Helical antenna operating at different resonant frequencies

摘要

A helical antenna capable of operating at a plurality of different resonant frequencies comprises two capacitively coupled helical antenna elements 11A, 11B, the first element 11A being disposed coaxially and within the second element 11B. The first element 11A is directly electrically connected to a metal connection 13 to feed the antenna. The first element 11A is fed through the connection to function as a quarter-wave antenna. The second element 11B is unconnected and is fed from the connector via capacitive coupling with the first element to function as a half-wave antenna. The helical antenna may be disposed at the end of a rod conductor 15 in an extendable mobile phone aerial. When the retractable aerial is extended the rod antenna is operational and when retracted the helical antenna is operational.