Miniaturized Omnidirectional Horizontally Polarized Antenna

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 10, OCTOBER 2015

Miniaturized Omnidirectional Horizontally Polarized Antenna Hatim Bukhari and Kamal Sarabandi, Fellow, IEEE

Abstract—A novel miniaturized impedance matched antenna with omnidirectional horizontally polarized radiation pattern is presented. The antenna structure resembles a circular loop formed by a circular array of shunt miniaturized n-fold resonant dipole antennas, which will be referred to as miniature composite wireloop antenna (MCWLA). The proposed antenna does not require an external matching network and can easily be matched to balanced ports with any desired impedance. MCWLAs can be made to be a very small fraction of the wavelength and yet to provide a relatively high radiation efficiency. The antenna input impedance can be adjusted by the number of array n-folded dipole elements, the number of folds and the diameter of the composite loop. The input impedance can be lowered by increasing the number of elements and/or by decreasing the loop diameter. Conversely, the input impedance, for a fixed number of elements and diameter, can be increased by increasing the fold number. The antenna efficiency increases with increasing the loop diameter. To demonstrate the proposed concept, a MCWLA is designed with diameter λ/8 and is designed and fabricated using the standard PCB approach on a thin substrate (λ/500). To achieve omnidirectional radiation pattern in H-plane, a minimum of three elements fed in-phase around the inscribing loop are used. It is shown that impedance match to a 50-Ω line can be achieved using fourfold miniaturized dipole elements. The simulation shows strong uniform current distribution on the A transformer balun that is used to connect the balanced port of the antenna to a small coaxial line at the center of the loop. The measured gain, including the loss through the balun, is to be 0 dBi and the gain variation in the H-plane is shown to be less than 0.5 dB for the prototype MCWLA. Index Terms—Composite loop antennas, dipole antennas, folded antennas, miniaturized antennas, omnidirectional antennas.

I. I NTRODUCTION

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INIATURIZED antennas are essential part of many modern wireless devices. They became a hot topic due to the extortionate demand in different communication systems. Most of the wireless system devices have space limitations and since the antenna is considered to be the largest component in most of the wireless systems; therefore, compact antennas are highly needed and desired. Lot of studies with different approaches in designing miniaturized antennas with efficient radiations and bandwidth that can meet industry’s

Manuscript received December 30, 2014; revised April 21, 2015; accepted June 28, 2015. Date of publication July 16, 2015; date of current version October 02, 2015. The authors are with the Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109-2122 USA (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2456971

requirements and can be easily integrated in different communication systems has been documented. This will improve the system reliability, better mobility, and reduce the circular array of miniaturized n-folded dipole composited wire-loop cost. The relationship between the antenna size and antenna bandwidth and its radiation were studied and reported [1]–[3]. The subject of omnidirectional and horizontally polarized antenna arrays is discussed in [4]. A number of array configurations that can produce horizontally polarized omnidirectional radiation pattern are presented there. However, the sizes of these antenna arrays are relatively large compared to the wavelength. Devices that operate at UHF band especially benefit from antenna miniaturization methods as the wavelength at these bands is relatively low. Challenges in antenna miniaturization include: 1) bandwidth; 2) radiation efficiency; and 3) impedance matching. In addition to these three factors, generation of a desired polarization or a specific radiation pattern for many applications, where polarization or radiation diversity is utilized, is important. If a horizontal polarization with omnidirectional radiation is desired, a small loop antenna, which is the dual of vertical short dipole antenna, can be utilized [5]. However, small loop antennas similar to small dipole antennas are difficult to impedance match to a 50-Ω line. Use of external matching network may lead to poor radiation efficiency or a bulky antenna structure. Different approaches to reduce antenna size have been reported [6]. One technique to design miniaturized dipole antennas is to meander the wire [7], however, this approach leads to poor radiation efficiency as the opposite flow of electric current in the meander wire cancel the radiated field from adjacent segments. From [8] and [9], it is shown how the meandering approach can be applied to slot antennas. However, using meandering technique will make the antenna very hard to match to a 50-Ω line. Very small slot antennas with very high radiation efficiency that do not require external matching network are reported in [10], [11], and [12]. Theoretically slot antennas (with infinite ground plane) should produce omnidirectional radiation pattern in E-plane. However, small size slot antennas with finite ground planes produce a null radiation along the ground plane direction. It is the intent of this paper to present a small size, low profile, horizontally polarized antenna with omnidirectional radiation pattern that does not require external matching network. Recently, the concept of composite slot loop antenna was introduced to develop a low-profile antenna that can produce a vertical polarization [13]. This antenna design is inspired from a small magnetic loop using a slot configuration on metallic small ground. For miniaturization purposes, the meandering approach employed in [10] and [11] was utilized to

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BUKHARI AND SARABANDI: MINIATURIZED OMNIDIRECTIONAL HORIZONTALLY POLARIZED ANTENNA

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reduce the size of six driven elements around the circumference of a small circle. The size of this omnidirectional miniaturized antenna can be as small as λ/10 and its height can be as small as λ/60. Since the input impedance of short slot antennas is rather high, the elements could be fed in parallel, but series capacitors at the feed point had to be used to achieve the impedance match. In this paper, a small composite wire-loop antenna with horizontally polarized omnidirectional radiation without the need for external matching is proposed and designed. The composite loop itself is formed by three isolated resonant miniaturized dipole antennas placed around the circumference of a small circle and fed from the center in a series fashion. Folded dipole methodology was applied to improve the characteristic impedance. Fig. 1. Composite wire-loop antenna.

II. C OMPOSITE W IRE -L OOP A NTENNA A. Antenna Design It is well-known that a small circular loop can produce horizontally polarized omnidirectional radiation pattern. However, a major drawback of this antenna is its extremely low radiation 4 resistance, proportional to λa , which is given by [5] Rloop = 320π 6

a 4 λ

where a is the radius of the loop. This low radiation resistance leads to very poor radiation efficiency. Although the radiation resistance can be improved using multiturn loop (by factor of N 2 ), the additional ohmic resistance and the resulting higher inductance keep the radiation efficiency low and impedance matching more difficult. To increase the radiation resistance, the radiated power, which is proportional to the loop current squared, the current in the loop must be increased. This can be accomplished by packing resonant dipole antennas within a circle. To ensure omnidirectional radiation, the current around the perimeter of the enclosing circle must be almost a constant and not canceled by any opposing current. Applying the meandering approach and folding the edge of a half-wavelength dipole antenna into a spiral-like shape reduce the dipole length significantly. Placing these meandered dipole antennas around the circumference of a small circle creates the desired wire-loop antenna as shown in Fig. 1. It is noted that the circumferential currents, constituted from three diploes, are all in the same direction and almost uniformly distributed around the circle. This is due to the fact that the electric current on a dipole at resonance is strong and almost constant near the feed point. Also feeding the dipoles identically with proper polarity, not only the circumferential currents are uniform around the circle but also the radiations from the spiral arms of the adjacent dipoles cancel each other. This will cause radiation to only come from the strong circumferential currents as in the case for small wire loop antennas. The challenges are in the feed network and impedance matching. Short dipole antennas with end spirals have low radiation resistance and rapidly varying reactance at the resonance as shown in Fig. 2(a). The terminal is placed right at the middle of the dipole and its physical parameters are as follows:

Fig. 2. Real and imaginary parts of the input impedance of: (a) one short dipole with end spirals and (b) three short dipoles.

l = 308 mm, t = 0.3 mm, and h = 0.5 mm where l is the dipole length, t is the trace width, and h is the substrate thickness with dielectric constant of 2.2. Due to the proximity of the endspirals, the input impedance and the resonant frequency of a short dipole in the presence of the other dipoles in the composite loop are quite different than the isolated antenna. The input impedance of the dipole of Fig. 1 in the presence of two other similar antennas terminated with matched load of 50 Ω is shown in Fig. 2(b). One advantage of adjacent elements is that they significantly improve the input resistance and decrease the rate of change of reactance with frequency, which leads to

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Fig. 4. Folded composite wire-loop antenna. Fig. 3. Three layers composite wire-loop antenna.

higher bandwidth. The most convenient way to feed the antennas is the parallel feeding. Unfortunately, feeding the antenna in parallel fashion will further reduce its input impedance, and matching this antenna to a 50-Ω-line becomes very difficult. A better approach is to feed the three antennas in series. By this way, the radiation resistances are added and get closer to the transmission line characteristic impedance. In addition, the line thickness and the length of the two wire transmission lines between the dipole terminals and the feed point in the center of the loop can be used for impedance matching as well. To facilitate series feeding approach, the antenna can be fabricated on three layers as shown in Fig. 3. Metallic via holes can be used to connect the transmission lines to the dipole traces on different layers. Through extensive numerical optimization, it is found that the structure of Fig. 3 cannot be matched to a 50-Ω line exactly. The main problem is the low radiation resistance of the dipoles. One approach is to increase the number of dipoles within the circle. For a given loop radius, each dipole has to be further miniaturized, but this will lead to a very small bandwidth if the antenna could be matched. A better approach for increasing the input impedance is to increase the input impedance of each element using the folded antenna topology. For ordinary dipole antennas, the input impedance can be increased with the square of the number of folds. So if there are N parallel wires forming the multifolded dipole, the input impedance is increased by N 2 [14]. The geometry of Fig. 3 is designed and optimized for two-, three-, and four-folded short dipoles for the composite loop antenna topology. Fig. 4 shows the antenna geometry for the four-folded short dipole configuration. Also Fig. 5 shows the input reflection coefficients at the feed point of the antenna for two-, three-, and four-folded configurations. As observed, the input impedance of the folded structure can be better matched with higher bandwidth when the number of folds increased. The shift in the resonant frequency is the result of change in the mutual coupling between the adjacent elements. It should be emphasized that the diameter of four-folded configuration is less than λ/9 at the resonant frequency. The simulated gain of the antenna is 0.2 dB and its radiation efficiency is 90% in which the order of magnitude higher than the conventional small loop antennas.

Fig. 5. Reflection coefficient of the composite wire-loop antenna with different folding numbers.

Fig. 6. Current distribution over the composite loop antenna.

B. Antenna Feed Connecting a dipole antenna directly to a coaxial cable induces electric current over the outer layer of the coaxial cable, which in turn can cause radiation and unbalanced current distribution on the dipole arms (Fig. 6). A transformer with a center-tap terminal of its secondary coil connected to the outer connector of the input coaxial line, which is connected to


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Fig. 7. Composite wire-loop antenna with the transformer and MMCX jack connector pads.

the primary coil of the transformer, produces a balance terminal (the transformer secondary) for the dipole. The advantages of transformer-type baluns are their compact size and light weight, which make them ideal for application at HF-UHF bands [14], [15]. This is important as the size of the antenna is very small compared to the wavelength. For lower frequency applications, the size of transmission line baluns is quite large (> λ/4) and cannot be used. Therefore, an isolated balanced to unbalanced transformer is suggested and used. In this design, a 3.8 mm × 3.8 mm mini wideband transformer from coilcraft, WBC1-1TLB, with 1:1 impedance ratio and 0.58 dB insertion loss, is used. Due to space limitation, a MMCX jack connector is used instead of a regular SMA connector. This connector is then connected to MMCX to SMA adapter for cable connections during measurements. C. Antenna Measurements The designed antenna is fabricated on a 0.5-mm thick RT/Duroid5880. This substrate has a dielectric constant of 2.2 and loss tangent of 0.0009. Two parts of the proposed antenna were fabricated on opposite sides of one substrate. The third part of the antenna was fabricated on a second substrate, which is then limited to the first substrate. Fabricated antenna’s overall diameter is 77.6 mm with a height of 1 mm. Fig. 7 shows the top view of the antenna final design with dimensions. Fig. 8 shows the final design prototype. The figure shows each layer individually since each layer is on a different substrate surface. The three layers are placed on top of each other. The top and middle layers are placed facing upward while the bottom layer is placed facing downward. The reflection coefficient of the fabricated antenna is measured using a calibrated HP8720D vector network analyzer. Fig. 9 shows the measured and simulated reflection coefficients. It is shown that the measured result is in a good agreement with the simulation. However, the deviation can be attributed to the fabrication process. The radiation pattern is measured using the University of Michigan Radiation Laboratory anechoic chamber. Fig. 10 shows the measured radiation pattern of the composite wire-loop antenna in the E- and H-planes and as expected the far-field radiation pattern very closely resembles

Fig. 8. Prototype of the composite wire-loop antenna.

Fig. 9. Measured and simulated reflection coefficients of the composite wireloop antenna.

that of a small dipole. The radiation pattern in E-plane is uniform within ±0.25 dB. Also the antenna gain is measured to be 0 dBi. Comparing the measured gain with the directivity of an ideal small loop antenna (1.75 dB), the antenna radiation efficiency (including the balun loss) is found to be 67%, which is far better than conventional small loop antennas.

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[6] R. C. Hansen, “Fundamental limitations in antennas,” Proc. IEEE, vol. 69, no. 2, pp. 170–182, Feb. 1981. [7] J. Rashed and C. T. Tai, “A new class of resonant antennas,” IEEE Trans. Antennas Propag., vol. 39, no. 9, pp. 1428–1430, Sep. 1991. [8] J. M. Kim, J. G. Yook, W. Y. Song, Y. J. Yoon, J. Y. Park, and H. K. Park, “Compact meander-type slot antennas,” in Proc. Antennas Propag. Soc. Int. Symp. (AP-S) Dig., Boston, MA, USA, Jul. 2001, vol. 2, pp. 724–727. [9] J.-M. Kim, K.-W. Kim, J.-G. Yook, and H.-K. Park, “Compact striplinefed meander slot antenna,” Electron. Lett., vol. 37, no. 16, pp. 995–996, Aug. 2001. [10] K. Sarabandi and R. Azadegan, “Design of an efficient miniaturized UHF planar antenna,” IEEE Trans. Antennas Propag., vol. 51, no. 6, pp. 1270– 1276, Jun. 2003. [11] R. Azadegan and K. Sarabandi, “A novel approach for miniaturization of slot antennas,” IEEE Trans. Antennas Propag., vol. 51, no. 3, pp. 421– 429, Mar. 2003. [12] N. Behdad and K. Sarabandi, “Bandwidth enhancement and further size reduction of a class of miniaturized slot antennas,” IEEE Trans. Antennas Propag., vol. 52, no. 8, pp. 1928–1935, Aug. 2004. [13] W. Hong and K. Sarabandi, “Low profile miniaturized planar antenna with omnidirectional vertically polarized radiation,” IEEE Trans. Antennas Propag., vol. 56, no. 6, pp. 1533–1540, Jun. 2008. [14] J. D. Kraus and R. J. Marhefka, Antennas, 3rd ed. New York, NY, USA: McGraw-Hill, 2002. [15] Y.-X. Guo, Z. Y. Zhang, L. C. Ong, and M. Y. W. Chia, “A novel LTCC miniaturized dualband balun,” IEEE Microw. Conf., vol. 16, no. 3, pp. 143–145, Oct. 2005.

Fig. 10. Measured radiation pattern of the composite wire-loop antenna. (a) E-plane. (b) H-plane.

III. C ONCLUSION A new approach of designing a miniaturized, horizontally polarized, and omnidirectional antenna is presented. In this approach, a composite wire-loop antenna composed of three miniaturized resonant-folded dipole antennas arranged around the circumference of a small circle is conceived. It is shown that by a serial feed network of proper length and line impedance, the antenna can be easily matched without any external matching network. A prototype was fabricated and its radiation pattern and input impedance were measured. The measurements show a very good agreement with simulated results. Omnidirectional radiation pattern in E-plane with ±0.25 dB variations was demonstrated. This antenna has a diameter of λ/9 and a height of less than λ/500. The measured gain and radiation efficiency are 0 dBi and 67%, respectively. These figures include the loss of the balun.

R EFERENCES [1] H. A. Wheeler, “Fundamental limitations of small antennas,” Proc. IRE, vol. 35, no. 12, pp. 1479–1484, Dec. 1947. [2] R. Azadegan and K. Sarabandi, “A novel approach for miniaturization of slot antennas,” IEEE Trans. Antennas Propag., vol. 51, no. 3, pp. 421– 429, Mar. 2003. [3] L. J. Chu, “Physical limitations of omni-directional antennas,” J. Appl. Phys., vol. 19, pp. 1163–1175, Dec. 1948. [4] R. W. Landee, D. C. Davis, and A. P. Albrecht, Electronic Designer’s Handbook. New York, NY, USA: McGraw-Hill, 1957, pp. 21–37. [5] R. E. Collin, Antennas and Radiowave Propagation. New York, NY, USA: McGraw-Hill, 1985.

Hatim Bukhari received the B.S. degree in electrical engineering from King Abdulaziz University, Jeddah, Saudi Arabia, in 2003, and the M.S. degree in electrical engineering from the University of Michigan, Ann Arbor, MI, USA, in 2008. Currently, he is pursuing the Ph.D. degree in electrical engineering at the University of Michigan. His research interests include ultra-wideband antenna design and electromagnetic wave propagation.

Kamal Sarabandi (S’87–M’90–SM’92–F’00) received the B.S. degree in electrical engineering from Sharif University of Technology, Tehran, Iran, in 1980, the M.S. degree in mathematics from the University of Michigan, Ann Arbor, MI, USA, in 1989, the M.S. and Ph.D. degrees in electrical engineering from the University of Michigan, Ann Arbor, MI, USA, in 1986 and 1989, respectively. Currently, he is the Director of the Radiation Laboratory and the Rufus S. Teesdale endowed Professor of Engineering with the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA. He possesses 25 years of experience with wave propagation in random media, communication channel modeling, microwave sensors, and radar systems and leads a large research group including two research scientists, 16 Ph.D. students. He has graduated 43 Ph.D. and supervised numerous postdoctoral students. He has served as the Principal Investigator on many projects sponsored by the National Aeronautics and Space Administration (NASA), Jet Propulsion Laboratory (JPL), Army Research Office (ARO), Office of Naval Research (ONR), Army Research Laboratory (ARL), National Science Foundation (NSF), Defense Advanced Research Projects Agency (DARPA), and a large number of industries. Currently, he is leading the Center for Microelectronics and Sensors funded in 2008 by the Army Research Laboratory under the Micro-Autonomous Systems and Technology (MAST) Collaborative Technology Alliance (CTA) program. He is also leading a newly established center in Microwave Sensor Technology funded by King Abdulaziz City for Science and Technology (KACST). He has authored many book chapters and more than 235 papers in refereed journals on miniaturized and on-chip antennas, meta-materials, electromagnetic scattering, wireless channel modeling, random media modeling, microwave measurement techniques, radar calibration, inverse scattering problems, and microwave sensors. He has also had


more than 570 papers and invited presentations in many national and international conferences and symposia on similar subjects. His research interests include microwave and millimeter-wave radar remote sensing, metamaterials, electromagnetic wave propagation, and antenna miniaturization. Dr. Sarabandi served as a Member of NASA Advisory Council appointed by the NASA Administrator for two consecutive terms from 2006 to 2010. Currently, he is serving as the President of the IEEE Geoscience and Remote Sensing Society (GRSS). He was a Member of the Editorial Board of the P ROCEEDINGS OF THE IEEE, and an Associate Editor for the IEEE T RANSACTIONS ON A NTENNAS AND P ROPAGATION and the IEEE S ENSORS J OURNAL. He is a Member of Commissions F and B of URSI and is serving as the Vice Chair of the USNC URSI Commission F. He was the recipient of the Henry Russel Award from the Regent of the University of Michigan. In 1999, he was the recipient of a GAAC Distinguished Lecturer Award from the German Federal Ministry for Education, Science, and Technology. He was also a recipient of the 1996 EECS Department Teaching Excellence Award and a 2004 College of Engineering Research Excellence Award. In 2005, he was the recipient of the IEEE GRSS Distinguished Achievement Award and the University of Michigan Faculty Recognition Award. He also was the recipient of the Best Paper Award at the 2006 Army Science Conference. In 2008, he was the recipient of a Humboldt Research Award from The Alexander von Humboldt Foundation of Germany and the Best Paper Award at the IEEE Geoscience and Remote Sensing Symposium. He was the recipient of the 2010 Distinguished Faculty Achievement Award from the University of Michigan. The IEEE Board of Directors announced him as the recipient of the 2011 IEEE Judith A. Resnik Medal. He was recognized by the IEEE GRSS with its 2013 Education Award.

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