Characterisation and optimisation of a coplanar waveguide fed logarithmic spiral antenna

A cavity backed coplanar waveguide (CPW) to coplanar strip (CPS)-fed logarithmic uniplanar spiral antenna, which covers a 9 to 1 bandwidth with a return loss better than 10 dB from 0.4 to 3.8 GHz is presented. A wideband balun is used to accomplish the transition from the unbalanced CPW transmission line to the balanced CPS transmission line. The balun exhibits an insertion loss of less than 3 dB in a frequency band from sub-100 kHz and up to a frequency of 3.85 GHz. The numerical results presented are based on simulations using the IE3D Version 6.03 for Windows 98. The obtained numerical results are in good agreement with experimental data. The spiral antenna is designed and prototyped for the FR-4 substrate. By placing the antenna on the FR-4 substrate it yields a low cost solution. Simulations have shown that the input impedance remains essentially constant over a bandwidth which is larger than 11 to 1. The simulated as well as the measured input impedance for the spiral antenna is 80 Ω. Measurements in an anechoic chamber have been made in order to measure the radiation pattern and the directivity of the antenna. The cavity backed spiral antenna exhibits a unidirectional radiation pattern, due to the absorbing material. Only half of the input power is transformed into radiated power due to the presence of the absorber. The simulated performance of the spiral antenna is very promising. The simulations indicated that the antenna has a radiation efficiency of more than 70% and an axial ratio and a return loss better than 3.5 dB and 10 dB, respectively, in the frequency band from 0.4 to 3.8 GHz