Ultra-Wide-Band Printed Circular Dipole Antenna

The design of a UWB dipole antenna with circular arms presented at ICU 2005 in Zurich [1] has been modeled and simulated with CST MICROWAVE STUDIO® (CST MWS). The dipole has been chosen for its omnidirectional radiation pattern - a wide bandwidth is obtained by using circular arms. The special feeding removes any unwanted radiation pattern disturbances.

The dimensions of the dipole arms and strip-line feed, shown in Figure 1, were taken from the paper [1]. Some dimensions concerning the exact transition from SMA to the stripline were not given so a rough estimate of these dimensions was made as depicted in Figure 2. It should be noted that the feeding and transition region is very critical with respect to the antenna's performance.

The antenna was parametrically modeled and embedded in an open air environment using open boundary conditions. For the calculations, perfectly conducting material is assumed for all metal parts and a lossy dielectric of FR4 applied with a tangent-delta = 0.025 at 5 GHz using a Debye characteristic. A magnetic half symmetry condition along the elevation plane was used. The inital mesh was created using the default settings. Through the implementation of the PERFECT BOUNDARY APPROXIMATION (PBA)® technique in CST MWS,  the initial mesh could be kept very coarse without neglecting any small details.

CST MWS' time-domain solver is perfectly suited to this type of broadband analysis. The required run-time for the inital mesh was around 63 seconds on a 2 GHz laptop. To gain confidence in the results, an adaptive mesh refinement simulation was performed to refine the mesh density at critical sections in the model. Within a few passes a broadband S-parameter deviation with a 2% tolerance was achieved. The runtime and S-parameter convergence versus passes is shown in Figure 3. The reflection coefficient, S11, for the various passes is given in Figure 4: the result is in good agreement with the simulation result of S11 given in [1].

The radiation pattern results were plotted in two planes: azimuth and elevation in polar plot form. The azimuth has an omnidirectional radiation pattern up to 6 GHz. For higher frequencies the difference between the minimum and maximum levels is greater than 3 dB. Figure 5 shows the co- and cross-components of the directivity in dB.

In the elevation plane the connector effects are visible. For frequencies 8 GHz and above the pattern is tilted towards the opposite direction of the SMA connector as plotted in Figure 6 (left). All curves are normalized to their own maxima for comparison. Figure 6 (right) shows a 3D radiation pattern together with the azimuth and elevation plane definition. The maximum computed gain of 3 dB is in good agreement with the value of 2.63 dBi in [1]. 

Three dimensional plots reveal a much better insight into the behaviour of the antenna's radiation. This is demonstrated for the farfield plot at 8 GHz in Figure 7: the tilt of the pattern can be easily observed.

 The Time-Domain solver of CST MWS togther with the unsurpassed PBA meshing technique are well suited to compute all relevant antenna parameters quickly and accurately even for complicated antenna structures. Simulation results given in [1] were compared to the results presented here: the agreement is good, particularly considering some details for SMA, transition and material properties were not given in the reference. For a closer comparison between the measurement and simulated results, the reader is advised to consult [1].

References:

[1] E. Gueguen, F. Thudor, P. Chambelin, "A low cost UWB Printed Dipole Antenna with High Performances," Conference Proceedings of the 2005 IEEE International Conference on Ultra-Wideband, Zurich, Sept. 5-8, 2005.