The Radome Scenario

At Elettronica SpA, we design and produce systems for electronic warfare. Each system has to meet different requirements, depending on the platform and its purpose (jamming, support measurement, etc).

In this case, we consider a terrestrial system for signal intelligence, using more than 40 ultra-wide band (UWB) antennae to detect threats. The peculiarity of this system is that a single radome has to protect all the antennae. In this scenario, the radome design represents a great challenge from both a mechanical and an electromagnetic (EM) point of view. In fact, the shape of the radome is, approximately, a cylinder with a diameter of 1 m and a height of 0.8 m, as shown in Fig. 1. It has to be strong enough to withstand difficult environmental conditions while it also has to be electromagnetically transparent from DC to 40 GHz.

The Radome Design

The first step in the EM design consisted of maximizing the transmission coefficient of the sandwich stratification, given the mechanical constraints. In our case, the constraints were related to the total thickness of the radome and the thickness of the inner and outer layers of preimpregnated composite fibres (prepreg).

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.

Solution identification

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.