Circularly Polarized Aperture Coupled Microstrip Antenna with Resonant Slots and a Screen

Transcription

1 RADIOENGINEERING, VOL. 19, NO. 1, APRIL Circularly Polarized Aperture Coupled Microstrip Antenna with Resonant Slots and a Screen Georgi S. KIROV 1, Desislava P. MIHAYLOVA 2 Dept. o Radio Engineering, Technical University o Varna, Studentska Str. 1, 91 Varna, Bulgaria 1 gkirov@abv.bg, 2 de_c@abv.bg Abstract. A broadband circularly polarized (CP) Aperture Coupled Microstrip Antenna (ACMSA) is described herein. In order to decrease the back radiation o the antenna due to resonant coupling slots (a cross-slot) in the ground plane, a three-layer structure with a screen is proposed. As a result, the back radiation o the antenna is reduced by more than 12 db and its gain is increased by about 1.3 db compared to the conventional two-layer ACMSA with nonresonant coupling slots. The antenna is designed to operate within the Ku-band. Keeping its simple and compact construction and high mechanical characteristics it can be used as an element o CP microstrip antenna arrays with various applications in the contemporary communication systems. A comparison with two similar CP antennas with resonant slots, a two-layer ACMSA and a three-layer ACMSA with a patch relector is accomplished. Keywords Aperture coupled microstrip antenna, back radiation reduction, circularly polarized microstrip antenna, cross-slot coupled, microstrip antenna with resonant slots. 1. Introduction The microstrip antennas (MSAs) are the most rapidly developing area in the antenna ield in the last years due to their light weight, low volume, thin proile coniguration and low abrication cost. Because o these advantages they are widely used in the contemporary communication systems such as personal communication systems, mobile satellite communications, direct broadcast television, wireless local area networks, etc Additionally in these applications the circular polarization (CP) contributes to the high link s reliability and spectral eiciency [1]. The main disadvantage o the CP MSAs is their narrow bandwidth characteristic. The impedance bandwidth o a typical microstrip patch antenna, deined by 1 db return loss, is several percent [2], while its 3 db - axial ratio (AR) bandwidth is less than 1% [3]. Since the AR bandwidth o the CP antenna is usually less than its impedance and pattern bandwidths it limits the requency bandwidth o the antenna. Thus the approach to enhance the bandwidth o the CP MSA consists o a broadbanding o its AR bandwidth. There are some methods to enlarge the AR bandwidth o the CP MSA: 1) By multiple resonances due to parasitic patches [4]; 2) By means o complex (usually balanced) eeds [4] [6]; 3) Using an air gap or a oam with low dielectric constant as a patch substrate [3] [6]; 4) By using photonic bandgap structures and other approaches. The CP MSAs designed according to the methods listed above have good electrical parameters but possess a complex coniguration and low mechanical characteristics. The goal o this study is a design o a broadband CP MSA with simple and compact coniguration and high electrical and mechanical characteristics. In [7] a broadband two-layer CP aperture coupled MSA (ACMSA) with a resonant cross-slot is investigated. An aperture coupled eed is chosen because it allows an independent optimization o the both parts o the antenna the eed part and the radiating one. The use o a resonant cross-slot enlarges additionally the AR bandwidth (bw AR ) o the antenna. The result obtained in [7] or this parameter is bw AR = 3.24% in Ku-band. Unortunately the investigated antenna has a high level o back radiation (BR) about (9-1) db. In [8] the BR level o a CP ACMSA is reduced using a three-layer ACMSA with an additional third substrate and relector. The above approach is not suitable or a CP ACMSA with a resonant cross-slot because it reduces the antenna BR level in Ku-band by only to 9 db but also decreases its AR bandwidth by 2% (rom 3.24% to 2.9%) [8]. In order to reduce the BR level due to the resonant cross-slot a three-layer CP ACMSA with a screen is designed and investigated by simulation in this study. 2. Description o the Antenna Fig. 1 shows the geometry o the antenna with a screen and its dimensions are listed in Tab. 1. Three substrates are used in the antenna coniguration as ollows: patch substrate Taconic TLX-7: ε rp = 2.6, tanδ P =.19; eed substrate Taconic RF-6A: ε r = 6.1, tanδ =.28; screen substrate Arlon AD 6: ε rs = 6.1, tanδ S =.3.

2 112 G. S. KIROV, D. P. MIHAYLOVA, CIRCULARLY POLARIZED APERTURE COUPLED MICROSTRIP ANTENNA WITH h P t S L a2 L S t g L a1 W a2 Cross-slot Feed substrate Microstrip eed line Ground t W a1 Fig. 1. Geometry o the antenna with a screen: (a) cross section, (b) ront view. b) In order to obtain a maximum radiation eiciency and requency bandwidth a thick patch substrate with a low dielectric constant is chosen, while the eed substrate is thin with a high dielectric constant or maximum eed eiciency. The insertion o a second resonance (except the irst resonance connected to the patch) due to the resonant cross-slot enlarges the requency bandwidth o the antenna more than two times in comparison with the conventional two-layer CP ACMSA [9]. The higher BR level due to the resonant slot is limited by a metal screen. The thickness h S o the screen substrate and the value o its dielectric constant ε rs are chosen in compliance with the ollowing undamental phase condition ( 2 / )( h t h ) / 2 a z x a Patch Patch substrate a) x where λ a is the corresponding equivalent wavelength in the space between the ground and the screen. Then ra ( h t hs ) / 4, (2) r h t rs hs / 4 (3) where λ is the ree space wavelength. W Screen substrate W P S Screen y h S t P h y a L P (1) Dimension [mm] Description a 3 Antenna Length (Width) L p.6 Patch Length K p 1.3 Patch Ratio L p /W p L a 4.8 Average Aperture Length W p L p /K p Patch Width K s 1.2 Slot Ratio L a1 /L a2 L s 1.6 Stub Length L a1 2L a K s /(K s +1) Aperture 1 Length W a1 L a1 /1 Aperture 1 Width L a2 2L a /(K s +1) Aperture 2 Length W a2 L a2 /1 Aperture 2 Width W.77 Feed line Width h p 1.7 Patch substrate Thickness t p, t s.3 Patch and Screen Thickness h.63 Feed substrate Thickness t g, t.17 Ground plane and Feed line Thickness h s 1.9 Screen substrate Thickness Tab. 1. Dimensions o the antenna with a screen. The radiation mechanism o the antenna is the ollowing: the electromagnetic energy provided by the eed microstrip line penetrates via the cross-slot into the patch resonator ormed by the patch and the ground. By means o a suitable choice o the patch and slot dimensions the both orthogonal modes TM 1 and TM 1 excited in the resonator obtain equal amplitudes and phase quadrature. The energy o the so ormed CP electromagnetic ield is radiated into space in the broadside direction o the antenna. In this case the let-hand CP corresponds to a copolar radiation, while the cross-polar one is presented by the right-hand CP. The metal screen relects the BR energy rom the resonant cross-slot in the direction o the basic radiation energy lux o the antenna improving its gain. The results obtained in this study are compared with the results o two similar MSAs designed in the same requency range: The irst antenna is a two-layer CP ACMSA with a resonant cross-slot investigated in [7]. Its coniguration diers rom the antenna coniguration shown in Fig. 1 only in the absence o a screen substrate and a screen. The antenna uses the same substrates and has the ollowing dimensions and parameters obtained by an optimization technique: L P = 6.1 mm, K P = 1.14, K S = 1.6 and L S = 1.2 mm. The other dimensions are the same as in Tab. 1. The second antenna is obtained rom the irst one by addition o a relector substrate (identical with the screen substrate rom Fig. 1) and the patch relector [8]. The relector is a square patch with length (width) L r = W r = 1. mm, centered below the radiation patch. The antenna diers rom the antenna with a screen only in the dimension L P =. mm. All dimensions and parameters o the antenna with a relector are also obtained by an optimization.

3 RADIOENGINEERING, VOL. 19, NO. 1, APRIL Simulation Two steps are accomplished in the design o the antenna: the irst step is a suitable choice o the substrates with standard values o their thickness and dielectric constant; the second one consists in an optimization by simulation o the rest antenna dimensions and parameters. As criteria in the optimization the AR bandwidth and the BR level are chosen. The simulation o the antenna model is carried out by the sotware package CST Microwave Studio. The inal results are veriied once again using the Ansot sotware HFSS 1. Return Loss [db] Five independent dimensions and parameters are chosen in the optimization: 1) Patch length L P : it inluences on the patch resonance; 2) Patch ratio K P = L P /W P : it acts on the amplitude and phase conditions necessary to obtain CP operation; 3) Average aperture length L a : this dimension determines the slot resonance; 4) Slot ratio K S = L a1 /L a2 : this ratio acts on the conditions required or CP operation; ) Stub length L S : it inluences on the impedance matching and the requency bandwidth o the antenna. There are still three dimensions W P, L ai, i = 1, 2 and W ai, i = 1, 2 related to L P and K P, L a and K S, L ai, i = 1, 2, respectively by corresponding interrelations given in Tab. 1. Several iterations are done to ind the optimum values L P =.6 mm, K P = 1.3, L a = 4.8 mm, K S = 1.2 and L S = 1.6 mm denoted in bold in Tab Fig. 2. Return Loss versus requency o the three antenna models (dotted two-layer antenna, dashed antenna with a relector, bold antenna with a screen). Axial Ratio [db] Numerical Results Figs. 2 to show the main simulated electrical characteristics o the antenna with a screen (bold). For comparison the same characteristics o a two-layer antenna (dotted) and an antenna with a relector (dashed) are also shown in the igures Fig. 3. Axial Ratio AR versus requency o the three antenna models (dotted two-layer antenna, dashed antenna with a relector, bold antenna with a screen). Fig. 2 shows the Return Loss (the module o the relection coeicient S 11 ) o the three antennas. The irst resonance at the lower requency is that o the patch and the second one at the higher requency is that o the crossslot. It must be noted that these two resonances are not proper resonances but mutual resonances o the patch and the cross-slot. Fig. 3 shows the axial ratio AR versus requency o the three antennas. The AR bandwidths deined rom this igure are the inal bandwidths o the antennas. Back Radiation [db] Fig. 4 shows the BR level versus requency o the antennas. This is the most important characteristic in this study. Fig. shows the gain G versus requency o the antennas. It is seen rom the igure that the antenna with a screen provides higher gain than the both other antennas, which is due to the presence o the screen in its construction Fig. 4. Back Radiation BR versus requency o the three antenna models (dotted two-layer antenna, dashed antenna with a relector, bold antenna with a screen).

4 114 G. S. KIROV, D. P. MIHAYLOVA, CIRCULARLY POLARIZED APERTURE COUPLED MICROSTRIP ANTENNA WITH Gain [dbi] Fig.. Gain G versus requency o the three antenna models (dotted two-layer antenna, dashed antenna with a relector, bold antenna with a screen). The basic electrical parameters o the three antennas are summarized in Tab. 2. In the table the central requency and the 3 db-relative requency bandwidth are deined as ollows ( AR).( min( AR) max( AR) ), (4) Electrical Characteristic max( AR) min( AR) bw ( AR) 2 1, %. () max( AR) Two-layer Antenna min( AR) Antenna with a Relector Impedance Bandwidth Antenna with a Screen min [GHz] max [GHz] [GHz] BW [GHz] bw [ %] Axial Ratio Bandwidth, Back Radiation and Gain min_ar [GHz] max_ar [GHz] AR [GHz] BW AR [GHz] bw AR [ %] BR min [db] BR max [db] G min [dbi] G max [dbi] Tab. 2. Electrical characteristics o the three antenna models. The subscripts min and max or the BR level and the gain G are not related to the minimum and maximum requencies min(ar) and max(ar) but correspond to their minimum and maximum values in the antenna requency bandwidth. As seen rom the table, the design with a screen has a considerable advance regarding the antenna back radiation. Co- and cross-polar radiation patterns in φ = 4 - plane o the three antennas are displayed in Figs. 6 to 8. Radiation pattern (dbi); Phi = 4 deg Radiation pattern [dbi]; Phi = 4 deg Fig. 6. Radiation pattern o the two-layer antenna in φ = 4 -plane (bold co-polar, dashed cross-polar), = GHz Fig. 7. Radiation pattern o the antenna with a relector in φ = 4 -plane (bold co-polar, dashed cross-polar), = 11.9 GHz. Finally, the co-polar radiation pattern o the antenna with a screen in φ = 9 - plane at requency = 12.1 GHz carried out by means o the sotware package CST Microwave Studio (bold) and the Ansot sotware HFSS 1 (dashed) is shown in Fig. 9. A good agreement between the both curves is seen. In particular, there is a ull coincidence between the two simulations in the ront hemisphere, which indicates a high credibility o the obtained results.

5 RADIOENGINEERING, VOL. 19, NO. 1, APRIL Radiation pattern [dbi]; Phi = 4 deg Fig. 8. Radiation pattern o the antenna with a screen in φ = 4 -plane (bold co-polar, dashed cross-polar), = 12.1 GHz. Radiation pattern [dbi]; Phi = 9 deg Fig. 9. Radiation pattern o the antenna with a screen in φ = 9 -plane (bold CST Microwave Studio, dashed HFSS 1), = 12.1 GHz.. Points o Discussion The antennas with a relector and a screen may be obtained rom the two-layer antenna by addition o a relector or a screen, respectively. Because o this reason the two-layer antenna is chosen as a basis or comparison between the three antenna conigurations in this section. Comparing the three optimized antenna models the ollowing eatures have been ascertained: The two-layer CP antenna possesses a 3.24% bandwidth with a BR level less than 9.4 db; The presence o the patch relector decreases the bandwidth by 2% (rom 3.24% to 2.9%) with a BR level less than 14.4 db (a decrease by db); The antenna with a screen keeps almost the same bandwidth (a decrease by only 2.%, rom 3.24% to 3.16%) with a BR level less than 22.3 db (a decrease by 12.9 db); The patch relector improves insigniicantly the antenna gain by about.3 db (rom 4.9 db to.2 db), while the screen increases the same parameter by about 1.3 db (rom 4.9 db to 6.2 db). The patch relector and the screen inluence insigniicantly on the cross-polarization characteristics o the antenna. 6. Conclusion A circularly polarized aperture coupled microstrip antenna with resonant slots and a screen with simple and compact coniguration and high electrical and mechanical characteristics is designed. It has a gain higher than 6.2 db and a back radiation level less than 22.3 db in the 3 db axial ratio bandwidth o 3.16% in Ku band. The axial ratio bandwidth o the designed antenna is more than two times the respective value o the conventional microstrip antenna. The proposed antenna can be used as an element o microstrip antenna arrays in the contemporary communication systems. Acknowledgements The authors wish to acknowledge the Bulgarian Ministry o Education, Youth and Science and the Technical University o Varna or inancial support under the Research Project No NP-7/29 Study o Circularly Polarized Microstrip Antenna Arrays. Reerences [1] WATERHOUSE, R., NOVAK, D. Printed Antennas or Wireless Communications. John Wiley & Sons, Ltd., 27, ch. 1. [2] GARG, R., BHARTIA, P., BAHL, I., ITTIPIBOON, A. Microstrip Antenna Design Handbook. Boston, London: Artech House, 21, ch. 9. [3] VLASITS, T., KOROLKIEWICZ, E., SAMBELL, A., ROBIN- SON, B. Perormance o a cross-aperture coupled single eed circularly polarized patch antenna. Electronics Letters, 1996, vol. 32, no. 7, p [4] POZAR, D. M., DUFFY, S. M. A dual-band circularly polarized aperture-coupled stacked microstrip antenna or global positioning satellite. IEEE Transactions on Antennas and Propagation, 1997, vol. 4, no. 11, p [] TARGONSKI, S. D., POZAR, D. M. Design o wideband circularly polarized aperture-coupled microstrip antennas. IEEE Transactions on Antennas and Propagation, 1993, vol. 41, no. 2, p

6 116 G. S. KIROV, D. P. MIHAYLOVA, CIRCULARLY POLARIZED APERTURE COUPLED MICROSTRIP ANTENNA WITH [6] ALONI, E., KASTNER, R. Analysis o a dual circularly polarized microstrip antenna ed by crossed slots. IEEE Transactions on Antennas and Propagation, 1994, vol. 42, no. 8, p [7] MIHAYLOVA, D. P., KIROV, G. S. Circularly polarized broadband microstrip antenna. E + E, 29, no. 9/1, p (In Bulgarian). [8] MIHAYLOVA, D. P., KIROV, G. S., SAVVA, M. N. Circularly polarized aperture coupled microstrip antenna with resonant slots and a patch relector. Journal o Applied Electromagnetism, 29, vol. 11, no. 2, p [9] HUANG, C.-Y., WU, J.-Y., WONG, K.-L. Cross slot coupled microstrip antenna and dielectric resonator antenna or circular polarization. IEEE Transactions on Antennas and Propagation, 1999, vol. 47, no. 4, p About Authors... Georgi KIROV was born in Malko Tarnovo, Bulgaria, in 194. He received his M.Sc. and Ph.D. degrees in radio engineering rom the Technical University o Varna, Bulgaria in 1968 and rom the Technical University o Soia in 198, respectively. In 1969, he joined the depart ment o radio engineering and, in 1992 became the head o the department. From 1981 to 1986, he was with the University o Seti, Algeria, as a lecturer in the ield o telecommunications. He specialized in microwave antennas at Moscow Technical University o Communications and Inormatics (in 1973 and 1978) and at the University o Luxembourg (in 26). He also was a visiting proessor at the University o Magdeburg, Germany (in 22-23). D-r Kirov s name is listed in some American and Great Britain encyclopedias such as Marquis Who s Who in the World, Marquis Who s Who in Science and Engineering, 2 Outstanding Intellectuals o the 21 st Century (IBC Cambridge, England), etc His research interests include high requency electromagnetism, antennas and wave propagation. Desislava MIHAYLOVA was born in She received the B.Sc. and M.Sc. degrees, both in Communication Technique and Technology rom the Technical University o Varna, Bulgaria, in 22 and 24, respectively. She has been working as a Ph.D. student at the same university since 2. Her research interests include studies in the ield o electrodynamics and antenna design.