The Measurement Result of Leaky-wave Antenna Arrays

The antenna pattern measurement of the leaky-wave antenna array was performed in the far-field antenna laboratory (is belonged to prof. Yu-De Lin) of department of Communication Engineering, National Chio-Tung University.

The test frequencies of the antenna arrays pattern measurement are 9.7GHz,

10.4GHz, 10.5GHz, 10.6GHz, and 11.0GHz. The antenna pattern measurement includes E-plane Co-polarization measurement, E-plane Cross-polarization measurement, and H-plane Co-polarization measurement.

The leaky-wave antenna array was realized by the printed circuit boards (PCB) process. Figure 4.9 displayed the photograph of the leaky-wave antenna array.

Fig. 4.9 The photo of the leaky-wave antenna array

The co-polarization represented that the observed signature when the transmitted and received polarizations are the same. The setup of the E-plane co-polarization measurement was shown in Fig. 4.10. The right side of Fig. 4.10 was a measured antenna array and the left side of Fig. 4.10 was a standard horn.

The standard horn was a transmitted antenna and the leaky-wave antenna array was received antenna. The polarizations of two antennas are the same. The

Fig. 4.10 Setup of pattern measurement for E-plane co-polarization

Fig. 4.11 The pattern of E-plane co-polarization of the leaky-wave antenna array

The cross-polarization represented that the observed signature when the transmitted and received polarizations are orthogonal. The setup of the E-plane cross-polarization measurement was shown in Fig. 4.12. The antenna pattern measurement of the E-plane cross-polarization was shown in Fig.4.13. The E-plane pattern was also an azimuth direction pattern.

Fig. 4.12 Setup of pattern measurement for E-plane cross-polarization

Fig. 4.13 The pattern of E-plane cross-polarization of the leaky-wave antenna array

The setup of the H-plane co-polarization measurement was shown in Fig.

4.14. The H-plane pattern was also an elevation direction pattern. The antenna pattern measurement of the H-plane co-polarization was shown in Fig. 4.15.

Fig. 4.14 Setup of pattern measurement for H-plane co-polarization

Fig. 4.15 The pattern of H-plane co-polarization of the leaky-wave antenna array

Since the pattern energy of H-plane cross-polarization of the leaky-wave antenna array was very low,which resulted in the measurement was omitted.

After the antenna pattern measurement, other important measurements are return loss and coupling efficiency measurement for the 8 stubs leaky-wave antenna array.

There are two leaky-wave antenna arrays in the measurement. The antenna arrays were labeled as A and B. The isolation measurement was adjusted the spacing of the two individual leaky-wave antenna arrays from the most left metal stripe of the antenna array B to the most right metal stripe of the antenna A (Fig. 4.16).

Fig. 4.16 The isolation measurement was adjusted the spacing of the two individual leaky-wave antenna arrays.

Fig.4.17 and Fig.4.18 represented the return loss measurement (from 1GHz to 20 GHz) for Antenna A and B. Since our system used the two antennas to isolate the coupling influence, we tuned the space between two antennas and measured the insertion loss (S21) for two antennas. Figs. 4.19-4.23 represented the coupling measurement for antenna arrays A & B Spacing from 0cm to 8cm.

Fig.4.17 Return loss measurement for the antenna array A

Fig.4.18 Return loss measurement for the antenna array B

Fig.4.19 Coupling measurement for antenna arrays A & B Spacing 0 cm

Fig.4.20 Coupling measurement for antenna arrays A & B Spacing 2 cm

Fig.4.21 Coupling measurement for antenna arrays A & B Spacing 4 cm

Fig.4.23 Coupling measurement for antenna arrays A & B Spacing 8 cm

It summarized the above measurements that the conclusions were derived the following description: the space of two antennas which increase over 4 cm, the insertion loss (S21) will be dropped to –48dB. And then the insertion loss (S21) of the space of two antennas is also represented the coupling effect of two antennas. Hence, the space of the leaky-wave antenna array is wider than one wavelength, and then the coupling effect can be neglected.

More significantly, the electromagnetic coupling of the two antenna arrays must be measured before combining the entire sensor system. Figure 4.24 plots the measured isolation of the two antenna arrays separated by only 5.0mm, revealing a coupling of less than 42dB. Table 4.3 shows the measured coupling versus the spacing between two antenna arrays, revealing that the coupling is insensitive to the spacing. Significantly, an attainable isolation value for a good circulator in the X-band is around 35dB, which is approximately 10dB below that obtained by the proposed two-antenna array approach. Figures 4.25 and 4.26 show the measured cut-plane on the main beam at 56° from the E-plane (yz plane) and the H-plane (xz plane) radiation patterns of the leaky-wave antenna array at 10.5GHz. The measurements in Fig. 4.24 demonstrate that the half power is about 15 dB and is bounded between -6.5° and +6.5°. Hence, the 3dB

beam-width of the antenna array was 13° in the E-plane, and that of the main-beam with a gain of 18.5dB was 56° from the broad side of the array in the H-plane. The first lobes of the E-plane radiation pattern were equal to 3.2 dB, because of the original design of the leaky-wave antenna. Since the path lengths of the differential feeding structure of the leaky-wave antenna were not equal, the side-lobes were not symmetrical. Nevertheless, the main direction of the beam is still 0° in the E-plane. In the paper, it applied the elevation pattern (H-plane) to achieve the range measurement. The azimuth resolution (E-plane) haven’t discussed in the dissertation. The radiation angle of the elevation pattern has no relation with range resolution. However, the radiation angle effects directly the echo power distribution and the signal-to-noise rate (SNR) of range measurement.

The FMCW front-end, which includes the CMOS transceiver and antenna arrays, was designed and experimentally characterized. The next section presents a practical example of obtaining the vehicle occupancy in TMS to indicate the capability of the proposed FMCW sensor system.

-80 -70 -60 -50 -40 -30 -20 -10 0

S21(dB) Antenna Array Antenna Array

S

Port1 Port2

TABLE 4.3 Coupling of Two Antenna Arrays in Different Spacing

Fig. 4.25 Measured E-plane radiation pattern of the cut-plane on the main beam at 56° of the 8-element antenna array.

-180 -120 -60 0 60 120 180

Fig. 4.26 Measured H-plane radiation pattern of the eight-element antenna array.

The csc²θ-type antenna pattern in system applications, originated with the surveillance radar with a fan-shaped beam. The azimuth (E-plane) beam angle is small, and the elevation (H-plane) beam angle is large [20]. The main lobe pattern of the H-Plane of the leaky-wave antenna is always at an oblique angle.

The oblique angle is then increased as the length of the antenna array. When the oblique angle was adjusted to a suitable value, the conditional cosecant-squared antenna pattern was realized. The characteristic curve of the cosecant-squared antenna pattern ranges between the two dashed-lines in Fig. 4 of another investigation [42]. The theory of the csc²θ pattern of a leaky-wave antenna has also been derived and proved elsewhere [42]. In the investigation, one element of the leaky-wave antenna is used to design a csc²θ pattern associated with the proposed method that the length of the antenna metal must be varied gradually to prove the design concept with reference to the numerical solution. When the lengths of the metallic antenna are larger than 80.0mm, the csc²θ patterns are obtained (Fig. 4.27).

-90 -60 -30 0 30 60 90

Elevation Angle θ (deg) -15

Fig. 4.27 Patterns of elevation angles vary with the stripe length of antenna from 25.0 to 150.0mm.

the antenna array is shorter, then the radiated energy cannot be immediately leaked to the space and the most of the energy was reflected the other end of the antenna array, that is, when the strip line of the antenna array is less than 80.0mm, resulting in the energy peak appears in the left side of Fig. 4.27.

We made a comparison with one antenna system in NCTU, our two antennas system had higher isolation (-45dB) than the mono-static system (-35dB of a circulator) and can add the external amplifier flexibly. Another important reason is the circulator designed by the CMOS technology did not provide enough isolation between the transmitting and receiving paths. In the paper, the power spectrum of the receiving path was 55dB less than that of the transmitting path at 10.5GHz. These measured results confirm that the proposed system obtained high leakage suppressions by using the CCS TL guiding structure throughout the entire chip design. However, two antennas system need more space to put the antennas. The radiation angle of the elevation pattern has no relation with the range resolution. But the radiation angle affects directly the echo power distribution and the signal-to-noise rate (SNR) of range measurement.

4.4 Integrating the Antenna Arrays and the CMOS Transceiver into a