Electronic Imaging And Antenna Arrays

Imaging systems such as the eye and electronic video cameras create images that are sampled in space. In other words, there is an array of light-sensitive elements at the imaging plane, and each element produces its own electronic signal proportional to the light intensity received at its location on the imaging surface. If the array is very finely grained, the image resolution is unaffected. If the array is not finely grained, a pixilated effect occurs.

Now, what if the "middle man" (i.e., the lens), is removed? Could electronics somehow process the radiation received by the sensor elements as a lens would, such that an image could be created? The answer is yes! Although the technology does not exist for such processing to be accomplished at visible frequencies, electronic imaging can be accomplished readily at radio and microwave frequencies. The key factor for electronic imaging is the ability to process all the information in the wave. Visible light sensing elements can only produce a signal proportional to the time-averaged amplitude (intensity) of the received light.

*The moon provides a gauge for understanding angular size. As seen from Earth, the moon subtends 1/2 degree.

Figure 13.8 A two-dimensional array of dipole antennas forms a rectangular aperture, sampled at discrete points in space.

Figure 13.8 A two-dimensional array of dipole antennas forms a rectangular aperture, sampled at discrete points in space.

All phase information is lost. In contrast, radio antennas capture the actual wave, providing instantaneous amplitude and phase of the radiation received at that location.

An antenna array is a group of antennas spaced such that they span an electrically large aperture. There are many variations on antenna array geometry. Some are very simple, composed of only two or three antennas, while others comprise hundreds of antennas. I will only discuss the basic premises of antenna arrays. Suppose you want to create an aperture for imaging at radio or microwave frequencies, but you don't want to use a dish. Instead you can create an electronic lens, by placing numerous antennas evenly across an aperture, as shown one-dimensionally in Figure 13.8.

In effect, this is a sampling of a continuous aperture at discrete locations, the spatial analogue to the time domain sampling performed in digital audio systems. Time domain sampling in audio systems puts a limit on the minimum time resolution (or equivalently, it puts a limit on the maximum frequency). The spatial case is somewhat more complex, and I will not cover the details. The processing to produce an image is quite simple—delay and sum. The electronics need to perform the delay that physically occurs in a lens or reflecting dish, such that all the rays are added together in phase. For example, to focus far-field radiation received from the direction broadside to the array, the antenna signals are just added together. To focus far-field radiation received from an angle, 0, the signals are each delayed by amount,

c to recreate the wavefront (places of equal phase) as shown in Figure 13.8. In this formula, X is the distance along the array, 0 is the angle in the direction of the source, and c is the speed of light. This technique allows for focusing at infinity. For focusing objects at a finite distance, each signal must be delayed to recreate the spherical wavefront of close objects. For example, to focus the antenna array at a finite distance, r, broadside to the array, you apply a delay,

c to each element. In this formula, X is the distance from the center of the array and r is the distance to the source from the center of the array. The delay can be implemented via LC filters or via computer programs in a microprocessor (assuming that the signals are digitized).

The application of antenna arrays is not limited to imaging. Antenna arrays can be used to focus signal reception from a specific direction, just like a reflecting dish. If the signals are narrowband, and therefore approximately single frequency, a simple phase shifter can be used instead of a delay function. Most communication signals fall in this category and are referred to as coherent sources. The process of using electronics to focus an antenna array is called beamforming. The beauty of the antenna array is that its beam direction can be changed electronically, in contrast to the reflecting dish, which must be physically redirected using motors. Another advantage of the antenna array is that it can simultaneously scan signals from different directions by operating multiple microprocessing algorithms or multiple circuits in parallel. The reflecting dish can only scan one direction at a time. The fact that antenna array signals are processed via electronics allows for adaptive antennas and so-called "smart antennas" to be designed. The last benefit of antenna arrays is that they can be used to create immense apertures, covering thousands of meters. All that is needed is a set of well-spaced antennas.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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