Hexagonal ferrites provide magnetic properties at microwave and millimeter wave frequencies without need for an externally applied magnetic biasing field. Sample 1 of this present paper was a hexagonal ferrite similar to that previously described but was designed for screening applications. The material composition was Ba CoδFe12-2δO19, where δ is the degree of substitution controlling the ferrimagnetic resonant frequency and was set in the range 0.79 to 0.98. Adjustments to the material composition can optimize the electrical parameters for antenna applications and position the material resonance to correspond to the operating bandwidth. The simulations in this present paper assume that the material parameters are not frequency dependent.
It has been demonstrated that periodically arranged composite dielectric blocks having different permittivities can create an averaging bulk effect. This was seen as a possible way of synthesizing a desired bulk ferrite material effect using slices of material that didn’t have the desired electrical parameters but was available. A 30 X 30 X 30 mm rectangular composite material coated antenna with central dipole excitation was comprised of lossless ferrite material (with a relative permittivity of 1.2) in one third of the central volume while the remaining two thirds were filled with lossless dielectric material. Simulations of the configuration were optimized for matched operation at 1800 MHz but the radiation efficiencies and bandwidths were significantly out-performed by the usual homogeneous arrangement as shown in Fig. 8.
Fig. 8. Simulated efficiency μ’ and bandwidth BW of composite 30 X 30 X 15 mm rectangular material coated antenna with central dipole excitation compared to homogeneous material antenna, showing variation with relative permittivity.
Other simulations involving multiple thinner slices were also investigated but consistently gave a lower antenna performance compared to the homogeneous case. The mismatched wave interface between the ferrite and dielectric materials is thought to create additional reflections within the materials accompanied by increased dissipation. It is concluded that a homogeneous material is needed in this present antenna application.
A tri-band antenna design based on a material with μ’ properties approximating those of Sample 2 (ε’ = 24.1, μ’ = 2, tanδε = 0.0001, tanδμ = 0.06, monopole length = 8mm) was simulated and is shown in Fig. 9.
Fig. 9. Layout of simulated handset showing tri-band, reduced μr , and rectangular material coated antenna with central dipole excitation mounted on 100 X 40 mm ground plane. (W1 = 30 mm, W2 = 20 mm, H = 8mm; monopole radius = 0.45 mm and monopole height = 8 mm; ring widths = 0.5 mm, the lower and the upper being 5 and 7 mm, respectively, above the ground, both of them extending 6 mm along the side walls of the block).
The dimensions of the block have been altered, including a useful reduction in block height H, and the antenna has been redesigned to account for the reduced μ’. The bands covered successfully at 6 dB are the GSM1800, 1900 and UMTS, as shown in Fig. 10.
Fig. 10. Simulated S11 characteristic of handset antenna of Fig. 9.
The corresponding efficiencies are 21%, 16%, and 12.5%. Even though they are decreased compared to their quad-band counterparts presented earlier, they are very encouraging considering that this design uses material designed for screening purposes. The reduction of the material loss for antenna applications will require adjustment of the material composition and processing. Note that the Bluetooth band was well matched but the efficiency could not be increased above a few percent due to the position of the rings.
The pattern cuts are shown in Fig. 11 and as for the quad-band antenna they have smooth overall coverage whilst maintaining the pattern nulls necessary for low SAR operation.
Fig. 11. Simulated radiation patterns of handset of Fig. 9. (a) yx plane without head and (b) yz plane without head. |
