Electromagnetic Wave Absorption in K Band and V Band with Carbon Microcoils

Kuan-Ting Lin1, Jian-Yu Hsieh1, Tao Wang1, Cheng-Hung Li2, Neng-Kai Chang2, Shey-Shi Lu1, Shuo-Hung Chang2, and Ying-Jay Yang1 1Graduate Institute of Electronics Engineering, National Taiwan University, Taiwan, R.O.C. 2Department of Mechanica Engineering, National Taiwan University, Taiwan, R.O.C.

Abstract— In this paper, an electromagnetic wave absorption component consisting of carbon microcoils is realized. An electromagnetic wave absorber operating in K band and V band is implemented by carbon microcoils which are enwrapped in PDMS. Samples containing carbon ﬁbers and carbon microcoils with diﬀerent lengths were used as contrasts in the absorption experiment. The measured absorptions of the carbon microcoils are 15 dB (97%) at 26 GHz and 20 dB (99%) at the region from 64 to 70 GHz. The experimental results show that the carbon microcoils are superior in electromagnetic wave absorption and may be considered as a useful tool in future EMI/EMC applications.

1. INTRODUCTION With the dramatic development of wireless communication technology in recent years, the safety of radiated electromagnetic (EM) wave becomes a more and more controversial issue. Regardless of the debate that whether radio signals are harmful to human bodies, relevant works on the prevention from EM wave exposure have been kept on going. In general, the parameter adopted to evaluate the safety of a wireless device is addressed by speciﬁc absorption rate (SAR), which is the ratio (W/Kg) of absorbed EM wave power (W) to human weight (Kg). When this value is greater than 4 W/Kg, the body temperature would raise appreciably. The SAR speciﬁcation of cell phones is under 1.6 W/Kg. In industry, copper-plating process is used to screen the EM wave of the cell phone and thus the electromagnetic compatibility (EMC) problem is solved. However such process would use a huge amount of chemicals, seriously polluting the environment. On the contrary, the process for the production of carbon microcoils is cleaner, where only acetylene and catalyzer (manganese and zinc) are used. The absorption ratio of commercial products such EM wave absorbing clothes, curtain, and paint are generally above 20 dB (that is, 99%). In this paper, comparable absorption ratios in the range of millimeter-wave by using carbon microcoils are achieved.

2. EXPERIMENT The experiment setup is depicted in Fig. 1, where a transmitter and a receiver were setup to determine the EM wave absorption rate of carbon-microcoils. On the transmitter side, a signal generator (SG) is connected to a rectangular horn antenna, which is directed to the other horn antenna on the receiver side. The receiving horn would collect the transmitted power and send it

Signal Spectrum generator analyzer DUT

Horn Horn Antenna Antenna

Anechoic chamber

Figure 1: Measurement setup. 2 to the spectrum analyzer (SA) for power detection [1]. Owing to the narrower beamwidth of the horn antenna, the alignment during the whole experiment was much simplified and the cross-section area of the device under test (DUT) was also greatly reduced. This experiment was performed in an anechoic chamber to minimize the effects of signal reflections and unwanted interferences. The powders of carbon microcoils were doped in the liquid of polydimethyl siloxane (PDMS) as DUT. The structure of the DUT is shown in Fig. 2(a). After the PDMS was jellified from liquid to a soft cushion, a transparent film full of carbon-microcoils was formed and ready to be tested. The picture of the DUT is shown in Fig. 2(b), where the length, width, and thickness are 4 cm, 3 cm, and 3 mm, respectively. Note that the whole DUT were composed of four 1 cm × 1 cm dies and two 2 cm × 2 cm dies, due to the limitation (2 cm × 2 cm) of our processing capability.

PDMS Microcoils

Transparency Film

(a) (b) Figure 2: (a) The structure of the carbon-microcoils, (b) photograph of the carbon-microcoils.

Prior researches on EM wave absorption of carbon microcoils absorbers were mainly focused on the frequency range from 20 GHz to 40 GHz [2–4]. However, considering the future trend of WLAN applications (60 GHz) and automobile radar application (77 GHz) [5, 6], it is meaningful to explore the practical usages of carbon microcoils in higher frequency band. In this manuscript, the frequency bands of interest include not only the K band (from 18 GHz to 26.5 GHz) but also the V band (from 50 GHz to 75 GHz). In this experiment, a signal with a constant power was emitted from the transmitter horn antenna, passing through the DUT and then received by the receiver horn. The diﬀerence of power levels (in dBm) detected by SA before and after inserting DUT was deﬁned as the power absorption ratio.

3. EXPERIMENT RESULTS AND DISCUSSION The EM wave absorption rate of the PDMS ﬁlm with and without microcoils dopants are shown in Fig. 3, where an intrinsic PDMS layer was adopted as calibrator to exclude the contribution caused by PDMS and extract the real portion of the absorption rate contributed by carbon-microcoils. The diﬀerence between PDMS and micro-coils is smaller than 6 dB from 18 GHz to 24 GHz. Nevertheless the absorption by micro-coils becomes more dominant as the frequency is beyond 24 GHz. Moreover, the absorption power of micro-coils is PDMS Microcoils Transparency Film 15 dB larger than that of PDMS at 26 GHz. The measured results in units of both dB and percentage are summarized in Table 1. The EM wave absorption rate reaches 97% at 26 GHz which is comparable to commercial products nowadays.

Table 1: Electromagnetic wave absorption rates of carbon microcoils in K band.

Frequency PDMS Microcoils (GHz) dB % dB % 18 2.2 39.74 0.8 16.82 20 0 0.00 5.8 73.70 22 1.4 27.56 4.3 62.85 24 1.9 35.43 4.1 61.10 26 2.4 42.46 15.3 97.05 3

Furthermore, microcoils with diﬀerent lengths were measured for comparisons. By manipulating the time of growth, we were able to control the length of these coils. Two samples with the same size (2 cm ×2 cm) but diﬀerent coil lengths were tested, where one of them was grown for 5 minutes and the other was grown for 15 minutes. The measured results are shown in Fig. 4. It is observed that both DUTs seem to show a similar trend in terms of absorption response. Both absorption curves roughly exhibit a frequency independent characteristic with longer coils possessing better absorption rates. Such results might be explained by the fact that longer coils were distributed more intensely in the PDMS, and hence more energy was absorbed in them accordingly.

4 16 PDMS Long 14 Microcoil Short (dB)

r 12

10 2 power (dB) d powe ed

be 8

4 ave absorb 0 2 crowave absor crowave Mi Microw 0

17 18 19 20 21 22 23 24 25 26 27 18 20 22 24 26 28 Frequency (GHz) Frequency (GHz)

Figure 3: Electromagnetic wave absorption rate in Figure 4: Electromagnetic wave absorption rate of K band. microcoils with diﬀerent length in K band.

The measured EM absorption rates of microcoils with diﬀerent length percentage are shown in Table 2. Compared with the limited absorption rate of short coils (< 20%), the long coils are more eﬃcient. Although long coils possess a better performance in absorption than short coils, their absorption rates still do not exceed 60%. The lower absorption rate of this experiment may be attributed to the size of DUT in this experiment: the area of the samples for length comparison is 2 cm × 2 cm while that of the samples of the former experiment is 4 cm × 3 cm. It means that the EM wave absorption power is proportional to the sample area. In addition, the absorption power can be raised by stacking the samples.

Table 2: Electromagnetic wave absorbed percentage with diﬀerent microcoils’ length in K band.

Frequency Long (dB) Long (%) Short (dB) Short (%) (GHz) 18 1.85 34.69 0.17 3.84 20 0.82 17.21 −0.1 −2.33 22 1.65 31.61 0.5 10.87 24 1.25 25.01 0.86 17.96 26 3.6 56.35 0.55 11.90

For further investigation, we extended the measurement frequency range to V band. In addition, samples with carbon fibers were also tested for contrast experiment, which were mixed with PDMS shown as in Fig. 5(a). The length, width and thickness of this sample are 7.5 cm, 6.2 cm and 1.2 cm, respectively. To calibrate the effects caused by intrinsic PDMS, a pure PDMS with the same dimensions was prepared. Fig. 5(b) shows the measured results. The ability of microcoils in power absorption is evidenced by the experimental results in V band frequency range, especially from 62 GHz to 72 GHz, where the absorption rates are greater than 15 dB. The peak value is 26.6 dB at 68 GHz, corresponding to 99.8% absorption. On the contrary the absorption rate of carbon fibers is relatively small, which is below 10 dB over the band of interest and is close to the measurement results of the pure PDMS sample. The measured data are shown in Table 3. 4

28 26 Microcoils Carbonfibers 24 PDMS 22

r(dB) 20

we 18 16 14 12 10 8 6 4

Microwave Absorbed Po 2 0 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 Frequency(GHz) (a) (b) Figure 5: (a) Photographs of Pure PDMS and carbon ﬁbers, (b) electromagnetic wave absorbed in V band.

Table 3: Electromagnetic wave absorbed percentage in V band.

Frequency Microcoils Carbon ﬁbers Pure PDMS (GHz) dB % dB % dB % 50 9.2 88.0 3.8 58.3 2.5 43.8 54 8.6 86.2 5.4 71.2 2.1 38.3 58 9.9 89.8 5.2 69.8 3.3 53.2 60 12.3 94.1 4.6 65.3 3.1 51.0 64 22.3 99.4 5.6 72.5 3.1 51.0 68 26.6 99.8 6.6 78.1 3.3 53.2 72 18.1 98.5 6.6 78.1 4 60.2 76 11.7 93.2 7.2 80.9 4.4 63.7 80 11 92.1 8 84.2 5 68.4

4. CONCLUSIONS A new structure for millimeter wave absorber is proposed, which can achieve a 99% power absorption in frequency range from 64 to 70 GHz. The absorption rate of microcoils is apparently more promising in V band than in the K band and the longer coils show better absorption rates than shorter coils. It is also found that the absorbed power of carbon ﬁbers sample is limited in V band. The experimental results reveal that the microcoil is very promising as an eﬃcient absorbing material for the future EMC application in 60 GHz radio system. In addition, the fabrication of microcoil is more environment friendly compared with process of copper plating.

ACKNOWLEDGMENT This work is supported by the National Science Council of the R.O.C. under Contract NSC-96-2628- E-002-200-MY3. The authors are also very grateful for the supports from the nanometer device laboratory (NDL), Taiwan and the NTU Wireless Communication Lab for measurement support.

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