Compact log‐periodic dipole array antenna with bandwidth‐enhancement techniques for the low frequency band
2016; Institution of Engineering and Technology; Volume: 11; Issue: 5 Linguagem: Inglês
10.1049/iet-map.2016.0611
ISSN1751-8733
AutoresKyei Anim, Dong‐Uk Sim, Young‐Bae Jung,
Tópico(s)Antenna Design and Optimization
ResumoIET Microwaves, Antennas & PropagationVolume 11, Issue 5 p. 711-717 Research ArticleFree Access Compact log-periodic dipole array antenna with bandwidth-enhancement techniques for the low frequency band Anim Kyei, Anim Kyei Electronics and Control Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon, 34158 R.O. KoreaSearch for more papers by this authorDong-Uk Sim, Dong-Uk Sim Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon, 34129 R.O. KoreaSearch for more papers by this authorYoung-Bae Jung, Corresponding Author Young-Bae Jung ybjung@hanbat.ac.kr Electronics and Control Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon, 34158 R.O. KoreaSearch for more papers by this author Anim Kyei, Anim Kyei Electronics and Control Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon, 34158 R.O. KoreaSearch for more papers by this authorDong-Uk Sim, Dong-Uk Sim Electronics and Telecommunications Research Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon, 34129 R.O. KoreaSearch for more papers by this authorYoung-Bae Jung, Corresponding Author Young-Bae Jung ybjung@hanbat.ac.kr Electronics and Control Engineering, Hanbat National University, 125 Dongseodaero, Yuseong-gu, Daejeon, 34158 R.O. KoreaSearch for more papers by this author First published: 17 January 2017 https://doi.org/10.1049/iet-map.2016.0611Citations: 15AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract A compact planar log-periodic dipole array (LPDA) antenna operating from 0.55 to 9GHz with high measured gain is proposed to be implemented in a reverberation chamber as a source antenna. To minimise the total size of the LPDA antenna, variants of top-loading techniques are utilised, and a much smaller spacing factor is opted to specify the spacing between the elements. However, miniaturisation of the LPDA antenna limits the antenna's wideband characteristics over the desired range of frequencies. Hence, a feedline meander and a trapezoidal stub are incorporated into the design as impedance matching techniques to effectively enhance the bandwidth performance especially for the lower band of the antenna's operating frequency range. The LPDA antenna exhibits radiation patterns with and without a radome having the measured gain ranging from 2.48 to 7.89 dBi and from −1.20 to 7.94 dBi, respectively. 1 Introduction Wideband antennas are extensively used in various wireless communication systems for industrial and military applications. For instance, the use of direction finding technique for military purposes requires antenna's ability to determine the angle in azimuth plane from which the desired signal originates, and to receive the signals over a wider range of frequencies [[1], [2]]. Antennas with such characteristics are used in various civilian applications to locate the sources of unknown signals in order to track and locate targets. The use of an antenna as a source of electromagnetic wave in a reverberation chamber in electromagnetic compatibility (EMC) measurement application is a subject of interest in this communication. The source antenna is used to set up a field distribution within the chamber. In this project, the chamber itself is basically a metallic box with mode stirrers which are articulated by motors in order to randomise the field received by the antenna under test with uniform amplitude and polarisation from all angles [[3]]. The reverberation chamber to be used for the EMC measurement is relatively small in order to achieve a higher lowest usable frequency for a ultra high frequency (UHF) band. Thus, an antenna with wideband characteristics, small size and low profile is required. Wideband antennas can also be used in applications such as surveillance, radar and imaging systems, energy harvesting from TV broadcast tower, communication systems, sensors and so on. Frequency-dependent antennas, particular class of wideband antennas, were first proposed by Rumsey [[4]]. Rumsey's analytical treatment of frequency-independent antennas laid the foundation for development of many wideband antennas such as log-periodic antenna [[5]-[7]]. Log-periodic antennas, in particular, are known for exhibiting relative uniform input impedance, voltage standing wave ratio and radiation characteristics over a wide range of frequencies and are primarily used in 10–10,000 MHz frequency band, as such make them suitable for the various wireless communication applications. The design of the conventional log-periodic antenna was adopted from the works of Isbell and Carrel [[8]-[10]]. The most commonly used log-periodic design for EMC applications is the log-periodic dipole array (LPDA) shown in Fig. 1. This type of antenna configuration depicts the frequency-independent concept, largely because its entire geometry is solely specified by the scaling factor (τ) which is not dependent on frequency. The longest element length (l1) resonates at half-wavelength of the lower frequency limit, whereas the upper frequency limit is a function of the shortest element. The spacing between elements, the width of each element, the gaps between poles of the elements and the apex angle are denoted by Sn, wn, gn and α, respectively, as demonstrated in Fig. 1. Fig. 1Open in figure viewerPowerPoint Basic LPDA antenna configuration A limitation of the LPDA is that the dipole element for the lowest operating frequency in the UHF range may become very large to be recommended for indoor wireless application such as EMC test in reverberation chamber. In pursuit of reducing the LPDA size, a planar log-periodic antenna with simple top loaded elements is proposed in this paper. The planar LPDA antennas are used in most EMC applications due to their attractive features including low profile, compact, lightweight, low cost and higher gain. Thus, different methods have been used for implementing planar LPDA antennas [[11]-[14]]. Nonetheless, miniaturisation of the LPDA antenna size using top-loading technique places limit on the maximum attainable impedance bandwidth of the antenna due to the higher quality factor (Q-factor) [[15]]. Thus, bandwidth-enhancement techniques such as feedline meander and trapezoidal stub are proposed for the antenna design. In this paper, a compact planar LPDA antenna operating from 0.55 to 9 GHz is proposed for implementation as a source antenna in a reverberation chamber. A top-loading technique and a much smaller spacing factor are used to reduce both the lateral size and antenna boom length, respectively, in comparison to [[1]]. Also bandwidth-enhancement techniques are utilised to effectively improve bandwidth performance. 2 Antenna design In this section, a planar LPDA antenna operating at 0.55 to 9 GHz frequency band is designed. The schematic of the LPDA antenna discussed in this paper is shown in Fig. 2. The shaded regions of the schematic are assumed to be on the upper side of the substrate, whereas the non-shaded portions represent the substrate lower side. Fig. 2Open in figure viewerPowerPoint Structure of the proposed planar LPDA antenna (a) Arrow-shaped balun, (b) Meander section of feedline, (c) Trapezoidal stub A conventional LPDA antenna as depicted in Fig. 1 is first designed. A scaling factor of 0.935 and a spacing factor of 0.174 relative to a directivity of 9.5 dBi are chosen from Table 1, taking into account the desired theoretical performance to be used for the conventional LPDA antenna geometry. Thus, the number of dipole elements (N) for the LPDA geometry can be determined using the following equations [[16]]: (1) (2) where and B are the designed bandwidth and desired bandwidth, respectively. Based on (1) and (2), the number of dipole elements for the LPDA is calculated to be 48. The length of the longest dipole element () resonating at half-wavelength of the lower frequency limit, 0.55 GHz is determined. Hence, the lengths (), and the widths () of the remaining elements are derived as follows [[17]]: (3) Table 1. Optimum design parameters for standard log-periodic antenna Directivity, dBi Scaling factor Spacing factor 7.0 0.782 0.138 7.5 0.824 0.146 8.0 0.865 0.157 8.5 0.892 0.165 9.0 0.918 0.169 9.5 0.935 0.174 10.0 0.943 0.179 10.5 0.957 0.182 11.0 0.964 0.185 Also, two extra dipole elements are added to the radiating elements at the front end of the LPDA antenna (hence, increasing the number of dipole elements to 50) in order to obtain a good radiation efficiency. 2.1 Antenna miniaturisation Next, the LPDA antenna is assumed to be printed on Rogers RO4003 substrate ( = 3.55). Hence, the free space length of the longest dipole element associated with the lower frequency limit, 0.55 GHz () can be scaled down using the following formula: (4) where is the effective permittivity of the RO4003 substrate ( = 3.55) and can be mathematically expressed as follows: (5) where w and h denote the width of the feedline and the thickness of the substrate, respectively. 2.1.1 Top-loading techniques To further reduce the lateral size of the LPDA antenna, variants of top-loading techniques are applied to some of the straight dipole elements of the LPDA antenna. In Fig. 2, the LPDA antenna consists of three different sets of dipole elements namely, traditional (straight), T-top loaded and hat-top loaded elements which are printed on both sides of the substrate to form a coplanar array. Before the design of the proposed LPDA antenna is described in detail, the resonant performance properties of the two top-loading techniques (i.e. T- and hat-top) considered in this paper are analysed and compared. Therefore, a hat-loaded and T-loaded monopoles are designed and simulated and their performances are compared with a straight (traditional) monopole radiator operating at a frequency of 1 GHz with equal physical length as the hat-loaded and T-loaded monopoles as shown in Fig. 3. Fig. 3 illustrates the geometries of the monopole counterparts of the hat-top loaded, the T-top loaded and the straight dipole radiators utilised in the proposed LPDA antenna. It can be observed that with the same physical length of 56 mm, the hat-loaded, the T-loaded and the straight monopoles have different resonant properties as plotted in Fig. 4. Although the straight monopole resonates at 1 GHz, the T-top and the hat-top loaded monopoles resonate at 0.8 and 0.7 GHz, respectively. It is evident that with hat-top loading, the physical length of a straight monopole operating at 0.7 GHz can be reduced by about 35%. It is also evident that with the same horizontal length (l1) of the T-top load, the resonant frequency can further be lowered with the vertical arm lengths l2 and l3 of the hat-top load. Furthermore, the hat-top load has the advantage of minimising the interference from adjacent loaded dipoles that may be caused by the long horizontal length (l1) of T-load for elements resonating at a very low-frequency band. Fig. 3Open in figure viewerPowerPoint Geometry configurations of the various monopole radiators with equal physical length Fig. 4Open in figure viewerPowerPoint Resonant performance characteristics of straight, T-top loaded and hat-top loaded monopole antennas 2.1.2 Reduced spacing factor Moreover, to reduce the total length of the LPDA antenna, the spacing between the elements were optimised to reduce the spacing factor from 0.174 to 0.06. A spacing factor value of 0.06 is not standardised but optimal to reduce significantly the boom length of the LPDA antenna. By utilising the dielectric material (RO4003 substrate = 3.55) and the top-loading techniques (T and hat) in the LPDA structure, the size of the dipole element resonating at 0.55 GHz of the initial standard LPDA design is reduced to a length of 194 mm as shown in Fig. 2. Hence, the final LPDA antenna lateral size is reduced by ∼27%. By reducing the spacing factor of the LPDA, the boom length is reduced to about 20% in comparison to the standard LPDA antenna [[9]]. 2.2 Bandwidth-enhancement techniques The downside of using top-loading technique to constrain the physical size the proposed LPDA design is that it has a higher tendency of limiting the maximum attainable impedance bandwidth of the LPDA antenna due to factors mentioned earlier in the introduction. Thus, two impedance matching techniques namely, feedline meander and resistive stubs are utilised in the LPDA design to effectively enhance the bandwidth performance of the LPDA antenna. 2.2.1 Feedline meander The following design procedure is proceeded to investigate and present the impact of the proposed impedance matching techniques on the bandwidth performance of the LPDA. From Fig. 5, the feedline of the proposed LPDA antenna is intermittently folded in-between loaded dipole elements to form a meander and its equivalent circuit is shown in Fig. 6. The parameter represents the effective capacitance that can be primarily attributed to self-capacitance between turns, denotes the effective inductance which can be mostly attributed to self-inductance of the meander section, and depicts the resistance of the feedline. The impedance seen from the input port of the feedline can be expressed mathematically as follows: (6) Fig. 5Open in figure viewerPowerPoint Close up view of the feedline meander and trapezoidal stub Fig. 6Open in figure viewerPowerPoint Equivalent circuit of the meander section of the feedline Based on (6), the input impedance () can be tuned by simply varying the parameters , and . To vary the capacitance () and the inductance () in (6), the optimisation design parameters and of the meander shown in Fig. 5 are varied and the input impedance () can be tuned accordingly as plotted in Fig. 7a. The improvement of the impedance matching performance, especially for the lower band of the antenna, comes virtually from the variation of the input impedance as demonstrated in Fig. 7c. It can, therefore, be extrapolated that as the parameters and of the meander vary, the value of changes in addition to to alter the impedance seen from the input port. At some point, the input impedance may match with the terminal impedance seen by the current signal surging into the radiating elements. Hence, the amount of signals reflected back to the input port is minimised and subsequently improves the bandwidth performance of the LPDA antenna. Fig. 7c shows that the parametric values = 1.0 mm and = 0.5 mm return the optimum impedance matching performance for the antenna's lower frequency band as all the desired frequencies fall below −10 dB mark. Since the meander technique is designed to improve the matching condition at the lower frequency band, the following reflection coefficient curves are plotted from 0.4 to 2 GHz band. Fig. 7Open in figure viewerPowerPoint Simulated parametric sweep of the main design parameters (a) Input impedance of the meander, (b) Input impedance of the trapezoidal stub width, (c) Reflection coefficient of the trapezoidal stub width, (d) Reflection coefficient of the trapezoidal stub width 2.2.2 Resistive stub In addition to the feedline meandering technique, trapezoidal stubs are inserted in-between the hat-top loaded elements and the meander feedline as shown in Fig. 5. The stub alters the resistive component to the meandered feedline via the variation of its width (). With increasing width of the resistive stub (), the input impedance can be tuned through the variation of as shown in Fig. 7b. It can be observed that the input impedance fluctuates more gently at 50 Ω as increases which subsequently improves the impedance matching condition of the antenna. However, it is evident that the more increases the lesser impact it has on the reflection coefficient of the LPDA antenna as illustrated in Fig. 7d. Meanwhile, the height of the stub () is kept constant throughout since the variation of the stub height has minimal impact on the bandwidth performance of the LPDA antenna. Incorporation of both feedline meander and trapezoidal stubs into the antenna configuration results in a more stable impedance bandwidth performance especially for the lower band of the antenna's operating frequency range. 2.2.3 Balun Finally, the fabricated LPDA antenna will eventually be connected to the coaxial cable for measurement. This may lead to an unbalanced feedline/transmission line of the LPDA antenna due to the quasi-transverse electromagnetic (TEM) of the cable and consequently degrades the antenna return loss performance. To maintain a balanced transmission line, an arrow-shaped balun is used to feed the LPDA antenna as depicted in Fig. 8a.With the balun structure, the coaxial cable transmission mode (quasi-TEM as microstrip transmission line) can gradually be converted to balance line transmission mode (full TEM). The concept of a balun is based on the Klopfenstein microstrip tapered line, which was used for impedance matching of microstrip transmission line in 1956 [[18]]. It is apparent in Fig. 8b that by tuning the design parameters bw and bl of the balun, the transition impedance between the feedline and the cable can be controlled and subsequently improves the return loss performance of the LPDA. Thus, a balun structure with optimal dimensions 1.5 mm × 1.0 mm ( × ) and 4.0 mm × 2.2 mm ( × ) is used to connect the balanced transmission line of the LPDA antenna to unbalanced coaxial cable [[19]]. Fig. 8Open in figure viewerPowerPoint Arrow-shaped balun design (a) Structure, (b) Combined parametric sweep of balun design parameters bw and bl The planar LPDA antenna shown in Fig. 9 was fabricated using Rogers RO4003 substrate ( = 3.55 and tanδ = 0.0027) measuring 282 mm × 194 mm × 0.508 mm (L × W × H). The LPDA antenna consists of 50 elements connected via feedline to a balun acting as an input port and are printed on both sides of the substrate with a thickness of 0.508 mm. A coaxial cable is used to feed the LPDA antenna via the balun, which is soldered along the lower side of the feedline and the inner conductor is connected to the upper side of the feedline through a hole at the end of the feedline as shown in Fig. 9. The fabricated LPDA antenna is measured with and without radome, and the results are analysed in this paper. The optimised design parameters used for the fabricated LPDA antenna are listed in Table 2. Table 2. Optimised design parameters of the fabricated antenna D, dBi τ σ .α, deg N , mm 9.5 0.942 0.06 13.58 50 194 Fig. 9Open in figure viewerPowerPoint Photograph of the fabricated planar LPDA antenna showing (a) Top view, (b) In transparent radome 3 Experiments The performance of the planar LPDA is analysed in terms of the radiation pattern, input reflection coefficient and gain. Fig. 10a compares the simulated and the measured input reflection coefficients and the gain of the proposed planar LPDA antenna. As illustrated in Fig. 10a, the measured input reflection coefficient ironically outperforms the simulated result. It can be observed that the simulated input reflection coefficient curve falls below −10 dB mark at 0.55 to 8.5 GHz frequency range, whereas the measured input reflection coefficient satisfies at 0.5 to 9.7 GHz frequency band. This higher impedance bandwidth performance, especially at the lower band of the antenna's operating frequency range, can be largely attributed to the incorporation of the feedline meander and resistive stubs into the antenna structure as impedance matching techniques. Fig. 10Open in figure viewerPowerPoint Measurement and simulation data of the planar LPDA antenna (a) Reflection coefficients, (b) Gain Figs. 11 and 12 show the measurement and simulation radiation patterns for E-plane and H-plane of the planar LPDA antenna at 0.55, 0.95, 5 and 9 GHz, respectively. They clearly show that the measured radiation patterns to a larger extent agree with the simulated results over the desired frequency range. It can be observed that the measurement radiation patterns of the LPDA antenna with radome have higher gain ranging from 2.48 to 7.89 dBi as compared with measurements without the radome and the gain ranges from to 7.94 dBi. The negative measured gain of the radiation pattern without radome at 0.55 GHz may be due to measurement error. The improvement of the measured gain of the antenna with radome is mainly due to increase in antenna directivity associated with the radome as demonstrated in Fig. 10b. The walls of the radome passively reflect the radiated power that may have been emitted backwards to the front of the antenna. Thus, the directivity of the radiated power is increased for a higher antenna measured gain performance. For this reason, the antenna in radome tend to have a much better radiation performance to match well with the simulated result as compared with the antenna without radome. It can, therefore, be said that the radiation performance of the proposed LPDA antenna is not degraded by a radome. Fig. 11Open in figure viewerPowerPoint Measurement and simulation E-plane radiation patterns of the planar LPDA (a) 0.55 GHz, (b) 0.95 GHz, (c) 5 GHz, (d) 9 GHz Fig. 12Open in figure viewerPowerPoint Measurement and simulation H-plane radiation patterns of the planar LPDA (a) 0.55 GHz, (b) 0.95 GHz, (c) 5 GHz, (d) 9 GHz 4 Conclusion A compact planar LPDA antenna operating from 0.55 to 9 GHz with high measured gain is designed as a source antenna to be implemented in a reverberation chamber. Variants of top-loading techniques and a very small spacing factor are utilised to miniaturise the proposed LPDA antenna size. In comparison to the standard LPDA antenna, the proposed LPDA antenna size is reduced by ∼27 and 20% in terms of the width and the length, respectively. However, miniaturisation of the LPDA antenna size with top-loading techniques compromises the wideband characteristics of the antenna. Thus, a feedline meander and resistive stubs are employed as impedance matching techniques to effectively enhance the bandwidth of the proposed LPDA antenna, especially for the lower frequency band. Furthermore, an arrow-shaped balun is used as an input port to realise a balanced transmission line of the LPDA antenna when fed by a coaxial cable. The proposed planar LPDA antenna demonstrates a high radiation performance when it is housed in a radome. 5 Acknowledgment This work was supported by ICT R&D program of MSIP/IITP. [B0194-16-1001, Study on Measurement and Evaluation Technology based on Reverberation Chamber]. 6 References [1]Yeo, J., Lee, J.-I.: ‘Miniaturized LPDA antenna for portable direction finding applications’, ETRI J., 2012, 34, (1), pp. 118– 121 [2]Harrison, R.W.S., Jessup, M.: ‘A novel log periodic implementation of a 700 MHz–6 GHz slant polarised fixed-beam antenna array for direction finding applications’. 9th European Radar Conf. 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