Advanced Modelling and Measurement of Wideband Horn Antennas

Dual-ridge horn antennas are commonly used as wideband gain standards for antenna measurements. The example shown in figure 1 is designed by SATIMO. The horn is connectorized and essentially an open, flared ridge waveguide with lateral bars. It is designed to harmonize the gain with the frequency curve. At low frequencies the bars appear as a closed surface and increase the boresight gain of the horn, whereas at high frequencies the bars are electrically transparent and the effective gain decreases. Carefully designed dual ridge horns have excellent return loss, cross polar and flat gain response (typically 7-15 dBi) in a 1:15 frequency range.

The antenna is modelled and simulated in CST MICROWAVE STUDIO® (CST MWS) using magnetic symmetry condition in the E-plane of the antenna and a rather coarse regular mesh with λ/10 spacing.

The used connectors are approximated with simple structures, and dielectric materials are ignored. For the calculations, perfectly conducting material is assumed for the metallic parts, and PML (Perfect Matching Layer) absorbing boundary conditions are used. The required CPU time for a full analysis is approximately four hours on a Pentium 4, 3 GHz PC.

The dual ridge horn was measured in the Satimo Stargate 64 spherical near field facility in Atlanta, USA as shown in Figure 2 over a frequency range of 800 MHz to 6 GHz.

The simulated and measured antenna return loss performance for both amplitude and phase components at the antenna reference port is in very good agreement as shown in Figure 3.

The results of simulated and measured boresight directivity are plotted in Figure 4. A excellent correlation between the two curves can be observed, and the differences fall within the measurement accuracy of the measurement equipment.

Figure 5 shows the comparison of simulated and measured E and H plane radiation patterns at 3.6 GHz. The polarisation definition is Ludwig III. It can be seen that the two curves are very close to each other. The performance CST MWS' Time Domain solver is excellent due to its ability to extract the farfield gain at a large number of frequencies from a single simulation run as opposed to having to carry out a large number of individual simulations at discrete frequencies in the Frequency Domain. The required monitors, 100 in this case, can be defined easily  using a dedicate standard macro.

The antenna's back radiated fields are often very difficult to determine with high accuracy due to coupling between the antenna under test and the antenna positioner. The SATIMO Stargate spherical near field system uses a Styrofoam positioner specifically designed for minimum interference with the antenna under test. The excellent agreement between the predicted and measured results for the radiated back lobe of the antenna (Figure 5) confirms the measurement accuracy in this difficult region.

The achieved correlation between the simulated and measured results confirms once again the validity of the numerical modelling of CST MICROWAVE STUDIO®. The ability to extract a high resolution of broadband gain data is a result of the Time Domain Solver ability to define and calculate a large number of farfield monitors in one single simulation run. This rerpesents a significant performance advantage compared to non-Time Domain methods which entail the simulation of a large number of discrete frequencies for the broadband gain extraction.

References:

Lars Foged, A. Giacomini, L. Duchesne, E. Leroux, L. Sassi, J. Mollet;  Advanced Modelling And Measurement Of Wideband Horn Antenna; Proceedings of the "11th Internationnal Symposium on Antenna Technology and Apply Electromagnetics (ANTEM 2005)", http://antem2005.ietr.org