Posted on October 2, 2013r=1 circle (picture from [1], but the color added by the author.) There are many circles which give constant resistance, but the most important is the “r=1 circle” (green). [1] http://www.arrl.org/files/file/Antenna%20Book%20Supplemental%20Files/22nd%20Edition/Smith%20Chart%20Supplement%20-%20Corrected%20Jan%202012.pdf)