A WIDEBAND WIDE-STRIP DIPOLE ANTENNA FOR CIRCULARLY POLARIZED WAVE OPERATIONS

Transcription

1 Prgress In Electrmagnetics Research, PIER 1, 69 82, 21 A WIDEBAND WIDE-STRIP DIPOLE ANTENNA FOR CIRCULARLY POLARIZED WAVE OPERATIONS L.-P. Chi and S.-S. Br Department f Electrical Engineering Feng-Chia University Taichung, Taiwan S.-M. Deng, C.-L. Tsai, P.-H. Juan, and K.-W. Liu Department f Electrnic Engineering Ming-Chuan University Taipei, Taiwan Abstract A thin diple antenna is a well-knwn antenna with linearly plarized wave peratin. In this wrk, a wide-strip diple antenna is prpsed fr circularly plarized wave peratins. T btain circularly plarized (CP) wave peratins, there are tw cnditins t be satisfied. One is that the antenna must have tw degenerated rthgnal mdes with different resnant frequencies. The ther is that the phase difference f tw rthgnal mdes is 9 degrees. T match the first cnditin, the slab width W is tuned t generate current distributins directed in tw different directins. In additin, the secnd cnditin is matched by asymmetric feeding pint by adjusting the verlapped square width C. The parametric study is cmpleted by the Ansft HFSS simulatr. Simulated results reveal that the CP wave is mainly influenced by the slab width W. The influences f the parameters C and d n the perfrmances f the prpsed antenna are als investigated in this paper. Taking 8 db as reference, there are tw wrking bands fr this prpsed antenna and the measured center frequencies are.66 GHz and 2.4 GHz, respectively, and the crrespnding bandwidths are.27 GHz (4%) and 1.78 GHz (87%), respectively. In additin, the measured center frequencies and bandwidths f the axial rati are 1.94 GHz and.53 GHz (27%), respectively. Crrespnding authr: L.-P. Chi (cp53122@ms23.hinet.net).

2 7 Chi et al. 1. INTRODUCTION Generally, a balanced-fed strip diple antenna r mnple antenna wns the linear plarizatin, mni-directinal radiatin pattern in the H-plane, and abut 2 dbi antenna gain. It has been used widely in the cnsumer prducts fr its gd characteristics and lw cst. Such facts have been cmprehensively reprted by sme papers [1 1]. On the ther hand, the circularly plarized (CP) antennas applied t the mdern cmmunicatin becme mre and mre ppular nt nly in the satellite cmmunicatin but als in the territrial cmmunicatins. Mrever, antennas with circular plarizatin can be used in radi frequency identificatin (RFID) systems and glbal psitining system (GPS) t reduce pwer lss resulted frm plarizatin mismatch. T btain circularly plarized wave peratins, there are tw cnditins t be satisfied. One is that the antenna must have tw degenerated rthgnal mdes with different resnant frequencies. The ther is that the phase difference f tw rthgnal mdes is 9 degrees. The ften utilized technique t generate CP wave in the micrstrip antenna is feeding asymmetrically in the micrstrip patch t generate tw degenerated mdes and t cmplete impedance matching [11 2]. Hwever, it is impssible t generate a circularly plarized wave in a single diple antenna because nly z-directed current distributin exists n the thin cylindrical cnductr. In recent, sme papers [21 23] try t intrduce circularly plarized radiatin by mdifying the cnventinal diple antenna. Tw diples [21, 22] are fed by tw transmissin lines, individually, with a ninety-degree phase difference and the same amplitude t prduce the circular plarizatin prperty. In paper [23], the circularly plarized peratin is achieved by cmbining a diple antenna with an artificial grund plane. In additin, the C-type feeding technique [24, 25] is a useful technique fr imprving the axial rati bandwith and quality f CP stacked micrstrip antennas. Hwever, t the authrs best knwledge, nly a few attempts have s far been made t generate circularly plarized wave by mdifying the cnventinal thin diple antenna. In this paper, the authrs try t mdify the riginal strip diple by increasing the width f the strip and bserve the behavir f plarizatin. The purpse f this prcess is t generate tw different directed current distributins, which are perpendicular t each ther. Finally, the authrs find that a wide-strip diple antenna can generate the circular plarizatin characteristics. Althugh a wide-strip diple antenna can generate the circularly plarized wave, we want t analyze and explain the reasn f it by the current distributins in the widestrip metal. Just fr balanced feed, nly ne strip s current distributin

3 Prgress In Electrmagnetics Research, PIER 1, is bserved. The current distributin can be divided int tw parts; ne is in the x-directin and the ther is in the y-directin. Owing t the asymmetric feed pint in the wide strip, this antenna can radiate circularly plarized wave. 2. ANTENNA CONFIGURATIONS A strip diple antenna with tw symmetric strips is shwn in Fig. 1. The length (in the x-directin) and width (in the y-directin) f the strip are L and W, respectively. There is a gap d (in the z-directin) between tw strips and the verlap dimensins between tw strips in the x- and y-directins are bth C. The Ansft HFSS high frequency simulatr based n the finite element methd is used as the simulatin tl. Gap surce is arranged between tw verlap regins. The feeding pint is at the center f the verlap regin. Frm the pint f view f simulatin, the antenna cnfiguratin with a gap surce is a symmetrical structure. It has a symmetrical radiatin patterns. But in L C W C W z y L L (a) x d L z C Figure 1. Gemetrical cnfiguratin f a wideband wide-strip diple antenna fr circularly plarized wave peratins: (a) Tp view; (b) side view. (b) x y

4 72 Chi et al. the real sample, a caxial cable is needed t feed the RF pwer int the tw strips. Hence, a rigid mini-caxial cable with a radius f.6 mm is adpted t feed int the tw strips. And then, ne strip is cnnected t the inner cnductr f the cable, and the ther strip is cnnected t the uter cnductr f the cable. Althugh the real sample is nt a gd balanced feed, the results f the reflectin cefficient and axial rati as well as the radiatin patterns match well with thse f the simulatin data. 3. RESULTS 3.1. Reflectin Cefficient After the ptimizatin f the antenna parameters, the ptimal data fr this structure are: L = 1 mm, W = 5 mm, d = 1 mm, C = 15 mm, respectively. The measured and simulated reflectin cefficients are shwn in Fig. 2. Taking 8 db as reference, there are tw wrking bands fr this prpsed antenna and the measured center frequencies are.66 GHz and 2.4 GHz, respectively, and the crrespnding bandwidths are.27 GHz (4%) and 1.78 GHz (87%), respectively. In additin, the simulated center frequencies are.68 GHz and 2.2 GHz, respectively, and the crrespnding bandwidths are.23 GHz (33%) and 1.75 GHz (86%), respectively. Bth simulated and measured reflectin cefficients are in gd cnsistency and the wide impedance bandwidth is achieved in this prpsed antenna. As shwn in Fig. 2, it seems that there exist multi bands in this prpsed Return Lss (db) Measurement Simulatin Figure 2. Measured and simulated return lss f the prpsed wide strip diple antenna with parameters: L = 1 mm, W = 5 mm, d = 1 mm, and C = 15 mm. Axial Rati (db) Measurement 11 1 Simulatin Figure 3. Measured and simulated axial ratis f the prpsed wide strip diple antenna with the same parameters in Fig. 2.

5 Prgress In Electrmagnetics Research, PIER 1, antenna. This fact results frm the reasn that there are multiple current paths in the wide strip Axial Rati T verify whether r nt the prpsed antenna can radiate circularly plarized wave, the axial ratis (AR) in the z-directin fr the simulated and measured data are illustrated in Fig. 3. As shwn in this figure, taking 3 db as reference, the measured and simulated center frequencies are 1.94 GHz and GHz, respectively, and the crrespnding AR s bandwidths are.53 GHz (27%) and.44 GHz (23%), respectively. Bth simulated and measured axial ratis match well and the prpsed antenna is a wide-ar bandwidth antenna due t the 27% axial rati bandwidth. Hence, the prpsed antenna wns bth wide impedance and wide axial rati bandwidths. As shwn in Fig. 3, the prpsed antenna can radiate circularly plarized wave in the z-directin. T realize the circularly plarized characteristics in different angles, the simulated axial ratis at 1.94 GHz in the x- z and y-z plane are shwn in Fig. 4. Taking 3 db as reference, the circularly plarized wave ranges frm 18 degrees t 6 degrees and frm 132 degrees t 16 degrees in the x-z plane. Mrever, the circularly plarized wave ranges frm 16 degrees t 14 degrees and frm 168 degrees t 162 degrees in the y-z plane. Owing t large acceptable angles fr circularly plarized radiatin, this prpsed antenna is suitable fr circularly plarized peratin y-z plane Axial Rati x-z plane Axial Rati Figure 4. Simulated axial ratis f the prpsed wide strip diple antenna at 1.94 GHz.

6 74 Chi et al Radiatin Patterns The measured and simulated radiatin patterns at 1.94 GHz in the y-z and x-z plane are shwn in Fig. 5 and Fig. 6, respectively. T bserve the circularly plarized radiatin, the radiatin patterns are divided int a right-hand circularly plarized (RHCP) wave and a left-hand circularly plarized (LHCP) wave. The tw figures indicate that the frward radiatin is dminated by LHCP and the backward radiatin is dminated by RHCP, respectively Measurement 15 RHCP LHCP Simulatin 15 Figure 5. Measured and simulated y-z plane radiatin patterns f the prpsed wide strip diple antenna at 1.94 GHz Measurement RHCP LHCP Simulatin 15 Figure 6. Measured and simulated x-z plane radiatin patterns f the prpsed wide strip diple antenna.

7 Prgress In Electrmagnetics Research, PIER 1, z 45 y x Figure 7. Current distributins at.65 GHz. ο z 45 y x ο ο ο Figure 8. Current distributins at 1.94 GHz Current Distributins In rder t analyze why the prpsed antenna has the circular plarizatin characteristics, the current distributins in nly ne metal strip at.65 GHz and 1.94 GHz are pltted in Fig. 7 and Fig. 8, respectively. As shwn in Fig. 7, mst f the current distributins are directed in the y directin fr fur different phase angles ωt =, 45, 9, and 135, respectively. Hence, the prpsed antenna generates linearly plarized radiatin in the lwer frequency.65 GHz. Hwever, at ωt = as shwn in Fig. 8 fr the upper frequency 1.94 GHz,

8 76 Chi et al. the current distributins can be divided int tw majr parts: ne is the x-directinal current flwing in the center strip t the end, and the ther is the y-directin current flwing frm the feeding pint t the end f width in the upper regin. But the dminant current distributin is directed in the x directin. The equivalent length f the current trace in the x-directin seems t be a half wavelength and that in the y-directin t be a quarter wavelength. At ωt = 45, the current distributins revealed in Fig. 8 bviusly wn tw dminant parts. Hwever, the current distributins at ωt = 9 shwn in Fig. 8 are dminant in the y-directin. Hence, Fig. 8 demnstrates that the currents are dminant in rthgnal current s directin with phase difference f ninety degrees. As a cnsequence, the circular plarized wave will be generated t prpagate int free space. Generally, the circularly plarized wave can be divided int tw types: ne is resnance mde and the ther is traveling-wave type. The prpsed antenna can be judged as the resnant type. It has tw resnance mdes: ne is half-wavelength resnance mde and the ther is a quarter-wavelength mde. Hence, the cncept f CP with tw resnance mdes (ne is half-wavelength and the ther is quarterwavelength) will give the antenna designer a very clear idea t design the wide-strip diple antenna with CP characteristics. As shwn in Fig. 8, it is bvius that the currents at any pint in the metal strip rtate clckwise s that the frward radiatin is dminated by LHCP and the backward radiatin is dminated by RHCP (see Sectin 3.3). 4. PARAMETRIC STUDY AND ANALYSIS 4.1. The Influences f the Slab Width W Althugh we have knwn the cncept f hw t generate the CP wave in the prpsed antenna, we still need t study and analyze the influences f the antenna parameters (such as the width and length f the strip, the feeding pint, and the verlap between bth metal strips) n the antenna characteristics such as impedance bandwidth and AR bandwidth. The parametric study is cmpleted by the Ansft HFSS simulatr. Firstly, the same parameters are taken as thse in Fig. 2 except the parameter W. If the slab width W is increased, the return lss shwn in Fig. 9 is imprved, which imply the better impedance matching is btained. In additin, Fig. 1 indicates that the increasing f the slab width W results in the circularly plarized radiatin. The cnventinal diple antenna can be regarded as a special case f the prpsed antenna if the slab width W is appraching zer. The cnventinal diple antenna radiates linearly plarized wave, because the current distributins n the cnductr are nly directed in a single

9 Prgress In Electrmagnetics Research, PIER 1, directin. Hwever, there are tw rthgnal current cmpnents n the prpsed antenna and the circularly plarized wave can be btained by increasing the slab width W. Mrever, the perfrmances f different slab width W are shwn in Table 1. The symbl NA in the Table 1 dentes that the axial-ratis are greater than 3 db fr all frequencies. Return Lss (db) W=5 mm W=4 mm W=3 mm Figure 9. Simulated return lss fr different slab width W. Axial Rati (db) W=5 mm W=4 mm W=3 mm Figure 1. Simulated axial ratis fr different slab width W. Table 1. The perfrmances f the prpsed antenna in the variatin f the parameter W. W (mm) AR Bandwidth (MHz) AR Center 3 NA NA NA 4 NA NA NA Bandwidth (%) Table 2. The perfrmances f the prpsed antenna in the variatin f the parameter C. C (mm) AR Bandwidth (MHz) AR Center Bandwidth (%)

10 78 Chi et al The Influences f the Overlapped Square Width C In this subsectin, the same parameters are taken as thse in Fig. 2 except the parameter C. As shwn in Fig. 11, the impedance matching is imprved if the verlapped square width C is increased. In additin, Fig. 12 and Table 2 indicate that the increasing f the verlapped square width C has little influence n the axial rati. Return Lss (db) C=15 mm C=1 mm -25 C=5 mm Figure 11. Simulated return lss fr different verlapped square width C. Axial Rati (db) C=15 mm 12 C=1 mm 11 1 C=5 mm Figure 12. Simulated axial ratis fr different verlapped square width C. Return Lss (db) d=1 mm d=12 mm -3 d=5 mm Figure 13. Simulated return lss fr different prt length d. Axial Rati (db) d=1 mm d=12 mm d=5 mm Figure 14. Simulated axial ratis fr different prt length d The Influences f the Prt Length d In this subsectin, the same parameters are taken as thse in Fig. 2 except the parameter d. As shwn in Fig. 13, the prt length d can be

11 Prgress In Electrmagnetics Research, PIER 1, Table 3. The perfrmances f the prpsed antenna in the variatin f the parameter d. d (mm) AR Bandwidth (MHz) AR Center Bandwidth (%) tuned t imprve the impedance matching. In additin, Fig. 14 and Table 3 indicate that the increasing f the prt length d can widen the AR s bandwidth. 5. CONCLUSION Thin diple antenna is a well-knwn antenna with linearly plarized wave peratins. In this paper, a wide-strip diple antenna is prpsed fr circularly plarized wave peratins. T btain circularly plarized wave peratins, there are tw cnditins t be satisfied. One is that the antenna must have tw degenerated rthgnal mdes with different resnant frequencies. The ther is that the phase difference f tw rthgnal mdes is 9 degrees. T match the first cnditin, the slab width W is tuned t generate current distributins directed in tw different directins. In additin, the secnd cnditin is matched by asymmetric feeding pint by adjusting the verlapped square width C. The parametric study is cmpleted by the Ansft HFSS simulatr. Simulated results reveal that the CP wave is mainly influenced by the slab width W. The studies f parameters C and d are als invested in this paper. There are tw wrking bands fr this prpsed antenna and the measured center frequencies are.66 GHz and 2.4 GHz, respectively, and the crrespnding bandwidths are.27 GHz (4%) and 1.78 GHz (87%), respectively. In additin, the measured center frequencies and bandwidth f axial ratis are 1.94 GHz and.53 GHz (27%), respectively. ACKNOWLEDGMENT The authrs are indebted t Jia-Rung Lu fr his assistance in simulatin and t Chung-sheng institute f Science and Technlgy fr financial and measurement supprt.

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