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Far field in arrays

 
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Mszazynski
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PostPosted: Tue Dec 26, 2017 3:43 pm    Post subject: Far field in arrays Reply with quote

I know the far field condition is related to antenna dimension and wavelength, but I was wondering, what if you had an enormous array operating at say 2.4 GHz, where the wavelength is ~ 12 cm. Do you take the max dimension of each antenna, or of the entire array when considering far field?
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admin
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PostPosted: Wed Dec 27, 2017 5:22 pm    Post subject: Reply with quote

The entire array. If you have a large array and aren't far away relative to the size of the array then the angle of arrival for each antenna will be different, so you can't use the traditional beamsteering approach or analysis
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Mszazynski
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PostPosted: Wed Dec 27, 2017 9:04 pm    Post subject: Reply with quote

I'm doing a simulation at 5.8 GHz with an array with dimensions 2.4 meters by 2.1 meters. Based on what you're saying, I wouldn't be able to make any accurate far field calculations unless I was outside of 2*(2.4 m)^2 / (5.17 cm), which would be about 220 meters away. Is this an accurate interpretation? Is it really impossible to usefully use such an array unless you're more than 220 meters away!?
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PostPosted: Wed Dec 27, 2017 11:46 pm    Post subject: Reply with quote

You didn't ask if it is impossible to use an array under the far field distance, you asked what conditions are required such that the far field approximations are accurate
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Mszazynski
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PostPosted: Thu Dec 28, 2017 2:00 am    Post subject: Reply with quote

Fair enough. Maybe I should just re-phrase the problem. I am trying to perform calculations for power transfer efficiency / power density with the array I described before. The peak gain is 37.65 dBi, but the gain even of the main beam is smaller at its outer edges. To get accurate power calculations, I feel that I need to take gain measurements (via simulation) at various angles cuts (both azimuthal and elevation) and average them to get an average gain for the beam. The result is, however, that I continue to get > 100% efficiency, which obviously cannot be right. Near-field vs. far-field obviously is important, but r isn't even part of this equation. Can you make any recommendations for calculation power density based on a gain that is not constant even with a single beam?
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