VHF/UHF Miniaturized Ferrite-EBG Composited Choke Structures

Parasitic currents on the sheaths of the coaxial cables will deteriorate the performance of the antenna, so the choke structures are needed. While a-quarter-wavelength choke sheaths at VHF and UHF are very long, miniaturized ferrite-EBG composited choke structures are proposed. The ferrite refers to ferrite magnetic beads. Electromagnetic bandgap (EBG) element is consisted of multi-section metal tubes with length relatively shorter than a-quarter-wavelength. The integrated structures are formed by arranging EBG elements and beads alternately along the coaxial line. The proposed choke structures combine the wideband choke performance of the ferrite beads and the high efficiency of the EBG structures. Furthermore, due to the difficulty of the quantitative measurement of the choke structures in an open space, a measuring setup designed in a transmission line model is devised. The choking effects of the proposed structures can be characterized by the $\vert \text{S}_{21}\vert $ parameter of the measuring device, as the direction of the currents flowing on the sheaths of feeding cables are the same as that of the currents on the inner conductor of the measuring device. Meanwhile, choking bandwidth, efficiency and size of the ferrite-EBG composited choke structures are compared with that of EBG structures and beads. The influence of different numbers of the proposed structures is studied. A prototype is fabricated and measured. The overall size of the ferrite-EBG composited choke structures is $0.25\,\,\lambda _{\mathrm {L}} \times 0.024\,\,\lambda _{\mathrm {L}} \times 0.024\,\,\lambda _{\mathrm {L}}$ (415 mm $\times40$ mm $\times40$ mm) ( $\lambda _{\mathrm {L}}$ is the wavelength of the lower end of the operating frequency band). The measured choking band ( $\vert \text{S}_{21}\vert < -10$ dB) is in the region of 181–343 MHz.