Characterisation and optimisation of a coplanar waveguide fed logarithmic spiral antenna

Abstract

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 I O 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 lE3D Version 6.03 for Windows 98 on an INTEL Celeron 338 MHz computer. 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, which indeed is an advantage. Simulations have shown that the input impedance remains essentially constant over a bandwidth, which is larger than 1 1 to 1. The simulated as well as the measured input impedance for the spiral antenna is 80 R. 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. It is noted that 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 I O dB, respectively, in the frequency band from 0.4 to 3.8 GHz, i.e., a 9 to 1 bandwidth.

A Wide Band Antenna for Multi-Constellation GNSS and Augmentation Systems

“…The planar cardioid arms antenna has bidirectional radiation patterns perpendicular to the plane of the antenna as expected for such planar antennas [17,18]. This phenomenon has been observed in the simulation studies as well (Fig.…”

“…From these measurements it is observed that the antenna has very low axial ratio (< ±3 dB) circular polarization characteristics over wide elevation angles. However, a commonly used criterion states that the axial ratio should be less than 6 dB [18]. The 3 dB beamwidth of the antenna is measured to be better than 100 • for all the frequencies.…”

“…The maximum size of the antenna along X-axis will be λ/4 which is less than the size of cavity backed Archmedian spiral antenna, i.e., λ/π. As the structure is solely defined by angle this will give frequency independent characteristics [15,17,18].…”

Circularly polarized antenna on SOI for the 60 GHz band

“…In our case, we take advantage from the balun design in [16], in order to implement the integrated balun as a first block of our feeding circuit.…”

“…The entire operation of the spiral antenna in its normal mode was postulated on the basis of an ideal, balanced, two-wire feed structure. Different feeding structure was proposed in the literature; [9] used external feed with integrated microstrip balun to overcome the problem of center feeding; [10] used an integrated balun circuit on a vertical substrate to assure feeding at the spiral centre; [1] used a cavity backed with coaxial feed vertically positioned under the spiral plan to assure balanced feed; Finally [16] used a radio frequency balun with integrated on substrate but positioned on the top of the spiral antenna. We note that, the feeding structure ( Figure 5) is sometime as large as the antenna, so that its design will maybe affect the radiation antenna properties and especially the axial ratio.…”