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Mutual Coupling

Mutual Coupling

Mutual coupling can APAS taken into account when the coupling coefficients (S-matrix) between an element and its neighbors are loaded into the program.  

For arrays with 500 elements or less APAS calculates for each element its own embedded element pattern. For larger arrays, the elements will be grouped in nine sectors or in two sectors. 

With nine sectors, eight of these sectors are positioned along the periphery of the aperture, sector nine, the largest one, is located in the center of the aperture and contains the majority of the elements. APAS computes for each of these nine sectors an embedded element pattern as affected by mutual coupling and associates this pattern to all elements belonging to the same group.

When two sectors are selected the user the elements are grouped in an circular inner sector and a circular outer sector. The user has to specify the size of the outer group by indicating the percentage of elements that must be contained by the this group. The remaining elements are allocated to the inner sector. 

The following plots shown in the Mutual Coupling Gallery illustrate the impact of mutual coupling on the far-field performance of a 2066 element array antenna having a circular shaped aperture. The array elements are of the open-ended waveguide type arranged in a triangular grid. The used S-matrix is obtained experimentally. The considered pattern is the sum pattern on receive and the used taper is a synthesized one for -44 dB sidelobes. The antenna beam is positioned at broadside and the operating frequency is 10.99 GHz.    

Mutual Coupling Gallery

One can see that mutual coupling has hardly any impact on the peak sidelobe performance even as the embedded element pattern is corrupted by a blind scan angle around θ = 55 deg.

Fig. 1 in the gallery shows the Phi = 0 deg cut of the embedded element pattern of eight  elements positioned near the periphery of the aperture, the 9th element is located in the center of the aperture. The X-Y coordinates across the aperture are indicated in the first plot.

Fig.  2 visualizes the position of the selected elements.

Fig. 3 shows the Phi =0 cut of the computed far-field in case of mutual coupling.

Fig. 4 shows the same cut but in absence of mutual coupling with the embedded element pattern equal to cosΘ. One can see that mutual coupling hardly degrades the peak side lobe level of the far-field patten. The increase of the peak sidelobe level due to mutual coupling is only 0.7 dB since the used taper is synthesized for -44 dB peak sidelobes.

Fig. 5 shows the Phi = 0 deg cut of the embedded element pattern of eight elements positioned at other places across the aperture; the 9th embedded element pattern still refers to centrally located element. The exact aperture locations of all nine elements are shown in Fig. 6. As can bee seen the aperture is divided in two ring sectors. The inner ring sector one contains 634 elements for which the same embedded element pattern is used.  The outer ring sector contains 1434 elements for which the individually embedded  element pattern is computed.

Fig. 7 shows the u-cut of the array far-field  in case of mutual coupling using for 1434 elements located in outer ring sector their individually computed embedded element pattern. One can see that in this case the peak side lob level of the whole pattern is raised to -42.3 dB an increase of only 1.7 dB. These computations/plots illustrate that mutual coupling has hardly any affect on low level peak side lobe performance of reasonable sized array antennas. 

Fig. 8 shows that for the Phi = 90 deg FF cut at 10.99 GHz the embedded element patterns of the considered array antenna hardly deviates from cosΘ.

Fig. 9 illustrates the impact of mutual coupling on the impedance mismatch losses when scanning the main beam at 10.99 GHz.  

Fig. 10 displays impact of mutual coupling on the array gain when the beam is scanned in the plane         Phi = 0 deg.