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Effect of Diffraction-Coupled Apertures on the Monopole Antenna Performance
I.V. Ivanchenko, Senior Member, IEEE, N.A. Popenko, Senior Member, IEEE, M.M. Khruslov,
Student Member, IEEE
Abstract — For the first time the concept of diffraction-coupled apertures is applied to the analysis of characteristics of the cylindrical monopole antennas. The information about the spatial distribution of electromagnetic fields in the radiating region of antennas is used for studying the influence of diffraction effects on the antenna performance. The optimal radius of the round hole in the ground plane center for coupling with the coaxial feeding line is determined for all the antennas under test. Based on the results of computational modeling the small-size monopole antenna is proposed. The experimental data on the antenna prototype are in good agreement with the simulations.
Index Terms — cylindrical monopole antenna, diffraction, interference, diffraction
I. Introduction
Recently, X-band UWB communication systems are in great demand. A good candidate for this purpose is the broadband cylindrical monopole antenna [1, 2]. The
advantage of this class of antennas is the ability to produce the different radiation patterns by changing the shape and size of the constituent elements of antennas [3-5], including multibeam radiation patterns [6, 7].
State-of-the-art applications of cylindrical monopole antennas require a special study of the influence of diffraction effects on the antenna performance. Many papers are devoted to this problem [3-6, 8-13]. For example, in [13] the influence of the conducting cube size as well as the position and the length of the cylindrical monopole located on the cube surface on such parameters as the peak of the input conductance, resonant frequency and quality factor are examined. The authors have found that the greatest peak of the input conductance is archived when the monopole is located in the
Manuscript received December 13, 2012.
Igor Ivanchenko is with the Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, 12 Ak. Proskura St.,
Kharkov, 61085, Ukraine, tel. +38 (057) 7203594, fax. +38 (057) 3152105 (email: ireburan@yahoo.com).
Nina Popenko is with the Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, 12 Ak. Proskura St., Kharkov, 61085, Ukraine, tel. +38 (057) 7203594, fax. +38 (057) 3152105 (e-mail: ireburan@yahoo.com).
Khruslov M.M. is with the Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, 12 Ak. Proskura St., Kharkov, 61085, Ukraine, tel. +38 (057) 7203594, fax. +38 (057) 3152105 (email: ireburan@yahoo.com).
cube center and decreases in case of the monopole moving to a face or to a corner of the cube. At the same time, the resonant frequency of the antenna tends to move towards higher frequencies due to the influence of higher-order effects. The results of studying the influence of the ground plane size on the cylindrical monopole antenna characteristics are presented in our previous publications [3-6] and the monopole antennas with the original design of the ground plane are proposed in [3, 5, 6].
It is worth noting that the knowledge of the influence of diffraction effects on the antenna beamforming is very important not only in relation to the individual antenna but also for the antenna array composed of such elements [14, 15]. For example, in [15] the authors point out an irregular changing the angle of maximum radiation of the cylindrical monopole antenna depending on the ground plane size. Moreover, they show that a similar situation occur in the case of the antenna array.
In this paper we study in detail the diffraction effects in relation to the X-band cylindrical monopole antenna and their contribution to the performance of the antenna..
II. ANTENNAS UNDER TEST AND RESEARCH TECHNIQUE
The antennas under test consist of the vertical cylindrical monopole as a segment of the central conductor of the coaxial feeding line with a diameter of 2a (in our investigations 2a= 1.4mm) and the circular metal ground plane with the radius R and thickness d (in our case d=0.5mm) (Fig. 1).
Fig. 1. Schematic view of antennas under test
In the framework of the research we focus on the two most interesting monopole heights dr1, namely: quarter-wavelength (dr1=X/4) and three fourth-wavelength (dr1=3X/4) monopoles
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for the wavelength X=30mm. The ground plane radius R varies from 7.5mm to 82.5mm with steps of 7.5mm. Furthermore, the round hole in the ground plane center for coupling with the coax has the radius r which varies from r=2.25mm, corresponding to the radius of the coaxial feeding line with the dielectric filling s=2.04, to r=0.8mm.
The input reflection coefficient S11, near-field distributions, radiation patterns and antenna bandwidth were selected by us for the analysis. The simulations were performed by using a software package for solving the scattering problems based on the standard finite difference algorithm with the “exact” absorbing conditions on the spherical artificial boundaries in free space and on the transverse artificial boundaries in the feeding line [16]. The measurements were carried out on the experimental equipment available at the LHFT [17, 18].
III. The central hole in the ground plane of antenna
It is quite clear that the most appropriate approach for studying the edge effects caused by the diffraction of EM waves on a finite aperture is the obtaining of information on the spatial distribution of EM fields in the inductive and radiating regions of antenna. The information like that allows one to study and to analyze the dynamics of the wave packets transformation in the process of antenna beamforming. In terms of studying the diffraction phenomena, the particular importance is acquired in the most complicated case when the typical dimensions of the objects under test become comparable with the operational wavelength. With respect to the cylindrical monopole antenna there are two typical areas of the edge wave sources, namely, the ground plane edge and the edge of the round central hole in the latter. The interference of these edge waves with the initial wave packet of the monopole will result in the interference picture formation in free space. Therefore, the study of regularities in the interference pictures formation becomes the key aspect allowing one to identify the relationship of the physical parameters of antennas with their radiation pattern shape. In addition, we can assume that the interaction of such the diffraction-coupled apertures will result in the need to take into account the diffraction correction in the resonant frequency of the antenna.
Based on the aforementioned concept, we have been analyzed the EM field distributions in the inductive region of monopole antennas with the different ground plane radii where the influence of the ground plane size on the antenna beamforming is manifested most clearly [19]. As a result it has been determined that for the monopole antennas with dr1=X/4 two types of spatial near-field distributions depending on the ratio of the ground plane radius and the wavelength is observed. The first type of the EM field distribution has two field variations along the ground plane radius with a minimum, located at a fixed distance from the ground plane edge for antennas with the ground plane radii divisible by X/2, and the second one looks like the circular interference picture for antennas with the ground plane radii divisible by odd number of X/4. We have called these distributions as the "spatial wave grating". The differences like those explain the oscillating dependencies of both the resonant frequency and
the elevation angle of peak directivity. In contrast to the antennas with dr1=X/4 the resonant frequency and elevation angle of peak directivity for the antennas with dr1=3X/4 are not changed virtually. The thresholds of the ground plane radii of antennas with dr1=X/4 and dr1=3X/4 corresponding to the transition from the mono-beam to multi-beam radiation pattern have been defined. Furthermore, and it has been shown that the number of beams increases when increasing the ground plane radius.
Following our concept of the diffraction-coupled apertures it is logical to assume that for each antenna configuration there is the optimal radius of the round hole in the ground plane center. To confirm this hypothesis, we performed the simulations of cylindrical monopole antennas performance with the monopole heights dr1=X/4 and dr1=3X/4 with different radii of the round hole. The minimum value of the return loss coefficient S11 was chosen as the criterion of the optimal radius of such a hole for each fixed-sized ground plane. Let us analyze the obtained results for the first and second groups of antennas separately taking into account the revealed above difference in the characteristics of antennas with the monopole heights dr1=X/4 and dr1=3X/4.
The comparative analysis of near-field distributions of antennas with the monopole heights dr1=X/4 and dr1=3X/4 shows that the field intensity under the ground plane of the antenna with the monopole height dr1=3X/4 is significantly less than that of the antenna with the monopole height dr1=X/4 at the same ground plane radii (Fig 2). The field intensity on the ground plane edge of the antenna with the monopole height dr1=X/4 is 5.7 times higher than that of the antenna with the monopole height dr1=3X/4. In the latter case the EM field distribution is concentrated mainly near the monopole. As a result of this, the contribution of diffraction fields to the antenna beamforming will be much smaller.
a
b
Fig. 2. Spatial near-field distribution of the electric field component E of the antenna with the monopole heights dr1=3X/4 (a) and dr1=X/4 (b) at a fixed time(R=X=30mm ).
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For the first group of antennas with the monopole height drI=X/4 we observe a small change in the optimal radius of the round hole when increasing the ground plane radius (Fig. 3a, curve 1). At the same time, the optimal radius of the hole for the second group of antennas increases when the ground plane radius increases from R=7.5mm to R=52.5mm. For R>52.5mm the optimal radius of the round hole decreases (Fig. 3a, curve 2). As a result of this, the dependence of optimal round hole size on the ground plane radius for the antennas with the monopole height drI=X/4 also demonstrates an oscillating form (Fig. 3b) similar to the aforementioned dependencies of the resonant frequency and the angle of maximum radiation [19].
Fig. 3. The optimal round hole radius depending on the ground plane radius: for antennas with drj=X/4 (curves 1 and 2) and dr=3X/4 (curve 3) (a); for all the antennas under study with the monopole height dr=X/4 (b)
The analysis of the similar dependence for antennas with the monopole height drI=3X/4 (Fig. 3, curve 3) suggests that the determination of the optimal round hole size has a meaning only for antennas with R <22.5mm, since for the antennas with R>22.5mm the optimal radius of the hole is r=2.25mm, which corresponds to the radius of the dielectric in the coaxial feeding line.
IV. The miniature cylindrical monopole antenna
Let us now consider in detail the behavior of the return loss coefficient Su of the antenna with parameters R=7.5mm and drI=X/4 when the round hole radius varies from r=2.25mm to r=0.75mm. The characteristics of antennas with the different
round hole radii are shown in the Figure 4 and the Table I. By analyzing the data obtained it should be noted that the optimization of the round hole radius gives rise to a significant improvement of the antenna performance. Namely, it leads to the expansion of the antenna bandwidth and to the improvement of the antenna efficiency. In particular, the antenna with parameters R=7.5mm, drI=k/4, and r=2.25mm has the return loss coefficient S11>-10dB over the entire analyzed frequency band. For the optimal value of the round hole radius r=1.15mm the parameter S11 reaches the minimum value S11=-32dB at the resonant frequency f=10GHz. In this case the -10dB impedance bandwidth of the antenna is equal to 16%. The further reducing the round hole size results in the increase of both the resonant frequency and the parameter S11 that leads to a deterioration of the antenna performance.
f, GHz
■ —r=o.75mm
• —r=o.85mm
• — r=0.95mm
• — r=1.05mm
• — r=l.l5mm
- —r=l.25mm
- — r=l.35mm
• — r=1.45mm
• —r=l.55mm
• — r=1.65mm
• —r=1.75mm
Fig. 4. Return loss coefficient £ц of the antenna with parameters R=7.5mm and drI=X/4 for different round hole radii.
For the antenna with R=7.5mm and drI=3X/4 the optimal radius of the round hole has been also determined. As one sees from the Figure 5 and the Table I, in this case the antenna efficiency also increases.
Fig. 5. Return loss coefficient Sn of the antenna with following parameters: R=7.5mm, dry=3X/4, r=1.6mm, and r=2.25mm
Thus, the optimization of the round hole radius allowed us to design the miniature broadband antenna. The prototype of such the antenna has been manufactured and tested (Fig. 6, Table I).
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TABLE I
Characteristics of the miniture monopole antennas
drI=7.5mm drI=22.5mm
r 2.25 1.15 theory experiment 2.25 1.6
fres, GHZ 9.8 1° 9.96 9.7 9.9
G - 1°.53 8.9 9.28 9.42
0max 74 74 69 49 49
Af, ° 16 15 1° 1°
%(GHz) (9.24-10.85) (9.2-10.74) (9.23-10.25) (9.41-10.4)
Fig. 6. Antenna prototype
Fig. 7. Return loss coefficient S11 of the antenna (a); measured (thick line) and simulated (thin line) radiation pattern of the antenna with parameters: dri=7.5mm, R=7.5mm, r=1.15mm (b)
The return loss coefficient Su of the antenna prototype is shown in the Figure 7 a along with the return loss coefficient Su of the antenna having the conventional round hole radius which corresponds to the radius of the dielectric in the coaxial
feeding line. The radiation pattern of the antenna is shown in the Figure 7b. As can be seen, this antenna produces monobeam radiation pattern with the maximum radiation at 0=69°. The experimental data are in good agreement with the simulations.
V. Conclusion
Thus, the physical nature of oscillating dependence of the optimal round hole radius on the ground plane radius of antennas with the monopole height drI=X/4 is explained by different conditions of the formation of spatial near-field distributions in two groups of antennas. We have established the dependence of the optimal round hole radius on the ground plane radius that indicates a validity of the approach applied by us for studying the radiation characteristics of this class of antennas in terms of diffraction-coupled apertures.
it has been determined that the variation in the round hole radius of the antenna with a fixed ground plane size results in the resonant frequency shift of the antenna and the antenna efficiency change. it has been established that for each ground plane size of the antenna with the monopole height dr1=l/4 one can choose the optimal round hole size at which the antenna performance is maximized. We have also shown that the optimization of the round hole size of the antenna with the monopole height drI=3X/4 has a meaning only for the ground plane radii R<3X/4.
The choice of the optimal round hole radius allowed us to offer the miniature cylindrical monopole antenna with parameters drI=X/4, R=7.5mm, and r=1.15mm which produces a wide-angle mono-beam radiation pattern in the -1°dB impedance bandwidth BW=15%. The antenna seems to be very attractive for using in wireless communications and data transmission such as WiMax.
References
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Igor Ivanchenko was born in Kharkov, Ukraine, in 1952. In June 1975 he graduated from the Kharkov State University and received the MS degree in Radiophysics. From 1975 to the present he works at the Institute for Radiophysics & Electronics of the National Academy of Sciences of Ukraine (IRE NASU). He received the Ph.D and D.Sc. degrees in Radiophysics in 1980 and 1997, respectively. From 1984 he is a Senior Researcher with the Department of Radio-Spectroscopy in the IRE NASU. He has authored and co-authored more than 100 publications in the fields of electromagnetics, non-destructive testing, low-temperature magnetic radiospectroscopy, and semiconductor physics. Currently he is a Head of the Laboratory of High Frequency Technology at IRE NASU. Prof. I.Ivanchenko is a Senior Member of IEEE and a Member of EuMa.
Nina Popenko was born in Gadyach, Ukraine, in 1948. In June 1971 she graduated from the Kharkov Institute of Radioelectronics and received the MS degree in Radio-technique. From 1971 to the present she works at the Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine (IRE NASU). She received the Ph.D and D.Sc. degrees in Radiophysics in 1981 and 1998, respectively. From 2007 she is a Leading Researcher with the Department of Radio-Spectroscopy in the IRE NASU. She has authored and co-authored more than 120 publications in the fields of low-temperature magnetic radio-spectroscopy, semiconductor physics, and antennas design. Prof. N. Popenko is a Senior Member of IEEE, and a Member of EuMa.
Maksym Khruslov was born in Kharkov, Ukraine, in 1982. He received the M.S. degree in radiophysics and electronics from Karazin Kharkov National University (Ukraine) in 2004. Since 2004, he works in the Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine, Kharkov, Ukraine, where he is currently as Junior Researcher with the Radiospectroscopy Department. His research interest includes the near-field technology, computational modeling of microwave antennas. Mr. Maksym Khruslov is a Student Member of IEEE, and a Member of EuMa.
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