As demonstrated in the previous section, an infinite array of two dipoles having different terminal loads, and therefore different individual resonant frequency transmission characteristics, can produce a frequency null in a determinate position in the spectrum. Although to simplify the calculations, dipoles of equal length are considered, the terminal loads can be varied by modifying the length of each dipole, thus, a finite array of two dipoles having different lengths can be used as a single-layer UWB chipless RFID transponder.
The geometry of the proposed DD-UWB chipless transponder is shown in Fig. 4.8. It consists two dipoles of lengths 𝑙 and 𝑙 , width 𝑤, thickness 𝑇, and separated a distance 𝑑 from each other. The dipoles are placed on a dielectric substrate of relative permittivity 𝜀 and thickness 𝑆𝑢𝑏𝑇 [58].
The DD-UWB chipless transponder is designed to work between the 3 – 9 GHz frequency band and be fabricated using available low-cost organic materials or flexible electronics. Aluminum with a thin film thickness of 35 𝜇𝑚 is considered for the conductive dipoles, commercial bond paper with 100 𝜇𝑚 thickness for the substrate and are fabricated according to the procedure described in section 3.3. To assess the influence of the substrate, the ratio between the free-space wavelength 𝜆 to the lower and upper limit of the UWB frequency band is calculated and presented in Table 4.1. The substrate thickness is at least 283 times greater than 𝜆 . Therefore no resonances due to the
Fig. 4.8: Geometry of the dipole-based UWB chipless RFID transponder 𝑇 𝑤 𝑤 𝑆𝑢𝑏𝑇 𝑑 Substrate Dipoles 𝜀 𝑧̂ 𝑦 𝑥
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interaction of these two materials should be expected within this frequency range and the final frequency signature is influenced by the dipoles geometry and the materials electrical properties, like conductivity and losses, as explained in chapter 3 and section 4.1.
Designing an UWB chipless RFID transponder with just one metallic DD array of different strip lengths and thin film thickness, as well as a substrate with different layer thickness and properties, increases considerably the problem complexity presented in previous subsection. Thus, a computer simulation software is required to perform the calculations and modeling of the UWB chipless RFID transponder frequency response. For the UWB chipless RFID transponders presented in this investigation work, Computer Simulation Technology (CST) Microwave Studio is used to obtain the RCS calculations. CST is also capable to model more complex structures, as the ones that will be presented in the following sections.
An UWB chipless RFID transponder based on a single double-dipole array with lengths 𝑙 = 35 𝑚𝑚, 𝑙 = 25 𝑚𝑚, width 𝑤 = 1 𝑚𝑚, separation distance 𝑑 = 5 𝑚𝑚, and the previous mentioned materials, is modeled and simulated using CST Microwave Studio. The calculated |RCS| results for a frequency band between 3 and 9 GHz are shown in Fig. 4.9. The default material properties for aluminum and bond paper provided by CST Microwave Studio are used to run the simulation. The obtained simulation results show a similar behavior generating a frequency dip at around 5 GHz, as the theoretical calculate ones for the infinite double-dipole arrays of Fig. 4.7 in section 4.2.1. However, unlike its theoretical calculation, no zero or unitary reflections are achieved since the array is no longer infinite.
The peak magnitude and frequency dip depth terms are introduced, to evaluate the performance and be able to compare between different UWB chipless RFID transponders frequency responses. The first one, as a measured of the reflectivity and the second one
Frequency (GHz) Free-space wavelength 𝜆 (cm) 𝜆 /𝑆𝑢𝑏𝑇 𝑆𝑢𝑏𝑇 = 100 𝜇𝑚 𝜆 /𝑇 𝑇 = 35 𝜇𝑚 3.1 10 968 2,785 10.6 3 283 809
Table 4.1: Free-space wavelength to substrate/dipole thickness relation
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of the resonance capabilities. The peak magnitude is given by the maximum value of the RCS at each specific peak. The dip frequency depth is the difference between the dip center frequency and the closest peak magnitudes. Both terms are also illustrated in Fig. 4.9. In general, peaks and dips are numbered in increasing order starting from the one located at the lower frequency. As can be seen, the first peak magnitude is located at 3.8 GHz and has a value of -23 dBm2 and the dip depth measured from the dip center frequency located at 4.8 GHz and the second peak magnitude at 5.3 GHz is 16.5 dB.
Additional frequency resonances can be generated varying the dipoles lengths. Table 4.2 presents four different length configurations to produce equal number of DD-UWB chipless RFID transponders, and their respective |RCS| simulation results are shown in Fig. 4.10. It can be noticed by simple visual inspection, that as the dipoles’ lengths are reduced, the observed frequency dip is shifted to the higher frequencies, and Fig. 4.9: Simulated |RCS| of a dipole-based UWB chipless RFID transponder, 𝑙 = 35 𝑚𝑚, 𝑙 = 25 𝑚𝑚, 𝑤 = 1 𝑚𝑚, 𝑑 = 5 𝑚𝑚, 𝑇 = 35 𝜇𝑚, 𝑆𝑢𝑏𝑇 = 100 𝜇𝑚 -50 -40 -30 -20 3.0 3.8 4.5 5.3 6.0 6.8 7.5 8.3 9.0 |R C S | d B m 2 Frequency (GHz) Dip depth 1stMaximum 2ndMaximum Transponder number Microstrip line 1 length (mm) Microstrip line 2 length (mm) Assigned binary code 1 35 25 00 2 32 22 01 3 28 18 10 4 23 13 11
Table 4.2: Double-dipole UWB chipless RFID transponder strips lengths [58]
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its stop band becomes wider. Furthermore, under this context, each UWB chipless RFID transponder can be considered as having its own unique frequency response, which is determinate by its whole spectrum characteristic, and with special emphasis on the different frequency dips generated at specific frequency ranges.
The UWB chipless RFID transponders coding principle is based on these frequency signatures, however, as explained in section 2.3, every author chooses a distinct characteristic to encode the information e.g. frequency dip/peak, phase, group delay, etc. In this work, a computer system and not the human eye must perform the detection. Thus, the coding technique must be developed not only from the transponders perspective but also considering the detector limitations. Therefore an arbitrary identification code will be assigned to each transponder having a unique frequency response, as done in Table 4.2, and further details regarding the coding techniques and detection capabilities will be discussed in detail in chapter 6.