## sábado, 13 de febrero de 2010

### Phased array

In wave theory, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.[1] Phased array transmission was originally developed in 1905 by Nobel Laureate Karl Ferdinand Braun who demonstrated enhanced transmission of radio waves in one direction.[2] During World War II, Nobel Laureate Luis Alvarez used phased array transmission in a rapidly-steerable radar system for "ground-controlled approach", a system to aid in the landing of airplanes in England. At the same time GEMA in Germany built the PESA Mammut 1.[3] It was later adapted for radio astronomy leading to Nobel Prizes for Physics for Antony Hewish and Martin Ryle after several large phased arrays were developed at the University of Cambridge. The design is also used in radar, and is generalized in interferometric radio antennas. DARPA researchers recently announced a 16 element phased array integrated with all necessary circuits to send at 30–50 GHz on a single silicon chip for military purposes.
An antenna array is a multiple of active antennas coupled to a common source or load to produce a directive radiation pattern. Usually the spatial relationship also contributes to the directivity of the antenna. Use of the term "active antennas" is intended to describe elements whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input. One common application of this is with a standard multiband television antenna, which has multiple elements coupled together.

RAF Fylingdales

Usage
The relative amplitudes of — and constructive and destructive interference effects among — the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft Company, Culver City, CA, in 1957 (see Joseph Spradley, "A Volumetric Electrically Scanned Two-Dimensional Microwave Antenna Array," IRE National Convention Record, Part I - Antennas and Propagation; Microwaves, New York: The Institute of Radio Engineers, 1958, 204-212). When phased arrays are used in sonar, it is called beamforming.
The phased array is used for instance in optical communication as a wavelength-selective splitter.

On VHF, phased arrays are used extensively for FM broadcasting. These greatly increase the antenna gain, magnifying the emitted RF energy toward the horizon, which in turn greatly increases a station's broadcast range. In these situations, the distance to each element from the transmitter is identical, or is one (or other integer) wavelength apart. Phasing the array such that the lower elements are slightly delayed (by making the distance to them longer) causes a downward beam tilt, which is very useful if the antenna is quite high on a radio tower.
Other phasing adjustments can increase the downward radiation in the far field without tilting the main lobe, creating null fill to compensate for extremely high mountaintop locations, or decrease it in the near field, to prevent excessive exposure to those workers or even nearby homeowners on the ground. The latter effect is also achieved by half-wave spacing – inserting additional elements halfway between existing elements with full-wave spacing. This phasing achieves roughly the same horizontal gain as the full-wave spacing; that is, a five-element full-wave-spaced array equals a nine- or ten-element half-wave-spaced array.
Naval usage

Space probe communication

The MESSENGER spacecraft is a mission to the planet Mercury (arrival 18 March 2011). This spacecraft is the first deep-space mission to use a phased-array antenna for communications. The radiating elements are linearly-polarized, slotted waveguides. The antenna, which uses the X band, uses 26 radiative elements but can gracefully downgrade.

Active Phased Array Radar mounted on top of Sachsen class frigate F220 Hamburg'sGerman Navy
superstructure of the

Now, adding a φ term to the $\begin{matrix}kd\sin\theta\,\end{matrix}$ fringe effect in the second term yields:

Taking the square of the wave function gives us the intensity of the wave.

Now space the emitters a distance $d=\begin{matrix}\frac{\lambda}{4}\end{matrix}$ apart. This distance is chosen for simplicity of calculation but can be adjusted as any scalar fraction of the wavelength.

Sin achieves its maximum at $\begin{matrix}\frac{\pi}{2}\end{matrix}$ so we set the numerator of the second term = 1.

Thus as N gets large, the term will be dominated by the $\begin{matrix}\frac{2\phi}{\pi}\end{matrix}$ term. As sin can oscillate between −1 and 1, we can see that setting $\phi=-\begin{matrix}\frac{\pi}{2}\end{matrix}$ will send the maximum energy on an angle given by

Additionally, we can see that if we wish to adjust the angle at which the maximum energy is emitted, we need only to adjust the phase shift φ between successive antennas. Indeed the phase shift corresponds to the negative angle of maximum signal.
A similar calculation will show that the denominator is minimized by the same factor.

## Different types of phased arrays

There are two main different types of phased arrays, also called beamformers. There are time domain beamformers and frequency domain beamformers.
A time domain beamformer works, as the name says, by doing time-based operations. The basic operation is called "delay and sum". It delays the incoming signal from each array element by a certain amount of time, and then adds them together. Sometimes a multiplication with a window across the array is done to increase the mainlobe/sidelobe ratio, and to insert zeroes in the characteristic.
There are two different types of frequency domain beamformers. The first type separates the different frequency components that are present in the received signal into different frequency bins (using either an FFT or a filterbank). When different delay and sum beamformers are applied to each frequency bin, it is possible to point the main lobe to different directions for different frequencies. This can be an advantage for communication links.
The other type of frequency domain beamformers makes use of so called Spatial Frequency. This means that an FFT is taken across the different array elements, not in time. The output of the N point FFT are N channels, which are evenly divided in space. This approach makes a simple implementation of several beamformers at the same time possible, but this approach is not flexible, because the different directions are fixed.

Emmanuel Rodriguez C.I. 17208374
Asignatura: CRF
Fuente: http://en.wikipedia.org/wiki/Phased_array