The First Prototype
A numerical optimization of the stratification was performed. The selected stratification alternated aramid fiber and honeycomb. The total thickness after optimization was 8 mm.
EM full-wave simulations were performed on the antennae behind the radome to evaluate the perturbations introduced in the radiation pattern. The radome dramatically increased the simulation’s spatial domain and, for some of the antennae, the problem was too complex for the available computational resources, so approximations were introduced in the simulation setup. In particular, a sinuous antenna working in the 1-18 GHz band was substituted by a field source radiating behind the radome. The simulation results showed an acceptable level of perturbation in the radiation patterns and so a first prototype was manufactured.
The radome prototype was evaluated but the result was unexpected: the sinuous antenna’s radiation pattern was deeply deformed, exhibiting a minimum at the boresight for some frequencies.
The perturbation in the radiation pattern was particularly strong in the higher part of the band where the radiating surface of the antenna was small compared to its physical size. This destructive phenomenon was a result of the contributions from the reflected field bouncing off the inactive part of antenna itself and radiating with the opposite phase.
A smart simulation environment had to be tailored to reproduce the pattern perturbation in order to create confidence in a successful redesign. The solution was found using ANSYS Savant, in cooperation with EnginSoft,. The ANSYS Savant software is based on a ray-tracing method. The setup is shown in Fig. 2. The equivalent field source is placed inside the radome. In order to reproduce the perturbation effect, the antenna structure was placed in its position (without being fed). This expedient allowed the antenna structure to reflect without overcomplicating the simulation.
The simulated radiation pattern at 17 GHz is shown in Fig. 3. This frequency was chosen to clearly demonstrate the described phenomenon. The simulation was in line with the measurement results.
The Second Prototype
With greater confidence from the simulation results, a second stratification was designed. This stratification design for the sinuous antenna favored transparency in the higher portion of the band. The simulation results are shown in Fig. 4. A prototype of this new stratification was manufactured and measured. The measurement results were in line with the simulations.