Planar square multiband frequency reconfigurable ... - Semantic Scholar

The notch disappeared, but the transfer function is not totally flat, like in the simulations. The reason of this difference between simulations and measurements is caused by the imperfections of the OSSB filter. After the experiments of the OSSB modulation, the effect of the mode filter is measured as well. The SMF patchcord theoretically filters off every mode groups except one. The simulations show that, the mode filter is very effective, so it should be tested by experiments as well. The transfer function of 1 km MMF is measured, and the impact of SMF patchcord is investigated. The results of these measurements are shown in Figure 7. As it can be seen, the transfer function of the 1 km MMF has a local minimum at 1 GHz and around 3 GHz. The SMF patchcord is capable to reduce the ripple of the transfer function, and it can emphasize the local minimums, and it makes the characteristics flat. This result shows that, SMF patchcord can improve the transfer function, and it can compensate the modal dispersion effects. According to these measurements, the purposed compensator is efficient and it can reduce the effect of the modal dispersion and the chromatic dispersion respectively in mm-wave over fiber systems. 5. CONCLUSION

A dispersion compensator is purposed in this article, which applies an SMF patchcord as a mode filter, and a BPF as an OSSB filter. The OSSB filter is placed at the receiver, so this filter can work as an OSSB filter and a mode filter together. To prove its efficiency simulations and measurements are made, which show that the purposed compensator works efficiently. Both transfer function and EVM simulations show strong improvement in the connection. The compensator reduces the EVM to 15% at notch point of the transfer function. The experiments demonstrate that the purposed compensator can compensate both chromatic dispersion and modal dispersion. Therefore, the investigated compensator could be an efficient solution in mm-wave over MMF systems. ACKNOWLEDGMENT

The authors acknowledge the METAFER for funding their research. REFERENCES 1. A. Nkansah, A. Das, N.J. Gomes, P. Shen, and D. Wake, VCSELbased single-mode and multimode fiber star/tree distribution network for millimeter-wave wireless systems, In: International Topical Meeting on Microwave Photonics MWP ’06, Grenoble, 2006, pp. 1–4. 2. A.M.J. Koonen, M. Popov, and H. Wessing, Optimizing power efficiency in radio-over-fiber systems, In: 2013 IEEE Photonics Conference (IPC), Bellevue, WA, 2013, pp. 527–528. 3. X. Liang, P. Shen, A. Nkansah, J. James, and N.J. Gomes, Full downlink indoor pico-cellular network coverage using a millimeterwave over fiber system, In: Asia Pacific Microwave Conference 2009, APMC 2009, Singapore, 2009, pp. 2344–2347. 4. S. Bhattacharya, B.R. Qazi, and J.M.H. Elmirghani, On indoor mobility performance limits for an engset radio-over-fiber model using prioritized general handoff process, In: 2010 IEEE International Conference on Communications (ICC), Cape Town, 2010, 1–6. 5. H. Schmuck and R. Heidemann, Hybrid fibre-radio field experiment at 60 GHz, In: 22nd European Conference on Optical Communication, ECOC ’96, Oslo Norway, 1996, pp. 59–62. 6. T. Cseh and E. Udvary, Cost effective RoF with VCSELs and multimode fiber, In: 2011 16th European Conference on Networks and Optical Communications (NOC), Newcastle upon Tyne, UK, July 2011, pp. 60–63. 7. A. Ng’oma, Radio-over-fibre technology for broadband wireless communication system, PhD Thesis, Eindhoven University of Technology, Eindhoven, Netherlands, 2005.

DOI 10.1002/mop

8. A.M. Matarneh and S.S.A. Obayya, Bit-error ratio performance for radio over multimode fiber system using coded orthogonal frequency division multiplexing, IET Optoelectron 4 (2011), 151–157. 9. I. Gasulla and J. Capmany, RF transfer function of analogue multimode fiber links using an electric field propagation model: Application to Broadband radio over fiber systems, In: International Topical Meeting on Microwave Photonics, MWP ’06, Grenoble, France, 2006, pp. 1–4. 10. L.A. Neto, D. Erasme, N. Genay, P. Chanclou, Q. Deniel, F. Traore, T. Anfray, R. Hmadou, and C. Aupetit-Berthelemot, Simple estimation of fiber dispersion and laser chirp parameters using the downhill simplex fitting algorithm, J Lightwave Technol 31 (2013), 334–342. 11. A. Hilt, E. Udvary, and T. Berceli, Harmonic distortion in dispersive fiber-optical transmission of microwave signals, In: Proceedings of International Topical Meeting on Microwave Photonics, 2003, pp. 151–154. 12. J. Siuzdak, L. Maksymiuk, G. Stepniak, and M. Kowalczyk, On the frequency response of multimode fibers, In: IFIP International Conference on Wireless and Optical Communications Networks, WOCN ’09, Cairo, Egypt, April 2009, pp. 1–5. 13. J. Siuzdak and G. Stepniak, Influence of modal filtering on the bandwidth of multimode optical fibers, Opt Appl 37 (2007), 31–39. 14. T. Cseh and T. Berceli, Improved receiver techniques for radio over multimode fiber systems, In: 2013 18th European Conference on Network and Optical Communication, Graz, Austria, July 2013, pp. 23–26. 15. U.-S. Lee, H.-D. Jung, and S.-K. Han, Optical single sideband signal generation using phase modulation of semiconductor optical amplifier, IEEE Photonics Technol Lett 16 (2004), 1373–1375. 16. G. Yabre, Comprehensive theory of dispersion in graded-index optical fibers, J Lightwave Technol 18 (2000), 166–177.

C 2015 Wiley Periodicals, Inc. V

PLANAR SQUARE MULTIBAND FREQUENCY RECONFIGURABLE MICROSTRIP FED ANTENNA WITH QUADRATIC KOCH-ISLAND FRACTAL SLOT FOR WIRELESS DEVICES Imen Ben Trad,1 Hatem Rmili,2 Jean Marie Floch,1 Wassim Zouch,2 and Mohamed Drissi1 1 €smes 35043, Rennes, IETR, INSA, 20 Avenue Buttes des Coe France 2 Electrical and Computer Engineering Department, King Abdulaziz University, Faculty of Engineering, P.O. Box 80204, Jeddah 21589, Saudi Arabia; Corresponding author: [email protected] Received 27 May 2014 ABSTRACT: A planarprinted multiband microstrip fed antenna with reconfigurable frequency performance was designed for multistandard wireless communication systems. The antenna consists on a square shaped patch with an optimized centered Koch-Island fractal slot. The antenna allows a reconfigurability of the frequency bands by incorporating 16 PIN diodes inside the fractal slot which is highly complex. That is why short and open circuits will be used to produce frequency agility instead of RF switches (for proof of the concept). The proposed antenna is capable to switch between 15 operating frequency bands centered at 1.7, 1.77, 2.36, 2.43, 3.30, 3.61, 3.67, 3.79, 4.05, 4.34, 4.59, 5.2, 5.27, 5.47, and 5.57 GHz through four operating modes M1–M4 over the wide range 1–6 GHz. Prototypes corresponding to different modes were manufactured and characterized. Simulated and measured results are C 2015 Wiley Periodicals, Inc. Microwave Opt presented and discussed. V Technol Lett 57:207–212, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28815 Key words: fractal slot; multiband antenna; frequency reconfigurable antenna; PIN diode

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 57, No. 1, January 2015

207

1. INTRODUCTION

Frequency reconfigurable antennas are gaining great attention because of the increasing need to provide antennas able to support different standards, such as GSM standards, UMTS, GPS, WLAN, WiMAX, LTE, and so on. The ability to readjust antennas operating frequencies is considered as a very lucrative feature to meet capricious requirements in wireless communication field. PIN diodes, varactors, and MEMS switches are commonly used techniques to ensure the frequency reconfigurable characteristic. Multiple designs have been studied to acquire the frequency agility property [1–4]. For example, a frequency reconfigurable planar inverted-F antenna was designed in [1], using two PIN diodes, three reconfigurable operating bands (2.3–2.4, 2.5–2.7, and 3.4–3.6 GHz) are achieved. In [2], a printed Yagi-Uda antenna was proposed; where frequency agility is obtained by loading the driver dipole arms and four directors with varactor diodes. Six operating frequencies were obtained over the band 478–741 MHz. In [3], a planar elliptic broadband antenna with reconfigurable dual stop-bands performance over the frequency range 0.75–6 GHz was suggested. The agility was produced by loading two varactor diodes, thus allowing high selectivity of the dismissed-bands, continuous reconfiguration and wide tuning range of the notched bands. However, among many types of antennas, printed slot ones are the most investigated for the design of reconfigurable structures [5–11]. Compared to conventional antennas, these structures have interesting advantages such as operating at lower frequencies by increasing the electric length of the patch without the need to increase its physical length as well as providing more possibilities to reconfigure the antenna by integrating RF switches inside slots. In [5], a rectangular patch antenna with square slot permits to achieve three operating bands over the frequency range 10.2– 10.5 GHz using two PIN diodes (strips are used instead of real switches). A circular monopole antenna with switchable slots have been designed for the switch of five bands (2.95–10.92 GHz in UWB mode; 2.24–2.72, 3.32–3.79, and 5.15–5.9 GHz in narrowband mode; and 2.11–2.8 and 5.14–5.9 GHz in dual-band mode) [6]. Six operating frequency bands situated between 2.2 and 4.75 GHz were achieved by a monopole antenna by positioning five RF PIN diode switches in the inverted L-shaped slot [7]. In this letter, we have designed a new printed slot-antenna with multiple reconfigurable bands and wide operating range. The structure is based on a square patch antenna with centered Koch-Island fractal slot and integrated switches (inside the slot) for frequency reconfigurable property. The use of fractal slots offers the opportunity to create larger slots in limited spaces, which increases the electric length,

Figure 1 The Koch-Island fractal shaped slot: (a) Original square of length a (iteration 0); and (b) Quadratic Koch curve after iteration 1. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

208

Figure 2 Geometry of the: (a) reference antenna; (b) frequency reconfigurable antenna

TABLE 1 Summarized Obtained Simulated and Measured Frequencies Simulated Frequencies (GHz) Mode M1 M2 M3 M4

Measured Frequencies (GHz)

F1

F2

F3

F4

F1

F2

F3

F4

2.36 1.6 1.7 3.79

3.30 2.41 3.6 4.34

4.03 3.65 3.87 5.31

5.25 4.4 5.43 –

2.36 1.77 1.7 3.79

3.30 3.67 2.43 4.34

4.05 4.59 3.61 5.47

5.27 5.2 5.57 –

allowing thereby the antenna miniaturization. The frequency agility is obtained through four operating modes M1–M4 ensured by 16 PIN diodes. The number of switches is great due to the high correlation between resonant frequencies. The proposed antenna is able to achieve 15 operating bands centered at 1.7, 1.77, 2.36, 2.43, 3.30, 3.61, 3.67, 3.79, 4.05, 4.34, 4.59, 5.2, 5.27, 5.47, and 5.57 GHz over the wide frequency range 1–6 GHz, with good impedance matching and a maximum realized gain of 8.48 dBi. Hence, by applying the fractal concept to the switched slot, a novel frequency reconfigurable antenna is successfully designed and characterized to be suitable for several wireless applications.

Figure 3 Simulated return loss of the frequency reconfigurable antenna. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]


DOI 10.1002/mop

Figure 5 The prototyped structures: (a) Mode M1; (b) Mode M2; (c) Mode M3; (d) Mode M4. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

area 70 3 70 mm2. The square patch of length Lp 5 Wp 5 50 mm was fed by a microstrip line of length Lf 5 10 mm and width Wf 5 3 mm, situated at a distance X0 5 19.5 mm from left edge of the patch [Fig. 2(a)]. Next, a fractal Koch-Island slot of width s 5 2 mm was centerd inside the square patch [Fig. 2(a)]. The shape of the slot is based on the fractal Quadratic Koch-Island curve, which was obtained with an iterative process generating a repetitive geometry at different scales. Starting from a square of original length a (iteration 0), each side is divided into four equal segments of length l, then the middle two segments are pushed out in opposite directions to create new squares (iteration 1) as shown in Figure 1. By continuing the process of breaking each line segment into four and pushing out in the same way, we obtain a fractal curve, called Koch-Island, which may be described by some parameters as the fractional length l(n), the overall length L(n), the circumference C(n), and the the area A(n), depending on the initial length a and the iteration order n. These parameters are given by: n 1 a 4 1 k X 1 2 1 12 n LðnÞ5 11 L5a12a 4 3 4 k51 lðnÞ5

Figure 4 Surface current distribution at: (a) M1: 2.36 and 4.03 GHz; (b) M2: 2.41 and4.4 GHz; (c) M3: 1.7 and 3.6 GHz; (d) M4: 3.79 and 5.31 GHz. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

2. ANTENNA DESIGN

First, a square patch was printed on the top layer of a metalized duroid substrate of relative permittivity 2.2, thickness 0.8 mm, and

DOI 10.1002/mop

(1) (2)

C54ð2n aÞ

(3)

2

(4)

AðnÞ 5 a

When the number of iteration n increases, the fractional length l(n) decreases, the overall length and the circumference of the fractal curve increase, whereas the area of the island still constant. We can note also that the overall length L tends to the limit (5/3a) when the number of iterations tends to the infinity (n ! 1).


209

Figure 6 Measured and simulated return loss of the proposed antennas: (a) Mode M1; (b) Mode M2; (c) Mode M3; (d) Mode M4. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

As it can be noticed from Eq. (3), applying of the fractal concept increases further the electric length of the antenna and allows reaching lower frequency without increasing the physical length of the patch. To design the reconfigurable structure, 16 PIN diodes were loaded on the fractal slot as shown in Figure 2(b). Depending on these switches states, four main modes are obtained and summarized in Table 1. First mode M1 is reached when all switches are disabled. By only turning on switches S1 to S8, the mode M2 is activated. Then, to move to mode M3, switches S9 to S16 must be activated. The last mode M4 is obtained when all switches are enabled. TABLE 2 Measured Achieved Bandwidths Measured bandwidth (MHz) F1

Mode

Figure 7 Measured return loss of the frequency reconfigurable antenna. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

210

M1 M2 M3 M4

40 62 45 80

(1.7%) (3.5%) (2.6%) (2.1%)


F2 50 80 25 70

(1.5%) (2.1%) (1.2%) (1.6%)

F3

F4

36 (1.0%) 100 (2.1%) 50 (1.4%) 220 (4.0%)

44 (0.8%) 470 (9.0%) 90 (1.6%) –

DOI 10.1002/mop

Figure 8 Measured three-dimensional radiation pattern at: (a) 4.05GHz (mode M1); (b) 4.19 GHz (mode M2); (c) 1.71 GHz (mode M3); (d) 3.76 GHz (mode M4). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] 3. RESULTS AND DSCUSSION

3.1. Simulation The frequency reconfigurable multiband antenna was simulated with the electromagnetic software HFSS v13 where PIN diodes were modeled by strip-lines; the On/Off state of switches is illustrated by the presence/absence of the corresponding striplines. From simulated results, we have concluded that resonant frequency bands are highly correlated; which means that their electronic control is not an easy task, and may explain the high required number of RF switches. Figure 3 shows the simulated return loss of the frequency reconfigurable antenna for different modes M1–M4. We can notice that the multiband antenna can switch between 15 frequency bands through four operating modes depending on the switches states integrated in the fractal slot. When the mode M1 is enabled, the multiband antenna operates at four frequency bands centered at 2.36, 3.30, 4.03, and 5.25 GHz. Then, the activation of mode M2 causes alteration of the surface current distribution map and the current flow passes through the gap. Four resonant frequencies (1.6, 2.41, 3.65, and 4.4 GHz) are then successfully reached. When the mode M3 is established, the path of the current flow is disturbed once again and the antenna resonates at 1.7, 2.89, 3.6, 3.87, and 5.43 GHz with a good input impedance matching as it can be seen in Figure 3. Finally, for the mode M4, resonant frequencies shift to higher frequencies and the antenna resonates at 3.79, 4.34, and 5.31 GHz. To better understand the RF switches effect on the antenna behavior, simulated surface current distributions (Fig. 4) are investigated for all modes at selected resonant frequencies. The

DOI 10.1002/mop

analysis of the current repartition presented in Figure 4, proves that the multiband antenna is able to redirect the current flow due to the presence of PIN diodes and change the resonating current paths, which affect the resonant frequencies of the structure and offer the possibility to operate with four modes and switch between 15 frequency bands. The basic mode M1 is characterized by the absence of striplines between the two regions of the square patch separated by the fractal slot. The surface currents are mainly concentrated between the slot and the four edges of the rectangular patch [Fig. 4(a)]; the current flow is stopped by the outer edge of slot; therefore, the central region of the patch is not involved in radiation of the antenna. To disturb the surface current distribution on the patch, we have added progressively switches within the fractal slot. We have remarked that, by adding 2, 4, 8, 12, and 14 switches, it’s not possible to act on the current distribution. The minimum number of switches which permits to modify the currents paths by keeping the symmetry of the structure is 16. It has been remarked also that integration of PIN diodes in parts of the slot where high surface currents are localized is with no interest. In fact, to redirect the current flow, switches must be placed in regions where surface current distributions are weak, which may facilitate the establishment of new current flow paths. For M2, M3, and M4 modes, the current flow passes through the gaps, and circulates inside the central region of the patch as it can be seen from Figs. 4(b)–4(d). Then, all parts of the square patch contribute to radiation. Depending on enabled mode, the migration of current is authorized via different paths. Hence, the


211

surface current distribution is constantly altered leading to the readjustment of the resonant frequencies. 3.2. Measurements Actually, important number of required PIN diodes (16) makes their related bias network extremely complicated and difficult to implement (even impossible). So, to prove the concept, strip lines are used instead of RF switches. Four prototypes, corresponding to the four modes M1–M4, were realized and experimentally characterized. Photos of the realized prototypes are presented in Figure 5. Figure 6 depicts measured and simulated return losses of the frequency reconfigurable antenna for different modes M1–M4. An excellent matching can be observed between simulation and measurements; the targeted frequencies are successfully reached. Indeed, when the first mode M1 is excited, the obtained measured frequencies are 2.36, 3.30, 4.05, and 5.27 GHz. Then, the activation of the second mode M2 leads to the achievement of targeted frequency bands centered at 1.77, 3.67, 4.59, and 5.2 GHz as predicted and shown in Figure 6(b). Enabling the third mode, M3 alters the surface current distribution map again thus driving the antenna to radiate at 1.7, 2.43, 3.61, and 5.57 GHz with a good input impedance matching as it can be seen in Figure 6(c). Figure 7 shows the return loss of the frequency reconfigurable antenna. The establishment of the fourth mode M4, actually, makes the planar square patch antenna shifting to higher frequencies and working at 3.79, 4.34, and 5.47 GHz as presented in Figure 6(d). Table 1 summarizes the achieved simulated and measured resonant frequencies for each mode, whereas Table 3 groups the measured bandwidths. The achieved bandwidths are narrow, thus, providing very selective bands for existing telecommunication applications over the 1–6 GHz frequency range, such as WiMAX (2.3/3.5 GHz), and eventually for future telecommunication standards. The radiation patterns have been measured in different frequency bands. Obtained patterns are directive as expected and a maximum of gain ranging from 3.1 to 8.48 dBi is well achieved (Fig. 8). Therefore, it can be concluded that the proposed antenna maintains stable radiation properties and good performances while toggling between the modes M1–M4. 4. CONCLUSION

A novel planar multiband antenna with centered fractal slot and frequency reconfigurable characteristic is designed and manufactured for wireless communication devices. By introducing 16 PIN diodes inside the slot, four modes M1, M2, M3, and M4 can be established, hence allowing the achievement of 15 operating frequencies while maintaining stable radiation properties. The maximum realized gain can reach 8.48 dBi. ACKNOWLEDGMENT

This article was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah. The authors, therefore, acknowledge with thanks DSR technical and financial support.

3. I.B. Trad, J.M. Floch, H. Rmili, L. Laadhar, and M. Drissi, Planar elliptic broadband antenna with wide range reconfigurable narrow notched bands for multi-standard wireless communication devices, Prog Electromagn Res 145 (2014), 69–80. 4. N. Ramli, M.T. Ali, A.L. Yusof, and N. Ya’acob, Frequency reconfigurable stacked patch microstrip antenna (FRSPMA) for LTE and WiMAX applications, In: Proceedings of Computing Management and Telecommunications (ComManTel), 2013, pp. 55–59. 5. G. Singh and M. Kumar, Novel frequency reconfigurable microstrip patch antenna based on a square slot for wireless devices, In: International Conference on Communication Systems and Network Technologies (CSNT), 2012, pp. 27–30. 6. H. Boudaghi, M. Azarmanesh, and M. Mehranpour, A frequencyreconfigurable monopole antenna using switchable slotted ground structure, IEEE Antennas Wireless Propag Lett 11 (2012), 655–658. 7. H.A. Majid, M. Kamal A. Rahim, M.R. Hamid, and M.F. Ismail, A compact frequency-reconfigurable narrowband microstrip slot antenna, IEEE Antennas Wireless Propag Lett 11 (2012), 616–619. 8. S.A. Hamzah, M. Esa, N.N.N.A. Malik, and M.K.H. Ismail, Reconfigurable harmonic suppressed fractal dipole antenna, In: AsiaPacific Microwave Conference Proceedings APMC 2012, Kaohsiung, Taiwan, pp. 800–805. 9. B.A. Cetiner, G.R. Crusats, L. Jofre, and N. Bıyıklı, RF MEMS integrated frequency reconfigurable annular slot antenna, IEEE Trans Antennas Propag 58 (2010), 626–632. 10. C.-Y. Chiu, J. Li, S. Song, and R.D. Murch, Frequency-reconfigurable pixel slot antenna, IEEE Trans Antennas Propag 60 (2012), 4921–4924. 11. K.-H. Chen, S.-J. Wu, C.-H. Kang, C.-K. Chan, and J.-H. Tarng, A frequency reconfigurable slot antenna using PIN diodes, In: AsiaPacific Microwave Conference Proceedings APMC, Singapore, 2009, pp. 1930–1933. C 2015 Wiley Periodicals, Inc. V

X-BAND 100 W SOLID-STATE POWER AMPLIFIER USING A 0.25 lM GAN HEMT TECHNOLOGY Dong Min Kang, Jong Won Lim, Ho Kyun Ahn, Sung Il Kim, Hae Cheon Kim, Hyung Sup Yoon, Yong Hwan Kwon, and Eun Soo Nam RF Convergence Components Research Team, Electronics and Telecommunications Research Institute, 218 Gajeong-ro, Yuseonggu, Daejeon 305-700, Korea; Corresponding author: [email protected] Received 27 May 2014 ABSTRACT: This article describes the successful development and the performance of X-band 100 W pulsed SSPA using a 25 W GaN-on-SiC high electron mobility transistor (HEMT). The GaN HEMT with a gate length of 0.25 mm and a total gate width of 8 mm were fabricated. The GaN HEMT provide a linear gain of 8 dB with 25 W output power operated at 30 V drain voltage in continuous wave (CW)-operation with a power added efficiency of 43% at X-band. It also shows a maximum output power density of 3 W/mm. The X-band pulsed SSPA exhibited an output power of 100 W (50 dBm) with a power gain of 53 dB in a frequency range of 9.2–9.5 GHz. This 25 W GaN HEMT and X-band 100 W pulsed SSPA are suitable for the radar systems and related applicaC 2015 Wiley Periodicals, Inc. Microwave Opt Technol tions in X-band. V Lett 57:212–216, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28814

REFERENCES 1. J.-H. Lim, C.-W. Song, Z.-J. Jin, and T.-Y. Yun, Frequency reconfigurable planar inverted-F antenna using switchable radiator and capacitive load, IET Microwaves Antennas Propag 7 (2013), 430–435. 2. Y. Cai, Y.J. Guo, and T.S. Bird, A frequency reconfigurable printed yagi-uda dipole antenna for cognitive radio applications, IEEE Trans Antennas Propag 60 (2012), 2905–2912.

212

Key words: X-band; GaN; HEMT; SSPA 1. INTRODUCTION

Radar is an object-detection system that uses radio waves to determine the range, altitude, direction, or speed of objects. It


DOI 10.1002/mop