The characteristics of an annular-ring microstrip antenna operating in TM11 and TM12 modes are determined experimentally. Our results confirm the main conclusions of a theoretical study published recently by Ali, Chew and Kong and show that: (a) the bandwidth of the TM11 mode is very small while that of the TM12 mode is relatively large, and (b) the lauching efficiency of the TM12 mode is highly sensitive to the feed position while that of the TM11 mode is virtually independent of the feed position.
