Introduction to Radar Systems – Lecture 6 – Radar Antennas; Part 3

Summary

This lecture on phased array antennas from the Introduction to Radar Systems course covers the fundamentals of antenna design, including radiation patterns, beam formation, and mutual coupling effects. It explains the mechanics of phased arrays, including the use of phase shifters to control beam direction rapidly, without needing to physically move the antenna. The lecture also discusses the trade-offs between phased array and reflector antennas, with a focus on performance, flexibility, and costs. The importance of element separation, beam width, and scan angles are also highlighted, offering a comprehensive introduction to radar antenna design.

Takeaways

• 😀 Phased array antennas allow for rapid electronic beam steering without mechanical movement, offering flexibility and agility for radar systems.
• 😀 The gain pattern of an antenna array depends on the spacing between elements and the phase alignment of signals, with optimal performance when elements are spaced less than half a wavelength apart.
• 😀 Constructive interference occurs when signals from multiple elements combine in phase, creating a stronger signal in the desired direction, while destructive interference reduces the signal in unwanted directions.
• 😀 Phased array antennas can electronically adjust the phase of each element to steer the beam towards different angles, making them ideal for quick target tracking and surveillance.
• 😀 The beam width of a phased array broadens as the scanning angle increases, which can result in reduced angular resolution. To minimize this effect, element spacing should be kept below half the wavelength.
• 😀 Mutual coupling between antenna elements can affect the radiation pattern and impedance, requiring careful consideration in antenna design to ensure optimal performance.
• 😀 Phased arrays can scan in multiple directions simultaneously, offering significant advantages over reflector antennas, which require mechanical movement to change beam direction.
• 😀 The main advantage of phased arrays over traditional reflector antennas is their ability to perform quick, simultaneous tracking of multiple targets over a wide range of angles without moving parts.
• 😀 Reflector antennas are generally cheaper and simpler to build compared to phased arrays, but they lack the speed and flexibility of electronically-steered phased arrays.
• 😀 In phased array design, the distance between elements (typically λ/2) is crucial to avoid grating lobes, which occur when elements are spaced too far apart, causing unwanted signal emission in multiple directions.

Q & A

What is the main advantage of using phased array antennas in radar systems?

The main advantage of phased array antennas is their ability to steer the beam electronically, allowing for rapid beam direction changes without the need for mechanical movement, enabling fast tracking of targets and improved performance in dynamic environments.

How does the phase control of elements in a phased array affect the antenna's radiation pattern?

By adjusting the phase of each element in a phased array, constructive interference can be achieved at specific angles, focusing the beam in a particular direction, while destructive interference suppresses radiation at unwanted angles. This allows the antenna to steer its beam electronically.

What are 'grating lobes' and how can they be avoided in a phased array antenna?

Grating lobes are unwanted side lobes that appear when the spacing between antenna elements exceeds half the wavelength (λ/2). They can be avoided by ensuring that the element separation is less than λ/2, preventing radiation in unwanted directions.

What is the effect of increasing the number of elements in a phased array antenna?

Increasing the number of elements in a phased array increases the gain and directivity of the antenna, narrowing the beam width and improving the resolution. The side lobes also decrease, leading to more focused energy in the desired direction.

What happens to the beam width and gain when the phased array antenna is scanned off broadside?

When the phased array antenna is scanned off broadside, the beam width increases (broadens), and the gain decreases with the cosine of the scan angle. This leads to a loss in angular resolution and a reduction in directivity as the beam moves further from broadside.

What is the significance of mutual coupling between elements in a phased array antenna?

Mutual coupling refers to the interaction between elements in the array, where the radiation from one element induces currents in nearby elements. This affects the impedance and radiation patterns, and needs to be accounted for in the design to avoid performance degradation, especially when elements are closely spaced.

How can phased arrays be used for multiple functions simultaneously?

Phased arrays can perform multiple functions simultaneously by electronically steering the beam in different directions. For example, one set of beams can be used for surveillance at lower angles, while another can be used for tracking targets at higher angles, allowing for efficient coverage of a wide field of view.

What are the trade-offs involved in choosing between phased array antennas and traditional parabolic dish antennas?

Phased array antennas offer the advantage of fast, electronic beam steering and flexibility, but they are more expensive and complex to design than traditional parabolic dish antennas, which are simpler and cheaper but lack beam agility.

What is the role of phase shifters in phased array antennas, and how do they work?

Phase shifters control the phase of the signal emitted from each element in the array. By shifting the phase of each element, the radiation patterns combine constructively in the desired direction, allowing the beam to be steered electronically without moving the physical array.

What are the challenges associated with designing phased array systems, particularly in terms of cost and complexity?

Designing phased array systems is challenging due to their high cost, complexity, and the need for precise electronic components like phase shifters and transmit/receive modules. These systems require extensive modeling and testing, making them more expensive and time-consuming to develop compared to traditional radar systems.