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The aim of this work is to get far field radiation patterns for any radiating source from transient acquisition, in a large frequency range. An outdoor transient Ultra-Wideband near-field measurement base will be installed, a single time pulse radiated by the source will cover the desired spectrum, and the accurate determination of far field radiations will be accomplished by means of cylindrical waves' modal development. This method uses simplified test equipments, easy to be installed, and it reduces measurement costs.

Interested in measuring radiations of antennas placed in their context of use, such as antennas placed inside and outside a vehicle, or radiations of electronic systems integrated in many functional parts of a vehicle, the near field (NF) to far field (FF) transformation technique presents many advantages [

The proposed method is based on transient metrology [

The transient outdoor measurement method presents many advantages [

In the following, we will characterize an Ultra-Wideband (UWB) antenna in order to validate the proposed method. The principle of the method and the used antenna will be presented in the first part, near field to far field transformation in cylindrical coordinates in second part, in order to explain the experimental setup and results in the last part.

The UWB transient measurement technique in outdoor is well suited to the characterization in near field. In fact, far field ranges of large antenna (as well as a radiating antenna placed on a vehicle) require a huge infrastructure and an important radiated peak power. In such cases, the elimination of the influence of the environment could be difficult.

Starting from near field transient measurements, the fields’ calculation for all frequencies is deduced from the Fourier transform, and the construction of the far field can then be made by the application of specific algorithms.

Near Field to Far Field transformations are based on two principles. The first is to measure the values of E and H fields on a closed surface (Huygens), and then to seek secondary radiating sources using integral equations. The second is to perform a modal development of the tangential near electric field, as a solution of the vector Helmotz equation of propagation, which verifies the boundary conditions on the measurement surface at Near Field as well as at infinity. The theoretical formulation of this transformation exists in three coordinate systems: planar [

Synoptic of the validation.

The modal development method [

The radiating source is a transmitting antenna which radiates ultra-short pulses with very low dispersion. The chosen antenna is the UWB SCISSOR antenna (Figure

Scissor antenna.

The scissor antenna is simulated in CST MICROWAVE STUDIO software (Figure

Simulated scissor antenna.

To analyze the radiation of this antenna, we used a transient solver, with a mesh dimensions going to a maximum of

Normalized FF

Normalized FF

This method deals with the reconstruction of Far-Field (

The measurement cylinder is chosen around the tested antenna (Figure

Experimental setup.

From these equations, we can get the modal coefficients

These equations relate the modal coefficients of the tangential fields on the NF measurement cylinder.

So, using the tangential components of the electric field, we can get the modal coefficients of the cylindrical wave expansion

It is very important that acquisition steps respect some criteria since we deal with Fourier Transform. Sample spacing must be such as to make aliasing error negligible. The standard sampling criteria adopted in cylindrical NF-FF transformation [

For the scissor antenna we used a shadow cylinder of radius 110 cm, centered at the antenna’s feeding point (Figure

The radiating source (scissor antenna) will be connected to a transient pulse generator through a matching balun. The generated pulse feeding the antenna is a Gaussian having a rise time of 116 ps (Figure

Generated time pulse.

Normalized spectrum.

The used probe is a transient electric field sensor AD-80 model (Figure

Transient E field sensor.

Time Windowing.

The sensor must be oriented in a way so that antenna’s radiations arrive orthogonal to the ground plane of the sensor.

In the following, we will measure only one component of electric NF,

The measured electric near field is a signal of two parts, one containing the energy of the radiating pulse, and the other is the reflections from the surrounding to be eliminated. Time windowing is a useful operation to be done on this stage (Figure

By applying Fourier Transform, we can get the field values at 1 GHz and 600 MHz; then applying the appropriate algorithms of the modal development method, we can construct the far field of this antenna.

Another measurement is done, for

The comparison of the constructed FF and the measured one, for a working frequency of 1 GHz and then at 600 MHz, is given in Figures

FF

These results show accordance between the FF values calculated from measured NF and the measured FF of the scissor antenna, in two different planes. It should be noted that this measurement is done on a prototype non automated measurement base. The validation of the method can be concluded by these comparisons.

By comparing the different results of Figures

FF

FF

FF

The FF construction in predefined planes can be achieved with selected NF data on the measurement cylinder.

The condition is to take the entire field spot presented on the cylinder. Figures 15, 16, 17, 18, and 19 explain better this idea.

In height, we are able to construct the FF values by taking measurements up to

Reduced height NF cylinder for

The constructed FF in the E plane for two different heights is shown in Figure

FF

The same idea works in the

Choice of data spot on NF cylinder for

FF

FF

Figure

These results have been validated for all planes defined by

This measurement base allows the Far Field determination of antennas and radiating sources from a single transient outdoor acquisition. It provides a notable gain in terms of measurement time and costs, while remaining nondisruptive to the environment with pulses of very short durations.

This measurement technique is more efficient in Near Field ranges. Using NF-FF transformation techniques, it allows us to obtain the radiation pattern of any radiating structure over a wide band of frequencies, from a single measurement followed by a mathematical calculation.

The next work will be oriented in two ways. First we will improve the modal development in cylindrical coordinates to get more accurate FF patterns, while the second way will be the determination of FF directly in time domain, without time to frequency conversion [