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# Modelling antennas – cutting out the clashes

### Hany E. Abd El-Raouf, R & D Engineer at Enginia Research describes his use of Integrated Engineering Software packages to successfully design & model antennas

Today the atmosphere around us is crowded with electromagnetic waves from a hundred different communication systems.

Each frequency band in the electromagnetic spectrum is used commercially for different wireless applications such as data transmission, radar and positioning.

Alongside this, the antenna designer faces many other challenges. The analysis of real antenna problems includes many complex structures, starting with the materials and the geometry complicity in the antenna structure itself extending to the surrounding media and objects.

Examples of such problems include antennas on a surface of an aeroplane, an antenna in front of a building or deep inside a building and then there’s the issues simulating a handset, including its antenna, in close proximity to a human body. Because of this variety of factors the analysis of real antenna problems becomes very complicated and difficult to handle using pure analytical methods, and the use of numerical techniques becomes essential. This is where using electromagnetic simulation software can help antenna engineers understand the most important elements.

By purely analytical we mean solving the antenna problem mathematically and expressing the solution entirely through sets of equations without numerical approximations. The numerical techniques use numerical approximations as part of the solutions, these gets adequate accuracy, and applicable way to simulate.

## Basic antenna simulation

As an example, the problem of an antenna in front of a building is shown in Figure 1. In this simulation we’ve used an electromagnetic solver (CHRONOS from Integrated Engineering Software), which is based on the Finite Difference Time Domain FDTD, to simulate the whole structure of the antenna and the building - including real objects inside the building such as tables, chairs, laptops, etc. The field distribution on the whole surfaces of the building is visualised in colours as shown in Figure 1.

##### Figure 1 Typical Field Distribution in a Building

In addition to the sources of interferences due to other communication systems or the interactions with other objects near the antenna, there are other sources of electromagnetic interferences (EMI) which may be generated due to natural effects and may affect the performance of the communication system and electronic equipment.

## Lightning

These sources include lightning [1] and electrostatic discharge ESD [2]. Lightning is caused by the accumulation of static charges on clouds which, when sufficient potential is reached with respect to the ground, breaks down the air insulation and a discharge spark results. It generally consists of multiple subsequent strokes, associated with impulse current, of different amplitude, steepness, polarity and duration. Electrostatic discharge is a transfer of charges between bodies at different electrostatic potentials. The energy produced by frictional forces between two insulators when these are rubbed together lifts charge carriers located at imperfection points in the materials to the conduction bands.

These charge carriers accumulate on the surfaces of the respective materials and a charge transfer takes place due to triboelectric effect. The ESD can occur with or without a fast initial current spike. In some applications the antenna and the electronic sensitive equipment needs to be shielded to reduce the coupling effect. As the shielding structure can be simulated using electromagnetic software, the results of the simulation will show its efficiency.

## Numerical techniques

Various electromagnetic numerical techniques are used in the different forms of commercial electromagnetic software. These techniques are categorised into time domain techniques and frequency domain techniques. The time domain methods include the finite difference time domain, FDTD, the time domain integral equations, the singularity expansion methods, SEM, the time domain finite element method, etc.

The frequency domain methods include the Method of Moments, MoM for solving the integral equations, the finite element method, FEM, etc. The FDTD, for example, is a direct solution of Maxwell’s equations and has advantages such as [3]: a high accuracy for a wide variety of RF and antenna problems; no storage in time and no matrix inversion needed; it gives the solution for broadband or ultra-wideband problems from a single execution of the problem using a short pulse; and it provides both the near-field and far-field results from a single run.

In addition, understanding the time-domain behaviour of antennas is important because processes in nature generally do not follow sinusoidal patterns, which is in general an advantage for the time domain methods.

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