Appropriate acknowledgment should always be given for the use of any material contained in or derived from this report. A detailed parametric analysis is done to investigate the effects of some of the important design parameters on the reflection coefficient and axial ratio. 𝐻 height between ground plane and patch, cm 𝜀r relative dielectric constant of substrate.
Background
A patch antenna can be easily printed on a substrate, either planar or non-planar, so it can be very inexpensive. However, one of the main disadvantages of using a patch antenna as a readout antenna is its very low bandwidth, usually around a few percent of the center frequency (Balanis, 2005).
Antenna Parameters
S-parameter and Bandwidth
The bandwidth of an antenna can be understood as the range of frequencies in which the antenna can operate with the desired characteristics. By default, the range of frequencies below -10 dB in the S parameter curve is the bandwidth of the antenna.
Gain
In a UHF RFID application, the antenna must have at least -10 dB from 860 MHz to 960 MHz for worldwide use.
Polarisation
AR is defined as the ratio of the major axis of the electric field to the minor axis of the electric field, or mathematically. Thus, the operating criterion is CP when AR is less than or equal to 3 dB in the desired frequency range.
Objectives and Motivation
Thesis Organisation
Review of Methods to Achieve Broadband Operation
Another method to achieve broadband operation is to use probe-fed meandered strip technique (Wang et al., 2010). First, both the radiating patch and the tortuous strip are printed on two separate FR4 substrates, with the radiating patch elevated by a distance above the tortuous strip. Then an SMA connector is connected to the strip and another probe is connected from the strip to the radiating patch.
They have shown that this feeding method can operate in the 865 MHz – 928 MHz band. Although this is slightly smaller than the universal UHF range, this method is simple and inexpensive. Finally, one common way to get high bandwidth is to use stacked elements.
In general, the antenna design consists not only of one radiating patch, but also of another parasitic patch placed above or below the radiating patch. examples include Boo et al. 2009) who used two rectangular patches in their design, while Liu, Liu, and Tentzeris (2015) proposed using two coin-shaped patches (a circle with a square slot in the middle). Similar to the aforementioned methods, the working bandwidth is in the universal UHF range (Sun and Lee, 2008).
Review on Methods of Generating Circular Polarisation
Review on L-probe-fed UHF Reader Antennas
Introduction
Single Patch
From equations (3.1) and equation (3.2), the length of the patch with a resonant frequency of 920 MHz can be calculated using Using the rough estimated physical dimensions of the patch, along with other parameters relating to the modified probe, the design is drawn in CST and simulated. The antenna is optimized by changing the parameters until the operating frequency is in the UHF range and also has a CP.
The details of all the parameters used will be explained much more clearly in Chapter 3.
Wilkinson Power Divider
Configuration of the Single-element Antenna
Reflection Coefficient
Radiation Patterns and Read Range
In the x-z plane, the gain is at a maximum of 9.7 dB occurs at 5° (𝜙 → 180°) and the gain decreases as the elevation angle between the marker and the antenna increases. As for the y-z plane, the general pattern is the same whereby both the gain and the reading range decrease as the angle increases. However, for this level there is a difference in the angle where the maximum profit is located.
The peak gain and maximum read range for this plane is the same as the x-z plane at 9.7 dB and 6.3 m respectively, but the angle at which it occurs is now changed to 5° (𝜙 → 0°). Another interesting feature in this plane is that the read range in the 𝜙 → 0° direction is greater than in the 𝜙 → 180° direction. As shown in Figure 4-5(a), the read range in the x-z plane is more or less symmetrical about the center point 0°.
Axial Ratio
Parametric Analysis
- Length of the Patch, 𝒍
- Width of the Patch, 𝒘
- Height of the Patch, 𝑯
- Vertical Length of the Probe, 𝑳 𝒉
- Horizontal Length of the Probe, 𝑳 𝒑
- Bending Length of the Probe, 𝑳 𝐰
As seen from the figure, the reflection coefficient is very sensitive to the height of the patch. For 𝐻 = 2 cm, there is no CP operation from 860 MHz to 960 MHz, since the magnitude of the axial ratio is more than 3 dB in this frequency range. For the reflection coefficient, changes in 𝐿ℎ significantly affect the magnitude of the reflection coefficient as shown in the figure.
With only 0.5 cm difference from the suggested dimension of 2.5 cm, the antenna match is very poor. The first length of the probe has a significant effect on the reflection coefficient, but the same cannot be said for the axial ratio. Therefore, 𝐿ℎ is one of the parameters used to adjust the reflection coefficient without affecting the axial ratio.
Compared to the proposed size of 5 cm, the reflection loss size of 𝐿𝑤 = 4 cm is considered larger. From the figure it can be concluded that the axial ratio is not affected by the third length of the probe 𝐿𝑤.
Configuration of the Wilkinson Power Divider
Starting from the 50 Ω middle line, a normal SMA connector is attached to the power splitter at a distance of 2.4 cm from the edge of the line. Next, the port is defined counterclockwise, so port 3 is defined in the upper left corner. Similar to port 3, port 4 in the lower left corner is placed 1.2 cm from the edge of the line, while for port 5, it is the same as port 2 with an offset of 0.5 cm from the edge.
The entire Wilkinson power distributor is printed on a 1.5mm thick FR-4 substrate and also has a copper ground pad on the underside. After each piece is finished, the individual parts are patched together using copper tape. This can be seen in Figure 5-2(a) where the boundaries between each piece are quite clear.
For the ground plane, the same patch of copper tape is used, so the entire ground plane is one solid conductive copper ground. The parameters and their descriptions together with their optimized value are shown in Table 5.1 and Figure 5.1.
Reflection Coefficient
In terms of size, the measured result shows a better fit with the size being smaller than the simulated result at almost all points in the surgical bandwidth. While the size of the resonance dip at 900 MHz is not as good as the simulated result (-22 dB and -27 dB, respectively), the size of the total reflection loss is better in that the prototype has a wider bandwidth. The measured result shows that the bandwidth is 48.6% as opposed to the simulated result of 22.4.
Furthermore, this bandwidth is calculated with the assumption that 700 MHz is the lower limit of the bandwidth. Similar to S33 discussed above, the impedance bandwidth of the measured result is better than the simulated one. The fabricated prototype has an S44 parameter of less than -10 dB at almost the entire frequency range of 700 MHz to 1200 MHz, whereas the simulated result only shows a bandwidth from 870 MHz to 990 MHz.
The comparison between the simulated and measured is very similar to that of S22, where the resonant dip occurs at 900 MHz with a magnitude of -27 dB and the bandwidth is much larger than the simulated bandwidth. A possible reason for the similarity of S22 with S55 and S33 with S44 is that both ports 2 and 5 are on the right side, while port 3 and port 4 are on the left side of the power divider.
Insertion Loss
It was found that the existence of an air gap, even if small, causes a significant loss of power received by the door. At around 920 MHz, both ports have a magnitude of -7 dB, which means that the power received from port one is 20. This result supports the fact that the air gap causes an additional loss of received power.
This further suggests that the air gap and patching also cause an additional 1 dB loss to the received power. In conclusion, the fabricated Wilkinson power divider prototype proves to be less efficient than a theoretical one due to the additional losses. Besides that, the power divider is not exactly an equal power divider, as the lower ports receive more current than the upper ports.
This is due to the limitation of the etching equipment not being able to etch the entire design in a single piece. If such equipment is available, the output of the power divider can be improved.
Array Configuration
Reflection Coefficient
Radiation Patterns and Read Range
Axial Ratio
In this thesis, a new modified L-shaped probe consisting of 3 sides is proposed as a means to excite a patch for UHF RFID reader antenna. The whole project consists of three parts: the single element antenna, the Wilkinson power divider and the 2 × 2 array. For the single-element antenna, the impedance bandwidth and CP bandwidth are found to be from 760 MHz to 990 MHz and 890 MHz to 940 MHz, respectively.
For the Wilkinson power divider, it was found that there is some additional loss due to the air gap and substrate, so that the insertion loss for the four ports varies from -9 dB to -7 dB in the UHF range. Finally, there is a 2 × 2 array, which is essentially a combination of a single-element antenna and a Wilkinson power divider. In the future, it is possible to work on increasing the bandwidth of the CP antenna so that it can cover the entire UHF range from 860 MHz to 960 MHz.
In addition, the bandwidth of the array can be slightly shifted to lower frequencies for universal use. A new circularly polarized coin-shaped patch and ring-shaped strip antenna for worldwide UHF RFID applications.
