A dielectrically loaded multifilar helical antenna having an operating frequency in excess of 200 MHz has an electrically insulative core with a relative dielectric constant greater than 5 occupying the major part of the interior volume defined by a three dimensional antenna element structure having, in one embodiment, eight coextensive helical tracks and, in another embodiment, six such tracks. The antennas are backfire or endfire antennas, all helical elements being phased so as to contribute to a circular polarization resonance at the operating frequency.