Simulation Results of Current Distributions

In the following, some modifications to observe the distribution on the test board are articulated first. Afterward, the simulated results of current distributions are given.

And then, the analysis methods in Sec. 5.3 are conducted.

In order to obtain a complete shape of standing wave, the length of board should be at least λ/2 long to make sure that peaks (maximum magnitude of current) and valleys (minimum magnitude of current) both appear at least once. Accordingly, the brass plate is elongated to be 240-mm long. Furthermore, for the sake of having an obvious standing wave, the reflection at the end of the plate should be as large as possible. As a result, the PIFA is removed to create total reflection (open boundary condition,  _L 1).

The schematic and the definition of coordinates are depicted in Fig. 5.5. The origin is set at the left-handed edge of the brass plate (the brass plate overlaps 10 mm with the ground of microstrip). To obtain the current flow to y-direction in the full-wave simulation, some loops are placed in parallel with the x-axis and enclose the brass plate.

As a consequence, the current flows can be acquired by integrating the magnetic fields

Fig. 5.6 shows the current distribution of the reference board at 2.46 GHz. As expected, the distribution has the shape of standing wave and the distances between the peaks or valleys are approximately λ/2 (61 mm) in free space. However, the radiation loss of the traveling waves propagating on the brass plate results in the decrease of peaks and valleys (The loss exists even if the brass plate is replaced with perfect electric conductor in simulation).

Subsequently, the current distributions for the different cases listed in Table 4.3 are considered. To be more specific, they are unidirectional absorber (the noise can be absorbed but will be reflected if it is excited from the opposite direction), 1^st-order reflector, 2^nd-reflector and 20-dB attenuator (the noise can be attenuated without consideration of the incoming directions). Since the resonant frequencies are slightly different for each case, hereafter, the cases with the same resonant frequencies will be put in the same figure to make comparisons.

On the one hand, for the region before the suppressors, SWRs of different cases are examined. Supposedly, for the reflective-type suppressors, their SWRs will enhance;

while for the absorptive type, their SWRs will reduce. It is noted that for the absorber and attenuator, in addition to their normal configuration, they will be put in the reverse direction (exchange the roles of port 1 and port 2) to test their ability to absorb noise regarding to the noise coming from an opposite direction.

On the other hand, for the region after the suppressors, the maximum magnitude of current is scrutinized. Since both types of suppressors can block the noise transfer efficiently, the magnitude of current after the suppressors are anticipated to be much lower compared with the reference board.

To begin with, the current distributions of the absorber and its reflective-type

counterpart (2^nd-order) at 2.46 GHz are shown in Fig. 5.7. From Fig. 5.7(a), where the current distributions before the suppressors are presented, it is obvious that, compared with the reference board, the SWR of the reflector is quite large while the SWR of the absorber is rather small. Moreover, it is worth mentioning that if the absorber is placed in reverse, its SWR will also increase. Since it is designated to dissipate the noise coming from the specified direction; hence, for the noise coming from the opposite direction, it will be reflected. Next, as for the current distributions after the suppressors shown in Fig. 5.7(b), it is apparent that the peak magnitude drops significantly when the noise suppressors are placed. Furthermore, the amount of reduction is related to their noise-suppressing abilities. For instance, the 2^nd-reflector has a better suppression level than the absorber by Table 4.3. Accordingly, in the area after the suppressors, the maximum magnitude of current is smaller for the 2^nd-reflector than that for the absorber.

Secondly, the current distributions of the attenuator at 2.425 GHz are presented in Fig. 5.8. In contrast to the unidirectional absorber, from Fig. 5.8(a), the SWRs of the attenuator and its reverse counterpart are both low, in comparison with the reference board. This phenomenon suggests that it can dissipate the noise coming from both directions.

Finally, the distribution of 1^st-order reflector at 2.47 GHz is given in Fig. 5.9. This case is chosen since it gives a very strong proof for our claim. It evidently illustrates the concept that the values of SWR are more related to the noise-blocking mechanism (reflective or absorptive) than to the suppression level. Since compared with the absorber and attenuator, the suppression level of the 1^st-order reflector is the lowest.

However, because of its reflective feature, its SWR is still high. In other word, the absorber and the attenuator can indeed achieve high suppression levels while at the

Fig. 5.5 Schematic of the elongated test board to observe SWR.

Fig. 5.6 Simulated current distribution of reference board at 2.46 GHz.

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Fig. 5.7 Simulated current distributions for the reference board, the absorber and the 2^nd-reflector at 2.46 GHz: (a) the region before suppressors and (b) the region after

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(b)

Fig. 5.8 Simulated current distributions for the reference board and the attenuator at 2.425 GHz:(a) the region before suppressors and (b) the region after suppressors.

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Fig. 5.9 Simulated current distributions for the reference board and the 1^st-reflector at 2.47 GHz:(a) the region before suppressors and (b) the region after suppressors.