RF devices, when operating frequency is increased to the microwave region, in addition to the RF circuit design, a good RFID tags also have a good antenna design. Factors to consider include the type of antenna, antenna impedance, paste the RF performance of the label items around the RF performance of the labeling items other items in the choice of antenna.
In addition, the choice of antenna type must make the impedance of free space and ASIC match the antenna's goal is to transfer maximum energy out of the tag chip, which requires careful design of the antenna and free space, so that it and connected to the tag chip. the best match. The folded dipole antenna through the choice of geometric parameters to achieve the desired input impedance. Termination, tilted, folded dipole antenna performance is excellent, compared with the half-wave dipole antenna size is much smaller, has a high gain, wide frequency coverage and low noise advantages, with the electrical terminals of the brazing and the balun, can maximize the gain, impedance matching and bandwidth.
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Second, the folded dipole antenna theory
Folding video (Shorted - Patch) antenna structure evolution shown in Figure 1. Folding video SP structure is called because the antenna evolved from the traditional microstrip piece. As we all know, the traditional rectangular piece antenna antenna length in the basic mode is λ0 / 2, as shown in Figure 1 (a) as shown. Around the piece of the electric field in the middle of taking into account the weakening of the metal shield to shorten the midline around the pieces, but not significantly change the natural frequency of antenna, the resultant length λ0 / 4 SP antenna, as shown in Figure 1 (b) below. We then reverse folded SP antenna video block and the ground plane as one, as shown in Figure 1 (c) below. Noting that little change of the electrical length of the folded SP antenna, but the antenna actual length will be halved to λ0 / 8. Finally, we added on the right a ground plane (the original ground has become antenna poorly at the block) and compression of the sheet as a whole, the formation of the standard folding video SP antenna shown in Figure 1 (d), the folding antenna using different methods integrated into the RF circuit board.
Figure 1 folded antenna structure evolution
The structural evolution of the folded dipole antenna shown in Figure 2, the structure of the folded dipole antenna shown in Figure 3. Seen two fold lines instead of the straight line of the dipole antenna, with the folded dipole antenna, the antenna is not a closed loop, folding the end is still open. The folded dipole antenna is essentially a non-inductive resistor connected a both ends of the folded loop antenna. The folded dipole antenna with a 5:1 or 6:1 frequency characteristics, which means that it can effectively work in the design of the lowest frequency to the frequency of 5 or 6 times. For example, the folded dipole antenna design frequency of 4.9 MHz, but can work until the 25MHz ~ 29 MHz frequency range. Such open end structure provides a impedance adjusted huge space, in particular, the input impedance Xin. Need to design changes in impedance is determined by the geometric parameters of the antenna L1, L2, L3, with little bandwidth relationship.
Third, the circuit design
The folded dipole antenna terminal resistance value is very critical. This value depends on the feed point impedance, usually need to be bigger. For example, if the use of 300Ω transmission line (or use a 75Ω coaxial cable, 4:1 balun), the appropriate value is 390Ω. If the use of the 450Ω transmission line, the correct value is 500Ω. If you are using a 600Ω transmission line, the value of 650 Ω is the best. The transmission line can be 300Ω ~ 600Ω flat feeder. Flat feeder nearby metal objects affect (water pipe, metal window frames, etc.), the better the transmission line is coaxial cable impedance matching balun. The recommended values of the folded dipole antenna is to use a 75Ω coaxial cable (RG - 59 or RG - 6 coaxial cable connector to the best selection of PL259), common 4:1 balun and 390Ω termination resistors. Terminal resistance must not use wirewound type, because the self-inductance will largely affect the performance of a 1/2W 1W carbon resistors very good.
Fourth, the parameter calculation
If the minimum frequency fmin (unit kHz), the unit of length m, the antenna geometry parameters calculated as follows: from one end to the other side length: L1 = (15240/fmin) × 3.28 (m)
The spacing of two lines:
L2 = (914.4/fmin) × 3.28 (m)
If the non-directional design, the folded dipole antenna cable-stayed angle is 30 °, but within the range of 20 ° ~ 40 ° is acceptable.
Five, the experimental test
The balun can be unbalanced port into a balanced antenna, by selecting the appropriate L1, L2, L3, can obtain the required impedance. By HP8753D vector network analyzer determination of this folded dipole antenna input impedance of the prototype the Xin simulation values and measured values. Figure 4 as a function of Rin and Xin and L1. When L1 from 24mm to 52mm (operating frequency of 2.45GHz, wavelength changes by 0.196λ0 to 0.425λ0), Rin, and Xin also increased. Figure 5 for L2 is the parameters of the impedance response, Rin, and Xin is still increasing with the increase of L2 is slightly different, but the curve trend. Figure 6 for L3 for the parameters of the impedance response shows that the L3 increases, Rin, is almost unchanged, but Xin decreases rapidly. Folded dipole antenna radiation parameters and the dipole antenna is similar to the design and analysis can be carried out by IE3D software.
Through the experiment, this folded dipole antenna return loss simulation and measurement results are in good agreement, as shown in Figure 7. Therefore designed a folded dipole antenna characteristics of the design of special impedance of the antenna is very useful.
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