Electrically small antenna with frequency tuning circuit for wideband applications

This study proposes the concept of frequency tuning for an electrically small antenna to be used in wideband applications in low frequencies. An electrically small antenna has too narrow bandwidth to be utilized for wideband systems. However, in such mobile TV services that the band is composed of each sub channel in VHF band, the narrow bandwidth can be appropriately utilized with wideband frequency tuning. That is, each service channel is covered by the narrow resonant band of a small sized radiator and the entire service band is satisfied by moving its frequency. To prove the idea of this study, a small helical antenna with the size of less than λ/10 is chosen as a sample and the T-DMB senice, a kind of the mobile TV services allocated to 200-MHz band, is selected for testing. With the designed tuning circuit constituted of lumped elements and a varactor. a low impedance of the small radiator is matched to about 50 Ω and the wide bandwidth is satisfied by moving its narrow resonant frequency. The validity of the proposed antenna is also proved by the receiving measured result.