current and voltage) is converted into electromagnetic energy and at the receiver side the electromagnetic energy is converted back into electric energy. In order to transmit efficiently, the physical size of the antenna must be at least one-‐tenth of the wavelength. It is the operating frequency rather than the bandwidth that determines the size even though they are linked[2].
Antennas fall into two categories based on the radiation pattern: omnidirectional and directional. In principle the omnidirectional antenna exists only in textbooks since the dipole is the simplest electromagnetic source. A directional antenna focuses energy in a certain direction. An antenna system provides the following functionality.
• Concentrates energy in the direction of the target.
• Collects the echoes reflected by the target.
• Estimates the angle of arrival (azimuth and elevation) from the received echo signal.
• Determines the range (distance) to the target.
• Acts as a spatial filter and resolves its scan area into angles. It receives signal only in the direction of the main lobe and rejects signals coming from the other directions [3].
Important antenna parameters are gain, directivity, input reflection coefficient. The directivity is an important parameter of an antenna and for radar applications the antenna needs to be very directive. The efficiency of an antenna is also a prominent figure-‐of-‐merit for the overall effectiveness of the system. Antennas have gone through great advancements due to the continuous research efforts on bandwidth, efficiency, gain and size. Many compact and light weight antennas are being developed for airborne applications.
In a phased array radar, it is not the single element’s radiation pattern that is essential but rather the radiation pattern of the whole array. The directivity of an array depends on the number of elements as well as the arrangement of the elements [4].
Antenna types
Parabolic reflectors are mostly used in mechanical steered antenna systems. The reflector dish reflects the field from a source at the focal point. A point source generates EM waves, which are then directed by the reflector. Similarly, during reception, the reflector collects and directs energy to its focal point. Parabolic reflectors can have large apertures, which allows the system to receive more energy. Similarly, it can concentrate energy in a very narrow beam during transmission and thus have large gain.
The broadside array is another type of antenna that is used in many radar systems. It has a number of radiating elements, placed a half wave length apart, with a flat reflector behind the elements. The radiation is essentially perpendicular to the array and the reflector directs energy in the desired direction.
Microstrip patch antennas are also used for radar applications. High directivities can be achieved since they are easy to fabricate. The number of patches in an antennas in an array can reach one thousand.
The microstrip patch antenna
The basic parameters of a microstrip antenna are covered in this part of the chapter. Step by step, the design process of the microstrip antenna is also explained.
In the quest for lightweight and compact antennas the microstrip antenna was a significant development. Microstrip patch antennas are, due to their size, weight, ease of manufacture and installation, very useful in medical, industrial and military applications. Unlike other types of antennas, prototypes can easily be manufactured.
Patch antennas have certain disadvantages; they have low power handling capability, a very wide beam and their bandwidth is also very narrow. In severe jamming situations, wideband antennas are very useful to reduce the jamming effect on radar and for this reason the antenna needs wideband impedance matching. Patch antennas are also developed for phased array applications. Space based radar systems have additional requirements on mechanical and thermal properties. 2D antenna arrays can be further divided into sub arrays for handling purposes. To achieve the required bandwidth, the dual stacked patch technique was used in combination with the probe feed technique. It was found that the beam steering in the azimuth plane with horizontal polarization was limited to 30 degrees with a return loss of 10 dB. When the scanning area was increased to 45 degrees the return loss dropped to 7 dB [2, 4].
Shapes of patch antennas
Some of the very common shapes of the patch antennas are shown in Figure 2.1.
(a) Square (b) Rectangular (c) Elliptical (d) Triangular
(e) Circular Ring (f) Ring Sector (g) Circle (h) Dipole
Figure 2.1: Shapes of patch antennas.
The most common shapes are circular and rectangular patches. These two shapes are favoured because of their mathematical models and radiation characteristic; hence they are easiy described and therefore used in this work. Their equivalent mathematical models can be found in various forms in the literature [2]. Along with the shape, the type of substrate is essential for the patch.
Substrate for the patch antennas
An important constituent in the design of a patch antenna is the substrate. There are two common substrate materials available in the market, FR-‐4 and Roger duroid.
The FR-‐4 is a cheap substrate material, and it is also easily available in the market. The problem with this material when compared with the Rogers is that it has a very high loss tangent. Its loss tangent also changes with the frequency; higher frequency generally leads to more losses in FR-‐4. This property of the FR-‐4 explains some of the differences between the measured and the simulated results. Therefore, FR-‐4 is not preferred when accuracy is essential. Roger duroid, on the other hand, has all the benefits which FR-‐4 lacks, but it is a very expensive material. In this thesis FR-‐4 is used due to its availability and price [5].
Antenna arrays
The antenna array is based on the principle that when antenna elements are arranged in an array, the directivity of the radiation pattern increases. Initially, antenna arrays were used to increase the directivity and gain of the resulting beam of energy, but it was later found that beam steering can also be managed by a delay at each antenna element. The number of antenna elements and their arrangement are important for the directivity and the beam steering capability of the radar. As a general rule, the directivity of the array increases with the number of elements since the total aperture gets larger.
Depending on the scan area, antennas can be arranged on 2D surfaces such as spheres and cylinders. These arrays have their own issues like impedance matching with a varying scan angle and mutual coupling between the antenna elements. Ring arrays have a less directed radiation pattern and large grating lobes [4].
The Phased array
The phased array has the capability of varying the phase or introducing time delay at each antenna element in order to steer the main lobe. Despite all the efforts and development in this area, the phased array is still a very expensive technology. Researchers are making efforts to reduce the price by using cost effective components in order to make it viable commercially. Ultrasonic phased arrays are used to detect faults in mechanical structures without deforming the material [6].
A phased array has certain limitations; one of its shortcomings is the practical scan angle. Typically, the scan angle of the phased array is between 45 and 60 degrees. There are two factors responsible for this limitation; one is the effective length of the antenna. The effective length of the array decreases with the increasing scan angle and it becomes zero at an angle of 90° according to Eq. (2.6.1).
Principle of the phased array
A phased array is composed of a number of radiating elements, each with a phase shifter. Deflection of the main beam can be achieved by introducing a phase difference between the elements. The electronic steering capability offers control of the directivity and gain. This eliminates the the need for mechanical rotation [7].
In the phased array transmitter/receiver configuration the beam is focused to the direction of interest. Possible interference from any other direction can be avoided by creating a null in that direction, thus providing a capability for avoiding interference.
Transmission and reception are linked by the reciprocity theorem. Figure 2.2 shows m transmitters having a delay element in each line. When an input signal !(!) is fed to each element, the signal is delayed by the multiple of ! and the resulting signal is given by Eq. (2.6.2).
Therefore, signals from the elements add coherently in a desired direction ! = sin!! ! »! , while in other directions they essentially cancel. d is the spacing between the elements and c is the speed of light. It is the coherent addition that increases the radiated power in the desired direction.
At reception, the signal arrives at different points in time to the antenna elements and the time difference is then used to find the direction of arrival. The direction of arrival estimation is a highly active research field both for radar and telecommunication.
The passive phased array
An array with only one transmitter and receiver is referred to as passive.The beam steering is done with phase shifters at the antenna elements as shown in Figure 2.3.
Figure 2.3: Block diagram of a passive phased array.
The passive array has a high power amplifier (PA) in the transmit path and a low noiseamplifier (LNA) in the receive path. This design is less reliable and requireshigh power handling capability in the transmit path. These shortcomings are eliminated in the active phased array [8].
The active phased array
The active array has a T/R module for each antenna element. The T/R module consists of a transmitter, receiver, phase shifter, TR switch and duplexer.
The system will not shut down, if one of the TR module stops working. Since the phase shifter precedes the transmitter the power is low. Another main advantage of the active phased array is the multiple beam handling capability. Since the elements are fed independently one may have multiple beams operating at multiple frequencies. The receiver is placed close to the antenna to reduce losses.
Array configuration
The array configuration is the geometrical arrangement of elements in an array. The simplest configuration has elements in a rectangular or quadratic planar grid. Another configuration described in the literature is the circular array. The structure looks like a big circular antenna. This type of antenna can be found inthe front of jet planes.
Array size
The array size determines the directivity, since directivity increases with the number of elements. The number of elements in an array may vary from a few to a thousand dependingon the application. In defence radars, the numbers of elements is extremely large [9].
Active Input Impedance
The active input impedance is one of the factors that limit the usefulness of the phased arrays. It comes into action when the array is scanning at large angles and it is due to induction from other radiating elements. The voltage source that feeds an element is not the only source acting on the element. The surrounding elements couple to the first element and this effect becomes severe as the angle of the beam increases. The coupling depends on the spacing between the elements [9].
Table of contents :
Chapter one: Introduction
1. Introduction
1.1. Background
1.2. Thesis motivation
1.3. Thesis objective
1.4. Thesis overview
Chapter two: Literature review
2.1. Antennas
2.2. Antenna types
2.3. The microstrip patch antenna
2.4. Shapes of patch antennas
2.4.1. Substrate for the patch antennas
2.5. Antenna arrays
2.6. The Phased array
2.6.1. Principle of the phased array
2.6.2. The passive phased array
2.6.3. The active phased array
2.6.4. Array configuration
2.6.5. Array size
2.6.6. Active input impedance
2.6.7. Spacing between adjacent elements
2.6.8. Beam steering
2.7. Radar
2.7.1. The radar equation
2.7.2. Radar cross section
2.7.3. The TR module
2.7.4. Literature about the TR module
2.8. System modeling and specification
2.9. Target modeling
Chapter three: System design and modeling
3.1. Antenna design
3.1.1. The design process
3.1.2. Design specification
3.1.3. Design parameters
3.1.4. Mathematical model of the patch antenna
3.1.5. Designing of a rectangular patch antenna
3.1.6. The Antenna array
3.2. Complete system model
3.2.1. The RF pulse transmitter
3.2.2. The RF power divider network
3.2.3. System level modeling of the TR module
3.3. 1D antenna array design
3.4. 2D antenna array design
3.5. Radar front end modeling
3.5.1. Signal analysis
Chapter four: Simulation results and analysis
4.1. The single element side feed patch antenna
4.1.1. Radiation pattern and gain
4.1.2. 3D radiation pattern
4.2. Simulation of a 1D phased array antenna
4.2.1. Radiation pattern and gain
4.2.2. 3D radiation pattern
4.3. Simulation of a 2D phased array antenna
4.3.1. Directivity and gain
4.3.2. S!! parameters
4.3.3. Efficiency and radiated power
4.3.4. 3D Radiation pattern
4.4. Simulation of beam steering of a linear array
4.5. Simulation of beam steering of a 2D array
4.6. The RF front end
Chapter five: Conclusion
5.1. Conclusion
5.2. Future work
References
