Ultra-Wide Band Radome CAE Optimization

Overcoming mechanical and electromagnetic challenges in refining a radome for electronic warfare

ABSTRACT

Elettronica SpA designs and produces systems for electronic warfare. Each system design is unique according to its platform and purpose. In this article, the company describes how it used CAE to approach the challenging design of a single sandwich radome to protect a large terrestrial system for signal intelligence that used more than 40 ultra-wide band UWB antennae to detect threats.

The radome was specified as roughly cylindrical shape with a diameter of 1 m and a height of 0.8 m, strong enough to withstand difficult environmental conditions while also being electromagnetically transparent from the DC band up to the 40 GHz band. During the design process, the engineers found that the model’s extensive spatial domain was too complex for their available computational resources. Th series of approximations they subsequently inserted in the simulation created simulation artifacts that distorted their initial design conclusions. The article describes how this simulation challenge was overcome.

<h5>Radome in Elettronica SpA’s anechoic chamber</h5>
Radome in Elettronica SpA’s anechoic chamber

We show here the design of a radome for electronic warfare covering an entire signal-intelligence system. The radome design was very challenging from both an electromagnetic (EM) and a mechanical point of view because of its size and bandwidth.
In particular, we want to show how the choice of the correct simulation environment can prevent simulation artifacts, leading to a successful design.I

Introduction

A radome is a cover placed above an antenna to protect it from the external environment. An ideal radome would protect the antenna from any physical damage and be electromagnetically transparent in its operational frequency band. Nowadays, radomes are widely used in ground, airborne and maritime applications.
The growth in the use of the electromagnetic spectrum has led to an improvement in radome performance. In particular, during the last decades, composite materials have become the preferred choice for antennae radomes due to their low thickness and high mechanical strength.
A radome can be designed in a monolithic or sandwich fashion. A monolithic radome is made of a single solid layer, while a sandwich radome alternates high-density (eg. aramid fiber, fiberglass) and low-density (eg. honeycomb, foam) materials. In this work, we focus on sandwich radomes.

A sandwich stratification can be electromagnetically modeled using an equivalent transmission line: each layer is represented by a shunt impedance, defined as the thickness of the layer and its characteristic impedance. Potentially, one can predict the transparency of the whole stratification based on the electric behavior of each layer. It is worth noting that a larger number of layers does not necessarily mean higher reflectivity. In fact, the sandwich stratification can be viewed as a shunt-inductance-coupled filter, where the goal is to insert a pole into the operational band.

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.

Conclusions

In this paper, we reviewed the design process of a challenging radome. Its operational band ranged from DC to 40 GHz and the radome had to cover an entire system for signal intelligence. The approximations introduced during the design process led to a strong perturbation in the radiation pattern of a UWB sinuous antenna, as was discovered in the measurement stage of the first prototype. After identifying a more effective simulation setup, a second version of the radome that demonstrated greater fidelity between the simulations and the measurements was successfully designed and manufactured,

Newsletter EnginSoft Year 14 n°4

Francesco Scattone, Antonio Manna | Elettronica 

Andrea Serra, EnginSoft

Fig. 2 - SAVANT simulation setup (cross section)

 

Fig. 3 - First design, simulated radiation pattern @ 17 GHz

 

Fig. 4 - Second design, simulated radiation pattern @ 17 GHz

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