Simulaciones y resultados
6.2. Proceso de calibraci ´ on
6.2.1. Calibraci ´ on de analizador de redes
In practice, signal fidelity is calculated for a given direction in space in order to fully characterise the spatial radiation properties of an antenna. When fed with an impulse, UWB antennas tend to radiate different signals in different spatial directions [19].
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Hence, it is extremely important to study the spatial properties of the radiated or received signal from a UWB antenna. The analysis can be done for a full plane to create a complete fidelity pattern. The fidelity pattern defines how the signal varies in different angular directions. By studying the fidelity pattern of an antenna, it is possible to determine the angular region around the antenna where the distortion effects of the antenna are minimal.
The system setup for the fidelity pattern measurements consisted of two identical miniature tapered-slot UWB antennas fixed 100 cm away from each other inside an Anechoic Chamber. Each antenna was connected to a port of an Agilent N5232A PNA-L vector network analyser, which was kept outside the chamber. The transmitter antenna was kept stationary and receiver antenna was fixed on a circular turn-table that was used to rotate the receiver antenna in small steps. Two different excitation pulses, fourth-order Gaussian and sine-modulated Gaussian were used to convolve with the measured system impulse response. An illustration of the measurement setup has been provided in Figure 5.12.
Figure 5.12: Experimental setup for fidelity pattern measurements inside the Anechoic Chamber.
The reference frame used to express the fidelity pattern measurements of the miniature antenna in vertical and horizontal planes is shown in Figure 5.13. For the fidelity pattern in the horizontal plane, the antennas were kept vertical with 𝜑𝜑 = 0°.
Vector Network Analyser
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Transfer function (S21) measurements were carried out between the two antennas,
with the transmitter antenna kept stationary and rotating the receiver antenna by steps of 𝜃𝜃 = 10° in the horizontal plane. One new S21 measurement was taken at each 𝜃𝜃
step. Hence, 36 different S21 measurements were done for a complete 360° scan.
Figure 5.13: Miniature UWB antenna position with respect to the coordinate frame used for the fidelity pattern measurements.
In order to deduce an antenna’s fidelity pattern in a particular plane, the fidelity factor needs to be calculated at every angle and the values can then be represented on a polar plot to visualize the fidelity pattern of the antenna. The fidelity patterns of the antenna were also obtained in the vertical plane. For this, the antennas were placed perpendicular from their normal upright position by tilting them 90° in the 𝑥𝑥𝑥𝑥 plane, by making 𝜑𝜑 = 90°.
The receiver antenna fixed on the circular turn-table was then rotated in small steps of 𝜃𝜃 = 10° to create the fidelity pattern in the vertical plane. The distance between the two antennas was kept constant at 100 cm. The deduced free-space fidelity patterns in the horizontal and vertical planes for two different excitation pulses have been illustrated in Figure 5.14, with the relative position of the miniature antenna displayed for reference.
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Figure 5.14: Fidelity patterns of the miniature UWB antenna in (a) Horizontal plane and (b) Vertical plane for free-space measurements.
It can be noticed from Figure 5.14 that for a sine-modulated Gaussian input, fidelity pattern is very linear for both the planes. The input pulse experiences minimal distortion in almost all angular directions around the antenna. With a fourth-order Gaussian input, fidelity pattern is better in antenna’s front side compared to the back side. Similarly, fidelity is better in the upper side of antenna structure than the lower side. Overall, in the horizontal plane, the highest and lowest fidelity values for a sine-modulated input are 0.9973 and 0.9576 respectively and for a fourth-order Gaussian input are 0.9653 and 0.7049 respectively. In the vertical plane, sine-modulated input gives the highest and lowest fidelity of 0.9996 and 0.9661 respectively, whereas fourth-order Gaussian gives 0.9923 and 0.7698 respectively.
Fidelity patterns have also been obtained with the receiver antenna mounted on a real human test subject. The measurement involved placing the receiver antenna on the test subject’s shoulder and making the test subject stand on the circular turn-table inside the Anechoic Chamber. The placement of the test subject’s arm was such that the antenna was precisely aligned along the circular turn-table’s axis of rotation. The transmitter antenna was kept at the same height as the receiver, at a distance of 100 cm. A 180° scan was done with the receiver antenna mounted on the test subject’s shoulder. The turn-table was rotated with steps of 𝜃𝜃 = 10° and S21 measurements were
done at each step. So, 19 measurements were done in total for a 180° scan to get a semi-circular fidelity pattern of the antenna in the anterior side of the body.
(a) (b) 0.2 0.4 0.6 0.8 1 30 210 60 240 90 270 120 300 150 330 180 0 0.2 0.4 0.6 0.8 1 30 210 60 240 90 270 120 300 150 330 180 0 Modulated Gaussian Fourth-order Gaussian
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Figure 5.15: Fidelity patterns of the miniature UWB antenna in (a) Horizontal plane and (b) Vertical plane for on-body measurements.
For on-body measurements, the vertical plane fidelity patterns were obtained by tilting the antennas by 90° in the 𝑥𝑥𝑥𝑥 plane, with the receiver antenna fixed on the test subject’s shoulder. The deduced on-body fidelity patterns in the horizontal and vertical planes for two different excitation pulses have been demonstrated in Figure 5.15, with the relative position of the miniature antenna displayed for reference. In these patterns, the position of the human body is on the back side of the antenna structure shown. It can be observed that the on-body fidelity remains almost identical to the free-space measurements in both the planes for a sine-modulated Gaussian input. With a fourth-order Gaussian input, the fidelity values do get affected by a slightly higher margin, with the lowest fidelity factor values being 0.7461 and 0.7041 in horizontal and vertical planes respectively and the average fidelity value is around 0.8, which is still very good. All these results clearly indicate that for the miniature UWB antenna, the human body has a very limited effect on the fidelity of the received pulses, which makes this compact and novel antenna suitable for wearable applications, intended to be deployed in high data rate and high efficiency wireless systems.