Figure 3-4-2 demonstrates the simulated and measured results of return loss against frequency for the proposed antenna. The measured result shows that the antenna has a fundamental resonant frequency of 2.4 GHz with the minimum return loss of -50.44 dB and the impedance bandwidth of 930 MHz (or 38.7%) covering the frequency range from 1.83 GHz to 2.76 GHz. The free space wavelength of 2.4 GHz is about 125 mm and approximately equals the mean circumference of square-ring slot (about 130 mm). The simulated polarization patterns are shown in Figure 3-4-3. The proposed antenna radiates right hand circular polarization (RHCP) in the upper half free space and left hand circular polarization (LHCP) in the lower half free space. Besides, the simulated results of axial ratio and CP gain against frequency for the proposed antenna are shown in Figure 3-4-4.
It is shown that the 3-dB AR bandwidth is 310 MHz or 12.9%, including the frequencies range from 2.21 GHz to 2.52 GHz, and the 1-dB AR bandwidth is about 100 MHz. The minimum AR value of about 0.1 dB is located at the frequency of 2.4 GHz. Besides, the CP gains are about 3.3 dBi and almost keep constant during 3-dB AR bandwidth. The measurement of polarization patterns in this study employs the rotating source method [54]. The measured results of polarization patterns at different frequencies of 2.3, 2.4, and 2.5 GHz are shown in Figure 3-4-5. These ripples in the polarization patterns are a consequence of the beam ellipticity, which occurs when a finite cross-polar component exists. The depth of the ripples defines the AR value. They present well circular polarization and also obtain good axial-ratio values over a wide angle range. From figure
4.5, the elevation-angle ranges with respected to lower 3-dB AR are -33 to 29, -43 to 47 and -49 to 9 degrees, respectively, for 2.3, 2.4 and 2.5 GHz in the upper half free space. It can be seen that the space distribution of the lower 3-dB AR value are more symmetric at the frequencies of 2.3 and 2.4 GHz than that at the frequency of 2.5 GHz. In order to further manifest the symmetric distribution of axial ratio, the calculated results obtained from the depth of the ripples in these measured polarization patterns are shown in Figure 3-4-6. It can be observed that the AR pattern with respect to elevation angle is nearly symmetric along the elevation angle of 0 degree at the frequencies of 2.3 and 2.4 GHz.
By using the three small triangles at the square-ring slot corners to perturb the magnetic current within the square-ring slot, the symmetric AR space distribution and the minimum AR position can be further controlled. In this proposed antenna, the length of c3 (4 mm) is not equal to the length of c1 and c2 (2 mm), and in this condition, a very symmetric AR space distribution can be achieved. In order to manifest this phenomenon, the simulated results of axial ratio against elevation angle for the proposed antenna with and without three small triangles at the frequency of 2.4 GHz are shown in Figure 3-4-7(a) and Figure 3-4-7(b) respectively. It can be observed that the AR distribution in the elevation direction for the proposed antenna with three small triangles is more symmetric along the elevation angle of 0 degrees than that without three triangles.
L1 L2 L3 L4 L5 L6 L7 c1 c2 c3
(mm) 64 40 25 20 11.5 2 2.75 2 2 4
W1 W2 W3 W4 W5 W6 W7 Wf g1
(mm) 60 40 25 20 3 3.5 1 3 1.4
Table 4.1
Y
X
Figure 3-4-1 Configuration of the proposed microstrip-fed circularly polarized slot antenna
1.0 1.5 2.0 2.5 3.0 3.5 4.0 -55
-50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0
2.4GHz 2.76GHz
Return loss (dB)
Frequency (GHz)
Return loss Simulation Measurement 1.945GHz
1.83GHz
Figure 3-4-2 Simulated and measured results of return loss against frequency for the proposed antenna
Figure 3-4-3 Simulated polarization patterns at the frequency of 2.4 GHz for the proposed antenna
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
Figure 3-4-4 Simulated results of axial-ratio and CP gain against frequency for the proposed antenna
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Figure 3-4-5 Measured polarization patterns at different frequencies of 2.3, 2.4 and2.5 GHz for the proposed antenna.
-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
Figure 3-4-6 Axial ratio against elevation angle calculated from the measured polarization patterns
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180 0
3 6 9 12 15 18 21
Axial ratio (dB)
Elevation angle (degree)
For Phi 0 90
(b)
Figure 3-4-7 Simulated axial ratio against elevation angle at the frequency of 2.4 GHz for the proposed antenna (a) with and (b) without three small triangles.
Chapter 5 Further Discussions
In this chapter, we further discuss these circularly polarized slot antennas.
According to excitation mechanism, these slot antennas can be divided into two types such as electric current excitation and magnetic current excitation. With respect to electric current excitation, the circularly polarized wave is mainly excited from the four segments of the square ring at the center of the square slot. However, for the magnetic current excitation, the CP radiation is excited from the four segments of the square-ring slot, and we can also consider the square-ring slot as a waveguide of magnetic current.
Besides, we must also consider the influence of the image effect by the ground plane. This effect has larger influence on electric-current excited slot antenna, but less influence on magnetic-current excited antenna. This is because the electric current traveling on the square-ring strip at the center of the slot induces several image antennas on the surrounding ground plane. These image antennas largely affect the polarization patterns of the original slot antenna. In addition, the slight change of feed configuration has also large influence on the radiation pattern and S parameter. Thus, the requirement of fabrication tolerance must be exactly controlled, or the S parameter and CP performance of the slot antenna have very large deviation from our design. In terms of magnetic current excitation, the image effect is very little influence on the S parameter and radiation pattern. This is because the magnetic current is confined within the square-ring slot and the confined magnetic current contributes to the most of the CP radiation. This type of slot antenna with microstrip fed and magnetic excitation is less sensitive to fabrication tolerance and feed configuration, and the main geometry factors of S parameter and radiation pattern can be separately controlled. Thus, the design flexibility is very attractive and convenient.
Figure 3-5-1 shows the photograph of the microstrip-fed square-ring square slot antenna, and (a) is back side and (b) is front side. We use 3D FEM simulator of Ansoft HFSS to simulate the slot antenna and also compare with using IE3D (which is 2.5D FEM simulator not 3D FEM simulator). The 3D solid model built by HFSS is presented in Fig. 5.2. The air box is used to monitor the space distribution of electric field and magnetic field. The boundary conditions of radiation are also applied to the surface of the air box. A wave port is loaded on the feed line. Figure 5.3 shows the simulated results of (a) scattering parameter, (b) axial ratio, and (c) radiation pattern. The simulated result of scattering parameter is in good agreement with the results shown in Figure 3-5-2. The difference between HFSS and IE3D is the higher order mode at the frequency of 3.25 GHz. The higher order mode maybe results from the waveguide mode of the substrate.
The polarized radiation patterns shown in Figure 3-5-3 and Figure 3-5-3(c), which are RHCP and LHCP, also show some difference between these figures. This can be explained that the HFSS takes account of diffraction effect from substrate edge and metal plane. Figure 3-5-4 shows the measured results of polarization pattern at various frequencies. It should be noted that the slot antenna presents very good circularly polarized radiation cover wide frequency bandwidth from 2.15 GHz to 2.5 GHz. Besides, the polarization patterns show slight symmetry on the upper free half space and the lower half space, if we correct the measured error due to the alignment between the under test antenna and the source antenna. Electrical field and magnetic field distributions on the x-y plane are shown in Figure 3-5-5 and Figure 3-5-6, respectively, while feed signal phase from 0o to 180o by step of 90o. We can observe the electric and magnetic field rotate along anti-clockwise which excites RHCP radiation. Besides, the magnetic field presents very smooth distribution over electric field, and this phenomenon results in the very good CP radiation performance due to its magnetic excitation mechanism. Figure 3-5-7 and Figure 3-5-8 illustrate the vector plot of electric field and magnetic field
distribution on the top of the air box while the feed signal phase from 0o to 180o by step of 90o. The phase of the magnetic field obviously leads one of the electric field by a quadrature phase.
The CPW-fed square-ring slot antenna is fabricated as shown in Figure 3-5-9. The 3D solid model is developed by Ansoft HFSS as shown in Figure 3-5-10. The simulated results of scatter parameter, axial ratio, and radiation patterns are presented inFigure 3-5-11. The slot antenna has the return loss with the minimum value of -28 dB at the frequency of 2.42 GHz and axial ratio with the minimum value of 1.8dB at the frequency of 2.55 GHz. These antenna parameters are some different from that by using IE3D simulator. This is due to the HFSS simulator accounting for the diffraction effect by the substrate and metal plane. The measured polarization patterns are demonstrated in Figure 3-5-12 at the different frequencies from 2.4 GHz to 2.6 GHz. It can be noted that the 2.5 GHz and 2.55 GHz have the better CP performance. In general, the radiation pattern for slot antennas should be symmetry at the upper half free space and lower half free space.
The non-symmetric distributions of polarization pattern along the line from 90 to 270 result from the diffraction and the measurement error. The misalignment between the source antenna and measured antenna contributes to the measurement error, and the non-fixed rotation of the measured antenna also contributes to the measurement error.
Electrical field and magnetic field distributions on the x-y plane are shown in Figure 3-5-13 and Figure 3-5-14, respectively, while feed signal phase from 0o to 180o by step of 90o. We can observe the electric and magnetic field rotate along clockwise which excites LHCP radiation. The most of the electric and magnetic field distribution concentrate at the square-ring strips, and thus it indicates the circular polarized radiation mainly results from the square-ring strips unlike magnetic current excitation using the square slot radiating. Figure 3-5-14 and Figure 3-5-15 illustrate the vector plot of electric field and magnetic field distribution on the top of the air box while the feed signal phase from 0o to
180o by step of 90o. We can observe that the plane at which the feed line is located has larger electric and magnetic field than other places. It manifests this type of electric excited slot antenna are larger influenced by the feed configuration. The slight deviation of the feed configuration form original design maybe leads to large undesirable results. In order to resolve this problem, we develop a new square-ring slot antenna as shown in Figure 3-5-17. The measured and simulated results of scattering parameter are shown in Figure 3-5-18. In order to manifest the less sensitivity to feed configuration, we fabricate two antennas with slight change of feed length to measure. The measured results indeed prove the consequence of less sensitive to feed configuration. In this antenna, the circularly polarized radiation mainly excites from the electromagnetic interaction between the square ring at the feed-line side and the square ring at the ground-plane size.
The simulated results of axial ratio against frequency and axial ratio against elevation angle are shown in Figure 3-5-19. Besides, the measured results of polarization patterns at the frequency of 2.45 GHz and 2.5 GHz are presented in Figure 3-5-20.
For the sake of further improving the circularly polarized response of CPW-fed square-ring slot antenna, we must make more effort to alleviate the image effect. We add two slits at the edges of the CPW feed to alleviate the image antenna forming. The proposed antenna is shown in Figure 3-5-21, and the simulated return loss is also shown in Figure 3-5-22. The minimum return loss occurs at the frequency of 2.2 GHz with the S11 value of -48.5 dB. Figure 3-5-23 presents the results of (a) axial ratio against frequency and (b) axial ratio against elevation angle at various frequencies. We can observe the AR bandwidth is very wide and the AR space distribution is very symmetry even if at different frequency. But the proposed antenna has narrower impedance bandwidth than AR bandwidth. This impedance bandwidth narrowing is incurred from the discontinuity of CPW feed line. Figure 3-5-24 demonstrates the measured polarization patterns at various frequencies. It also manifest the good CP response of the proposed
antenna
Figure 3-5-1 Photograph of microstrip fed slot antenna
Figure 3-5-2 3D solid model of microstrip fed slot antenna
(a)
(b)
(c)
Figure 3-5-3 Simulation of (a) S parameter, (b) Axial ratio, and (c) CP radiation pattern by using Ansoft HFSS simulator
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Figure 3-5-4 Measurement of polarization patterns of microstrip-fed CP slot antenna at different frequency.
Figure 3-5-5 Electrical field distributions on the x-y plane as feed signal phase from 0 to 180 by step of 90
Figure 3-5-6 Magnetic field distributions on the x-y plane as feed signal phase from 0 to 180 by step of 90
Figure 3-5-7 Vector plot of electrical field distribution on the top of Air box as the feed phase from 0 to 180 by step of 90
Figure 3-5-8 Vector plot of magnetic field distribution on the top of Air box as the feed phase from 0 to 180 by step of 90
Figure 3-5-9 Photograph of CPW-fed square-ring slot antenna
Figure 3-5-10 3D solid model of microstrip fed slot antenna
Figure 3-5-11 Simulation of (a) S parameter, (b) Axial ratio, and (c) CP radiation pattern by using Ansoft HFSS simulator
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Figure 3-5-12 Measurement of polarization patterns of CPW-fed CP slot antenna at different frequency.
Figure 3-5-13 Electrical field distributions on the x-y plane as feed signal phase from 0 to 180 by step of 90