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Yes. The concentric lines represent signal strengths.



In the image above ( freeloaded from http://www.hyperlinktech.com/item.aspx?id=297 )
the outermost blue ring is 3 db down from the highest measurement. The inner black rings are -10 db and -20 db from the peak measurement.

the intersection of the red graph and the blue rings blue rings demonstrate:

3 db down from the highest reading at about 10 degrees off axis ( outermost blue ring )

10 db down from the highest reading at about 10 degrees off axis

20 db down from the highest reading at about 40 degrees off axis