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K7JEB wrote in message . ..
On 12 Jun 2004 16:43:35 -0700, (David Harper) wrote: I had a simple question in regards to phased array antenna patterns. If a phased array is trying to send a narrow beam in a specific direction, how do the other side lobes get reduced and/or eliminated? Are the individual antenna transmitters/elements not omnidirectional themselves? If not, what are the characteristics of their patterns? There's really nothing special about a phased-array antenna. It is a radiating aperture, just like the mouth of a horn or the front of a parabolic "dish". The only difference is that the phase and amplitude of the excitation across the aperture can be much more finely adjusted to "squint" the beam. But the essential tradeoff remains: the larger the aperture, the narrower the beam. And, like horns and reflectors, one can reduce the sidelobes by not exciting the outside elements as strongly, but with a corresponding reduction in gain from the maximum obtainable for a given aperture size. This is called illumination tapering and is done on all antennas in one form or another to work the gain/sidelobe tradeoff. I ask this because I'm trying to understand how tracking radars can send narrow beams in the desired direction without significant secondary lobes to interfere with returns from the desired lobe. Most radars go for wider beams with lower sidelobes because the overall system performance is better. But the sidelobes are only reduced, not eliminated entirely. Oh, yeah, and about the patterns of the individual elements, they have to be as non-directional as possible. The deal is that the overall antenna pattern is the result of the PRODUCT of the array factor with the pattern of the individual elements. If the individual elements have reduced gain off boresight, the entire antenna gain is reduced off-boresight as well, limiting the angles at which the beam may be "squinted". Jim, K7JEB Thanks! You know any simple phased array configurations that are easy to mathmatically model? I've played with a 4-element array all located on the same axis just for fun to see what resulted (and to make sure my equations were right), but I'd like to find some real-world systems (hopefully without too many elements) to play with. Any digrams exist on the net? (dimensional relationship between elements, etc). Thanks again! Dave |
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Something to keep in mind is that phased arrays with a large number of
elements are simpler in one respect than ones with just a few elements. In the first case, you can make the assumption with reasonable accuracy that the feedpoint impedances of all elements are the same, since all are in essentially the same environment with respect to the other elements. For most simple phased arrays, you can't make this assumption. So while pattern analysis of simple phased arrays is easy once you assume equal element currents, actually getting those currents can be more difficult than you might assume. If you're interested only in the patterns and not how you'd actually get the currents you need, simple trig is enough. But that's not enough to enable you to actually design and build one that'll work as planned. Chapter 8 of the _ARRL Antenna Book_ describes what has to be done to make simple arrays work properly, as well as describing a few common simple arrays. There are also a few examples of phased arrays with the free EZNEC demo (http://eznec.com), where you can save yourself the math and immediately see the effect of changing element currents. Roy Lewallen, W7EL David Harper wrote: Thanks! You know any simple phased array configurations that are easy to mathmatically model? I've played with a 4-element array all located on the same axis just for fun to see what resulted (and to make sure my equations were right), but I'd like to find some real-world systems (hopefully without too many elements) to play with. Any digrams exist on the net? (dimensional relationship between elements, etc). Thanks again! Dave |
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On 14 Jun 2004 11:53:37 -0700, (David Harper) wrote:
Thanks! You know any simple phased array configurations that are easy to mathmatically model?... In a follow-up to your post Roy, W7EL, suggested modeling an array with his **FREE** EZNEC demo. I'll second that motion. EZNEC is well worth learning to use if one is doing any kind of antenna analysis, even back-of-the-envelope guesstimates. Here's what I would do to get a high-gain array with EZNEC: Place a large number of 1/10-wavelength wires in a row, end to end, and separated from each other by 1/2 wavelength. Place a current source in the middle of each, up to the maximum number allowed by the EZNEC demo version. Set each current source to 1 amp at zero degrees. Force each wire to consist of only one segment - this makes the current in it uniform. Have EZNEC evaluate the pattern in the same plane as the wires. This is a one-dimensional phased-array. You can vary the amplitude and phase of the individual current sources to experiment with beam steering. And you can change the spacing for the individual elements out to 1 wavelength and beyond to see what that does. It's a lot of fun to watch what happens. This should be a relatively easy configuration to write a far-field expression for if you are so inclined. Math never was my strong suite, so I'm out of my element and can't give you much help. (Hey, we're just a bunch of ham-radio operators here). Have fun! Jim, K7JEB |
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