A microstrip-fed cavity-backed circularly polarized horn antenna

A MICROSTRIP-FED CAVITY-BACKED CIRCULARLY POLARIZED HORN ANTENNA Nak-Seon Seong1 and Seong-Ook Park2 Department of Telematics/USN Electronics and Telecommunications Research Institute (ETRI) Daejeon, Korea 2 School of Engineering Information and Communications University (ICU) Daejeon, Korea 1 Received 4 May 2006 ABSTRACT: A 20 GHz 2 ⫻ 2 subarray radiator of microstrip-fed cavitybacked circularly polarized horn is presented. Since horn antennas incorporating waveguide types of circular polarizers have big size of radiator height, microstrip feeding and polarization is proposed and cavity-backed introduced to improve radiation performance. Two approaches of single and 2 ⫻ 2 subarray of horn are compared to be a basic radiation element of an array antenna. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2454 –2456, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21952 Key words: antenna; cavity; horn; microstrip; polarizer 1. INTRODUCTION Antennas used in mm–wave frequencies have some restrictions of performance such as radiation efficiency, size, and so on. So horn antennas have been widely used for the frequency bands because of its good performance in efficiency. But horn antennas have disadvantages of size and structural complexities, especially, when circular polarization is to be employed for phased array antenna. In our case of a development of 20-GHz active phased array antenna, one of the main concerns was the size reduction of radiator height, so eventually its volume. Since most horn antennas uses waveguide types of polarizer, the volume of the phased array antenna becomes inevitably bulky due to its long configuration along the radiator. However, in order to overcome the structural problems of size in the horn type, horn antennas incorporating microstrip structures have been proposed for their compactness and simplicity. But their uses are limited by structural complexity that feeder and polar- Figure 2 Geometry of microstrip-fed cavity-backed horn antenna: (a) side view, (b) front view Figure 1 Radiation patterns of horn antenna 2454 izer are separated [1], or limited to linear polarization [2]. Other approaches using microstrip feeding methods such as microstrip-fed cavity-backed slot antenna [3] and microstrip-fed horn antennas [4, 5] have been reported. The former one obtained a radiation gain of 8.7 dBi at 6 GHz for four-element subarray, while the latter obtained 5.5 dBi without horn and 9 dBi with horn at 5.8 GHz for a single radiator. Nasimuddin [5] tried to reduce planar size using high permittivity substrate. However, this paper proposes a combined method of the microstrip feeding and caivity-backed horn radiation to reduce the size and to obtain optimized antenna performances. Further, basic radiator elements for the 20-GHz phased array antenna are investigated between single and 2 ⫻ 2 subarray structure of horn. MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 48, No. 12, December 2006 DOI 10.1002/mop Figure 3 Feeding network of 2 ⫻ 2 subarray 2. SELECTION OF RADIATION ELEMENT In the design of a basic radiation element for phased array antenna, there are two alternatives for chosen interelement distance, for example, d ⫽ 2.4 ␭ as follows: ● ● ● a single horn with the aperture diameter d ⫽ 2.4 ␭ as the radiator a 2 ⫻ 2 subarray of four conical horns as the basic radiator element with dh ⫽ 1.2 ␭ Beam width of the conical horn with aperture diameter dh ⫽ 1.2 ␭ do not practically change for different heights of R ⱖ 29 mm at f ⫽ 20.5 GHz. The half power beam width is equal to ␪3dB ⫽ 53° ⫻ 53°, and the radiation gain G ⫽ 16.5 dB (Fig. 1). Predicted radiation pattern of a 2 ⫻ 2 radiation subarray of conical horns with aperture diameter dh ⫽ 1.2 ␭ is also shown in Figure 1. The width of the main beam of such radiator is ␪3dB ⫽ 22° ⫻ 22° and the gain G ⫽ 16.5 dB. For a single conical horn with aperture diameter dh ⫽ 2.4 ␭ for different heights of the horn R at f ⫽ 20.5 GHz, its radius is selected as R ⫽ 50 mm, for the main characteristics of the horn are stabilized Figure 5 Predicted and measured results of 2 ⫻ 2 subarray: (a) radiation pattern (E-plane), (b) return loss from that value. In this case, the width of this beam pattern is ␪3dB ⫽ 26° ⫻ 30° and the gain is G ⫽ 14.5 dB as compared in Figure 1. Analysis mentioned earlier draws a conclusion that the most useful structure to reduce the radiator height for the phased array radiator is a 2 ⫻ 2 subarray of four radiators than a single horn of double size in height and radius. The great decreasing of dimensional size of the radiator along its height is obtained. 3. ANTENNA DESIGN Figure 4 Picture of fabricated antenna. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] DOI 10.1002/mop Figure 2 is a crosssectional view of the proposed antenna element of microstrip-fed circularly polarized cavity-backed horn. The feed network and circular polarizer are implemented on the Duroid 5880 substrate (␧r ⫽ 2.22, h ⫽ 0.508 mm, tan ␦ ⫽ 0.0009). Height of the waveguide hh is selected based on R ⫽ 29 mm. Diameter of the cavity Dc is chosen by means of existence of the main wave type of TE11 mode in it. In our case, Dc ⫽ 0.75 ␭. For the optimal exciting of the wave-guide resonator, it is necessary that the distance between microstrip resonator and the bottom of the wave- MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 48, No. 12, December 2006 2455 guide one is about hc ⫽ ␭g/4 ⫽ 0.335 ␭, where ␭g is wavelength in a waveguide. In our case, hc⫽ 0.4 ␭, where ␭ denotes signal wavelength in the free space. Exciting circuit for subarray of four radiators is shown in Figure 3. It was designed on the base of symmetric stripline. For circular polarized field exciting in each horn, orthogonal resonators L6 and L7 with phase shift of ␲/2 are used, such phase shift is realized by different length of resonators L6 and L7. Ellipses of circular polarization of the radiators 1– 4 are rotated in space on ␲/2 one by one for increasing of polarization coefficient of total field of subarray. For this space rotation, different lengths in exciting circuit L1, L2 and L3, L4 are used and such difference of electric lengths L2 ⫺ L1 and L4 ⫺ L3 equals ␲/2. The fabricated radiator element of single horn is shown in Figure 4. Figures 5(a) and 5(b) present the simulated and measured radiation pattern and return loss of the microstrip-fed cavity-backed 2 ⫻ 2 four-element horn subarray, respectively. The measured antenna gain of the subarray is 15.2 dBi at 20.5 GHz and the 15 dB return loss is about 1.85 GHz. The measured radiation pattern well matches to the expectation, but the gain is slight smaller by 1.2 dB. This loss is attributed to the loss in power combining from each element of the 2 ⫻ 2 subarray in the air due to the unbalance of the four-way power divider in the feeding network, the loss in the dielectric substrate, and measurement errors. 4. CONCLUSIONS This letter has presented a microstrip-fed cavity-backed horn antenna for circular polarization. Two types of single and 2 ⫻ 2 subarray of four-element have been discussed for selection of a basic radiator element for phased array antenna. The designed circularly polarized horn antenna is structurally compact and exhibits good performances in bandwidth, radiation pattern, and gain. The height of the radiator is reduced as compared to using waveguide types of circular polarizer, on the other hand, the antenna gain increases when using flat microstrip antennas. REFERENCES 1. M. Sironen, Y. Qian, and T. Itoh, A 60 GHz conical horn antenna excited with quasi-Yagi antenna, IEEE MTT-S Int Microwave Symp Dig, 1 (2001), 547–550. 2. A. Abdel Rahman, A.K. Verma, and A.S. Omr, High gain wideband compact microstrip antenna with quasi-planer surface mount horn, IEEE MTT-S Int Microwave Symp Dig 3 (2003), 1721–1724. 3. Q. Li, Z. Shen, and P.T. Teo, Microstrip-fed cavity-backed slot antennas, Microwave Opt Technol Lett 33 (2002), 229 –233. 4. Y.-B. Jung, S.-Y. Eom, S.-I. Jeon, and C.-J. Kim, Novel Ka-band microstrip antenna fed circular polarized horn array antenna, IEEE MTT-S Int Microwave Symp Dig 3 (2004), 2476 –2479. 5. Nasimuddin, K. Esselle, and A.K. Verma, Compact circularly polarized enhanced gain microstrip antenna on high permittivity substrate, Microwave Conf Proc Asia–Pacific Conf Proc 4 (2005), 4 –7. A NOVEL DESIGN OF A CPW-FED SQUARE SLOT ANTENNA WITH BROADBAND CIRCULAR POLARIZATION Kow-Ming Chang,1 Ren-Jie Lin,1 I-Chung Deng,2 Jin-Bo Chen,2 Ke Qing Xiang,2 and Chang Jun Rong2 1 Department of Electronics Engineering and Institute of Electronics National Chiao Tung University Hsinchu, Taiwan 30050 Republic of China 2 Department of Electronics Engineering and Institute of Mechatronic Engineering Northern Taiwan Institute of Science and Technology Taipei, Taiwan 112, Republic of China Received 4 May 2006 ABSTRACT: A novel design of a broadband circularly polarized square slot antenna fed by a single coplanar waveguide is proposed and discussed. The circularly polarized radiation is achieved by means of using an outer rectangular ring and an inner rectangular ring at the center of a square slot. The proposed antenna has the fundamental resonant frequency of 2.44 GHz with the return loss of ⫺40.93 dB, and it has the bandwidth of 3 dB axial ratio of 420 MHz or (17.21%). Details of the results are presented and discussed. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2456 –2459, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21951 Key words: circularly polarized; square slot antenna; coplanar waveguide; axial ratio 1. INTRODUCTION Coplanar waveguide (CPW) was widely studied as an alternate to microstrip line for feeding square slot antenna in past few years, because they are compatible with the monolithic microwave integrated circuits (MMIC) and active device applications [1– 4]. Besides, the CPW-fed slot antenna can also have relatively much wider impedance bandwidth than the conventional microstrip antenna [5, 6]. They have enough bandwidth to support modern wireless communication. Thus, the designs of the CPW-fed antennas have recently received much attention, especially for the circularly polarized (CP) CPW-fed slot antenna. In order to obtain CP radiation using a single feed, many microstrip antenna designs have been reported [7]. The obtained CP bandwidth determined by 3 dB axial ratio (AR) bandwidth is usually narrow and less than 2%. It is also noted that most of the available CP wide slot antenna designs are with a microstrip line feed. However, few studies have been done on the design with CPW feeding [8, 9]. Furthermore, it is very difficult to achieve a good impedance matching and a good AR at the same dimensions for the CPW-fed slot antenna. In this study, we propose a novel design of a CPW-fed square slot antenna with broadband CP. The proposed antenna exhibits several advantages such as 3 dB AR profile over a wide frequency range, 3 dB AR space distribution over a wide elevation range, and broad impedance bandwidth. 2. ANTENNA CONFIGURATIONS © 2006 Wiley Periodicals, Inc. 2456 The geometry of the proposed CPW-fed CP slot antenna is depicted in Figure 1. We used an inexpensive FR4 dielectric substrate with a thickness of 1.6 mm, a relative permittivity of 4.4, and a loss tangent of 0.0245. In this suggested antenna, the lengths of L1, L2, L3, and Lf are fixed to be 49, 29, 17, and 11 mm, respectively, and the widths of W1, W2, W3, W5, and W6 are fixed to be 49, 23, 17, 15, and 10 mm, respectively. A 50-⍀ CPW transmission line, having a protruded single strip of width Wf ⫽ 6.3 mm and a gap of distance g1 ⫽ 0.5 mm MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 48, No. 12, December 2006 DOI 10.1002/mop