Radiation patterns of 10.6‐&mgr; infrared antennas have been obtained experimentally and interpreted by means of a simple theory. It is found that the effective antenna length is equal to the length of theetchedportionof the sharpened cat‐whisker antenna. This effective antenna length appears to be caused by decoupling of the electromagnetic field across a shape discontinuity of the antenna wire. The results suggest a simple means for defining shape and size of infrared antenna structures.