Technical Papers and Presentations

Introduction

A new circular polarized antenna model for mobile transmission has been implemented in COMSOL Multiphysics^® for needs related to shooting sports events on the move. The antenna was required to work in the UHF television frequency band (specifically, 600 MHz) and, if possible, in VHF (230 MHz). Moreover, the antenna should be compact enough to be hosted in the top case of the motorcycle used for shooting. Another implementation challenge is that, in order to allow the correct reception for any possible reciprocal orientation between the transmitting antenna with the receiving one on the helicopter, circular polarization (CP) must be adopted. This aspect is crucial for shooting sports events on the move. Taking into account all the above requirements, with the help of COMSOL Multiphysics^® several antenna shapes have been modeled and tuned to seek the required performances in the frequencies of interest. The simulated results have been confirmed in the prototypes by experimental measurements.

COMSOL Multiphysics® model

Several radiant system configurations have been considered. On one hand, some CP radiating structures need simpler feeding networks. Among these, flat double spirals based geometries provide fair CP performance over a wide frequency range, provided that the spiral parameters are tailored for the required operating range. On the other hand, structures relying on the presence of a more complicated 4-phase feeding network have proved to be more robust as CP radiators, although not well suited for dual-band operation.

The size requirement sets a hard challenge for the performance at VHF band: the antenna efficiency, bandwidth and size are strictly related by the Chou’s limit [1]. A first model achieving CP radiation on both the UHF and VHF frequencies is illustrated in figure 1. Here two inductive loaded compact dipoles, resonating at operating VHF frequency, are fed by the two-arm spiral. The latter works as CP radiator in UHF. The geometry is tuned in such a way that the four patches oscillate in 4-phase relationship to each other.

In the all COMSOL® models, the geometry has been fully parametrized, in order to achieve easy modification of the shape for the tuning and hence for the optimization. Intense simulative work, thanks to the possibility of using the COMSOL^® Optimization module, has allowed us to find the good trade-off between size, impedance matching, realized gain and axial ratio. The latter, very useful to obtain the quality of the polarization circularity of the antenna, has been recently introduced by COMSOL® RF module.

Results

The performance of the developed antennas have been evaluated in terms of impedance matching, realized gain and axial ratio. In fig.2 the antenna radiation pattern obtained from one of the models is reported.

The simulated results have been validated in laboratory on the realized prototypes by means of a Very Near Field Scanner, EMscan RFX2 [2] showing a good agreement between the numerical and experimental performance.

References

[1] L. J. Chu, “Physical limitations on omni-directional antenna,” J. Appl. Phys., vol. 19, no. 10, pp. 1163–75, 1948.

[2] https://www.keysight.com/upload/cmc_upload/All/EMSCANRoadshow.pdf