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Cylindrical phased arrays

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New for August 2018: Cylindrical phased arrays (CPAs) are an interesting cousin of planar phased arrays. By "cylindrical", we are referring to an array that is a cylinder about the z-axis, a generalization/extension of a circular array. They can be used for radar or communications applications, and usually have element-to-element spacings of close to ½ wavelength like their planar cousins. 

There are several key differences between circular arrays and planar arrays.  In planar arrays, an entire face is excited/weighted such that there is a linear phase progression in both dimensions; the elements’ “combine” coherently at a single scan angle, and have a pattern characteristic that depends on how far they are scanning from the face’s broadside direction. In a CPA, an arc of the cylinder is illuminated such that the center of the arc is radiating at broadside, and other elements are typically phased and weighted so that they provide coherent summation at broadside as well – a roughly parabolic phase progression in azimuth.  A feed network inside the array delivers or receives power to all the illuminated elements.  To change direction, the feed network has to rotate so that it its center is the element that is closest to the desired scan direction.  This is called commutation, a word that was lifted from electric motors where the commutator beings current to windings on the armature.

The grand-daddy of CPAs is the "Wullenweber" antenna (a circle of dipole elements), developed in Germany during the war and used until the 1970s for radio direction finding.

Wullenweber Array

Wollenweber antenna image from Wikipedia

In early efforts, a mechanically rotated feed network was used, a relibility nightmare. Check out this patent from 1984, which features a "coaxial waveguide commutator".

https://patentimages.storage.googleapis.com/97/22/3b/b86b1f8477f0a4/US4446463.pdf

Here's another patent from 1980, which has a loop coupler (no-contacting) that connects elements from the stator to the feed (rotor).

  

https://patentimages.storage.googleapis.com/75/58/ea/32ffa6f6afc69c/US4229746.pdf

An important property of the mechanical commutator was low mass, so that it can be quickly moved.

More recently, solid-state switches have been used to implement RF networks for commutation instead of mechanical devices.  However, the relative annoyance of such implementations compared to their planar cousins, along with the numerical and conceptual complexity of the electromagnetic analysis, have made large-scale cylindrical array systems rather rare.  Recent technological trends and application-specific demands are changing the tradeoff space.  Namely, digitization of transmitted and received signals at the column level significantly alleviate the annoyances of implementing a commutation network, and allows for nearly ubiquitous digital beamforming along all azimuth directions at once, if desired, given sufficient computational resources.  Soon we will add a page on direct RF-to-digital T/R modules for applications like this…

Unlike a planar phased array, a cylindrical phased array can provide nearly-identical performance in all directions at a given elevation angle.  While this is not necessarily a requirement for maintaining links, tracking, or detecting targets, it is incredibly attractive for scientific applications in remote sensing. 

For example, the University of Oklahoma recently demonstrated a Cylindrical Polarimetric Phased Array Radar (CPPAR) for weather applications, as can be seen here:

https://arrc.ou.edu/radar_cppar.html.  

Here's our Microwaves101 page on OU, and more information on the CPPAR can be found in publications like https://www.researchgate.net/publication/238017445_Polarimetric_Phased-Array_Radar_for_Weather_Measurement_A_Planar_or_Cylindrical_Configuration 

and: 

https://www.researchgate.net/publication/271227522_Optimizing_Radiation_Patterns_of_a_Cylindrical_Polarimetric_Phased-Array_Radar_for_Multimissions.

 

Author : Unknown Editor

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