A lossy ferrite core/dielectric shell structure effectively miniaturizes an axial-mode helical antenna (AM-HA) without sacrificing the antenna gain in the range of 2.85-3.45 GHz. Co(2)Z hexaferrite-glass composite was used as an inner core and acrylonitrile butadiene styrene as an outer shell. The volume of the AM-HA was reduced by 29% while maintaining realized gain of 9.0 dBic, compared to air-core AM-HA. The simulation also confirmed that further miniaturization of the AM-HA up to 43% is achievable with a slight decrease in realized gain from 9 to 8.7 dBic. The designed magnetic core/dielectric shell structure offers a vast selection of lossy ferrite for miniaturization of the AM-HA.