The dielectric-loaded, multi-filar helix antenna employs a unique construction process and is built around a dielectric core in such a way that the electromagnetic fields are intensified within the dielectric core and very much reduced in the surrounding space. This mechanism reduces the density of electromagnetic fields near the antenna (the so-called near-fields) whilst maintaining the ability of the antenna to radiate to the far-field.
The dimensions of the ceramic cores are determined by the frequency of operation. Typical dimensions are of the order of:-
1.6 GHz Ø = 13 mm h = 35 mm 5.8 GHz Ø = 6 mm h = 9 mm 7.7 GHz Ø = 3.5 mm h = 6.5 mm
The antenna is physically a metallisation pattern about a cylindrical dielectric ceramic core of relative dielectric constants in the range of 20<er<100. The design of the antenna is such that the electric-field dipole is always loaded by the dielectric core as it is caused to spin about the cylindrical axis such that at any instant it straddles the instantaneous positions of opposing voltage maxima across the centre of the radiating section.
The basic operation of the dielectric-loaded multi-filar antenna, shown in Figure 1, is a synthesised combination of two resonances – ① is a full guide-wavelength ring-resonance of the sleeve balun rim organised so that equal-amplitude phased currents are distributed to individual helices according to the mechanical angle of connection to the balun-rim and ② each individual helix resonates with a guide half-wavelength so that the ensemble of helices resonate in a phased sequence that commutate in a manner that synthesises the spinning dipole that is required for circular polarisation.
The feed structure of the generic dielectric-loaded multi-filar antenna topologies is shown in Figure 2. ① Dielectric core (shown halved to reveal the co-axial feed). ② Radiating section comprising of a multiplicity of phased resonant helical tracks. ③ Sleeve balun section which projects balanced current feed at the top plane of the antenna and also isolates the radiating section from the device housing. ④ 50 Ω co-axial feed cable. ⑤ Reactive match tile to match antenna to 50 Ω characteristic impedance.