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Array And Phased Array Antenna Basics ##HOT##


A phased array antenna is an array antenna whose single radiators can be fed with different phase shifts. As a result, the common antenna pattern can be steered electronically. The electronic steering is much more flexible and requires less maintenance than the mechanical steering of the antenna.




Array and Phased Array Antenna Basics



These phased array antennas consist of lines, which are commonly controlled by a single phase shifter. (Only one phase shifter is needed per group of radiators in this line.) A number of linear arrays arranged vertically on top of each other form a flat antenna.


These phased array antennas consist completely of single elements with a phase shifter per element. The elements are arranged like a matrix, the flat arrangement of all elements forms the entire antenna.


The frequency scanning array is a special case of the phased array antenna, in which the beam steering is controlled by the transmitter's frequency without use of any phase shifter. The beam steering is a simple function of the frequency. This type of phased array antenna was often used in older radar sets.


A vertical antenna array is fed serially by a so called snake feed. At the main frequency F1, all radiators get a part of the power of the same phase through structurally identical detours, which cause a phase shift of n 360. All radiators therefore radiate with the same phase. The resulting beam is thus perpendicular to the antenna's plane.


The book helps you to understand the fundamental antenna technology being deployed in modern systems and equips you to design problem-solving sparse array models proven by electromagnetic simulations that can reduce the cost and overall complexity of the existing systems. Numerous design studies are documented to validate the theories presented. The book takes into account the functional constraints in designing commercial and military systems while demonstrating provable techniques that are practical and achievable.


With the proliferation of digital phased arrays in commercial and aerospace and defense applications, there are many engineers working on various aspects of the design who have limited phased array antenna familiarity. Phased array antenna design is not new, as the theory has been well developed over decades; however, most of the literature is intended for antenna engineers well versed in the electromagnetic mathematics. As phased arrays begin to include more mixed-signal and digital content, there are many engineers who could benefit from a much more intuitive explanation of phased array antenna patterns. As it turns out, there are many analogies between the behavior of phased array antennas and the discrete time sampled systems that the mixed-signal and digital engineers work with every day.


These articles are not intended to create antenna design engineers, but rather to help the engineer working on a subsystem or component used in a phased array to visualize how their effort may impact a phased array antenna pattern.


Figure 2 shows this phased array arrangement using phase shifters rather than time delay. Note that we define the boresight direction (θ = 0º) as perpendicular to the face of the antenna. A positive angle θ is defined to the right of boresight, and a negative angle is defined to the left of boresight.


A linear array is a single element wide with N number of elements across. The spacing may vary, but often it is uniform. Therefore, in this paper, we will set the spacing between each element to a uniform distance, d, (Figure 5). Although simplified, this uniformly spaced linear array model provides the foundation for insight into how the antenna pattern is formed vs. a variety of conditions. We can further apply the principles of the linear array to understand two-dimensional arrays.


So how can we take the equations previously developed for an N = 2 linear array and apply them to an N = 10,000 linear array? Right now, it seems that each antenna element has a slightly different angle pointing to the spherical wavefront, shown in Figure 6.


For a small array (small D) or a low frequency (large λ), the far field distance is small. But for a large array (or high frequency), the far field distance could be many kilometers! That makes it hard to test and calibrate the array. For those conditions, a more detailed near model may be used, and then bridge this back to the far field, real-world use of the array.


The focus then is on the array factor, GA. The array factor is calculated based on array geometry (d for our uniform linear array) and beam weights (amplitude and phase). Deriving the array factor for a uniform linear array is straightforward, but the details are best covered in the references cited at the end of this article.


There are some variations in equations used across literature depending on how parameters were defined in the linear array. We use the equations from this article, which results in consistency with our definitions in Figure 2 and Figure 3. Since our primary concern is how the gain changes, it is often more instructive to plot the normalized array factor relative to unity gain. That normalized array factor can be written as Equation 11.


The previous section only considered the array factor. But to find the total antenna gain, we also require the element factor. Figure 14 illustrates an example. In this example, we use a simple cosine shape as the element factor, or normalized element gain, GE(θ). The cosine rolloff is common in phased array analysis and can be visualized if considering a flat surface. At broadside, there is a maximum area. As the angle moves away from broadside, the area visible reduces following a cosine function.


The array factor, GA(θ), was used for a 16-element linear array, with a λ/2 spacing, and a uniform radiation pattern. The total pattern is a linear multiplication of the element factor and array factor, so in a dB scale, they can be added together.


Up until this point, all the diagrams and text have described a signal that the array is receiving. But how would this change for a transmit array? Fortunately, most antenna arrays are reciprocal. Therefore all of the diagrams, equations, and terminology are the same for transmit as they are for receive. Sometimes it is easier to think of the beam as being received by the array. And sometimes, perhaps in the case of grating lobes, you may find it more intuitive to think of the array as transmitting a beam. In this article, we generally describe the array as receiving a signal. But if this is harder to visualize for you, then you can equally think of the same concepts on the transmit side.


This concludes Part 1 of the series. The concept of beam steering with a phased array was introduced. The equations to calculate phase shift across the array for beam steering were derived and shown graphically. Then array factor and element factor were defined with observations of how the number of elements, the spacing between elements, and the beam angle impacts the antenna response. Finally, a comparison of antenna patterns in cartesian vs. polar coordinates was shown.


While initially phased arrays would cost an arm, a leg, and a right kidney, now due to new advancements in the technology and higher frequencies, the phased arrays are becoming smaller and more affordable.


As you can see, in phased array antennas, we can manipulate the phase and amplitude of every single signal at each antenna. That results in the birth of a new composite beam that can vary in power level and can also be pointed in your desired direction.


By integrating a phase shift to the signals either transmitted or received by each antenna present in an array enables the creation of a new signal antenna that performs differently in its amplitudes and power.


Part one (lectures 1 to 7) covers adaptive antennas. Part two (lectures 8 to 16) covers phased arrays. Parts one and two can be studied independently (in either order). The intended audience for this course is primarily practicing engineers and students in electrical engineering. This course is presented by Dr. Alan J. Fenn, senior staff member at MIT Lincoln Laboratory.


Adaptive antennas and phased arrays, with rapidly scanned beams or multiple beams, are commonly suggested for radar and communications systems in ground-based, airborne, and spaceborne applications that must function in the presence of jamming and other sources of interference.


This lecture series begins with a discussion of the fundamentals of adaptive antennas pertaining to radar and communications systems, with an emphasis on consumption of adaptive array degrees of freedom from the jammer's viewpoint. Displaced phase center antenna array mutual coupling effects in the problem of adaptive suppression of radar clutter is discussed in Lecture 2. Next, in Lectures 3 through 5 a theoretical foundation for a focused near-field technique that can be used to quantify the far-field adaptive nulling performance of a large aperture adaptive phased array system is described. Simulations of focused near-field and focused far-field nulling performance for adaptive arrays are presented for arrays of isotropic elements in Lecture 3 and for arrays including mutual coupling effects in Lectures 4 and 5. Experimental testing of the focused near-field adaptive nulling technique for phased arrays is described in Lecture 6. An experimental high-resolution multiple-beam adaptive-nulling antenna system is described in Lecture 7.


Lectures 8 through 16 then concentrate on phased array antenna development for a variety of array elements. Lecture 8 provides an introduction to phased array antenna theory. In Lecture 9, finite and infinite array analyses and measurements for periodic phased arrays of monopole elements are presented. Lecture 10 describes the focused near-field polarization characteristics of monopole phased arrays as related to adaptive array testing in the near field. Next, in Lecture 11 a test bed phased array that implements the displaced phase center antenna technique, as related to the analysis presented in Lecture 2, is described along with the planar near field testing technique that is used to assess adaptive clutter cancellation performance. The planar near field scanning method for measuring low-sidelobe radiation patterns of phased arrays is described in Lecture 12. Experimental arrays of horizontally polarized loop-fed slotted cylinder antennas (Lecture 13), dual-polarized dipole arrays (Lecture 14), and ultrawideband dipole arrays (Lecture 15) are described. In Lecture 16, rectangular waveguide arrays are analyzed by the method of moments. 041b061a72


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