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Uniplanar Millimeter-Wave Log-Periodic Dipole Array Antenna Fed by Coplanar Waveguide
A uniplanar millimeter-wave broadband printed log-periodic dipole array (PLPDA) antenna fed by coplanar waveguide (CPW) is introduced. This proposed structure consists of several active dipole elements, feeding lines, parallel coupled line, and the CPW, which are etched on a single metallic layer of the substrate. The parallel coupled line can be optimized to act as a transformer between the CPW and the PLPDA antenna. Meanwhile, this transform performs the task of a balun to achieve a wideband, low cost, low loss, simple directional antenna. The uniplanar nature makes the antenna suitable to be integrated into modern printed communication circuits, especially the monolithic millimeter-wave integrated circuits (MMIC). The antenna has been carefully examined and measured to present the return loss, far-field patterns, and antenna gain.
With the development of the modern wireless communication systems with low transmitted power and high data rate demand, the wideband and high-directivity antenna becomes an essential component in the front-end system. The traditional log-periodic dipole array (LPDA) antenna in a free space environment is first introduced by Isbell . Because of its end-fire characteristics, such as the fixed peak radiation and the stable radiation pattern, and broad bandwidth, the LPDA antenna has been widely studied and applied in many communication systems, such as TV, radar, satellite, and indoor commutations [2–7]. However, the traditional LPDA antenna is a dimensional structure, so it is difficult to design, debug, and achieve mass productivity at high frequency band, especially at millimeter-wave band. More seriously, it cannot be integrated with modern planar commutation system.
In order to overcome the drawbacks of the LPDA antenna, the dipole elements and the feeding lines can be achieved by using the modern printed circuit board (PCB) technology; this antenna can be called printed log-periodic dipole arrays (PLPDA) antenna [8–13]. Therefore, the PLPDA antenna takes the advantages of low profile, low cost, high stability, and ease to be massively produced and deeply integrated into modern planar integrated wireless communication systems.
Similar to the printed quasi-Yagi , the performances and design complexity are mainly challenged by the selection of an appropriate feeding network. The PLPDA antenna was initially introduced by Camlbell et al. . The antenna is designed on a double-layer substrate, and its dipole elements, fed by the stripline in the middle metallic layer, are cross-symmetrically distributed on the top and the bottom metallic layer, respectively. But double-layer substrate configuration increases the cost of the antenna [8–11]. In order to reduce the fabrication cost, the single layer PLPDA antennas fed by coaxial cables have been proposed [12, 13]. Unfortunately, the coaxial cables should be wielded with the top and bottom feeding lines of the PLPDA antenna, which will bring significant fabrication error at high frequency band. Substrate integrated waveguide (SIW) [15–18], also named laminated waveguide  or postwall waveguide , has been deeply studied on the operation principle and widely applied in the design of the microwave and millimeter-wave antenna, filter, coupler, and systems [21–26]. Because of the intrinsic balun characteristics between the top and bottom metallic layer, the SIW can be applied to the feeding network design of the PLPDA antenna . However, the size of the SIW is larger than that of the traditional printed transmission lines, such as the slot line, microstrip, and CPW. More seriously, the dipole elements of the PLPDA antenna mentioned above are totally cross-symmetrically spaced along the parallel feeding line on respective sides of the dielectric substrate, which increases the PCB fabrication cost compared to the single metallic layer design.
It is well known that, for millimeter-wave integrated circuits, CPW has several distinct advantages over microstrip, which includes the surface-mount integration, lower phase velocity variation, and good characteristic impedance control . Specifically, the CPW features a wideband, low loss and fabrication cost, and simple uniplanar structure without the need for the vias and bottom metallic ground compared to the conductor-backed CPW structure [14, 29–32]. The slot PLPDA antenna can be fed by CPW , but the slot antenna suffers from high insertion loss and lower power handling capability.
In this paper, a new broadband CPW feed for the uniplanar PLPDA antenna is proposed. The advantage of the novel proposed antenna is that it avoids bond wires or air-bridge lines, and all the printed dipoles and the feeding lines as well as the CPW are printed on only one side of the substrate without the vias. Therefore, this novel uniplanar millimeter-wave PLPDA antenna alleviates the design complexity, reduce the cost, and can be easy to integrate with MMIC.
2. Antenna Design Principle
2.1. Geometrical Layout
The proposed PLPDA antenna with four dipole elements fed by CPW is shown in Figure 1. In the antenna design, the dipole elements are symmetrically distributed along the two feeding lines on the top metallic layer of the substrate. A gap is to lead the opposite current direction between the two feeding lines. , , and are the length, width, and spacing of the dipole elements for the PLPDA antenna. is the width of both the feed line and the central line of the CPW. The 50 Ω CPW is transited to the two feed lines by the two parallel uniplanar couple lines. One parallel line is connected to the central line of the CPW, and the other one is connected to one side of the ground of the CPW. The array is fed from the longest element. The proposed antenna is printed on a single layer dielectric substrate, and depresses its height. The dipole elements, the feeding lines, the parallel uniplanar couple lines; and the CPW are totally etched on the top side of the substrate.
2.2. Current Distribution
The magnetic distribution of the CPW at dominant mode, including the side view and the top view, is shown in Figure 2. It can be seen that a 180° phase difference is presented between the central line and the ground of the CPW. The CPW can perform a good balun line to feed the PLPDA antenna over wide operating frequency band because of the wide bandwidth of its dominant mode.
Sometimes the CPW should be balanced between the left and the right ground with air-bridge line, which greatly complexes the circuits design and increases the fabrication cost. In this design, the CPW is connected to the PLPDA antenna by the parallel uniplanar couple lines, which can be carefully designed to take the task of a balun to avoid the air-bridge line. Therefore, the length of the parallel uniplanar couple lines is the key factor for the proposed CPW PLPDA antenna.
Figure 3 shows the simulated current distribution of the dipole element at 35 GHz. The current is mainly concentrated in the region between the second and third dipole elements at 35 GHz. It can be seen that the currents of the second and third dipole elements on the left side are in phase with that printed in the opposite direction on the right side of the substrate, and the currents of other dipole elements on the left side are out of phase with that printed in the opposite direction on the right side of the substrate. Therefore, it is demonstrated that the proposed CPW PLPDA antenna can be radiated in the monodirection towards the shorter elements.
2.3. Parameters Determination
The proposed CPW PLPDA antenna is composed of three parts: dipole elements spaced with the feeding lines, parallel couple lines, and the CPW. At first, the parameters of the dipole elements spaced with the feeding lines can be primarily determined by the modified Carrel's method , including the consideration of the effective relative dielectric permittivity in computations to dipole lengths and spacing.
The length of each half wavelength dipole is depressed by , which can be given by and according to the PLPDA antenna design principle, the parameters , , and can be determined by the scale factor and spacing factor as given by  where can be calculated by where is the shortest effective working wavelength, is the effective dielectric permittivity, is the dielectric constant, is the substrate thickness, and is the width of the first dipole element.
The proposed CPW PLPDA antenna is fabricated on Fr-4 with a thickness of 0.5 mm, = 4.3, = 0.025, and then the parameters of the 5 Ω CPW and can be determined according to the transmission line theory.
The parallel couple line can take the tasks of the balun. It performs as a transformer between the CPW and PLPDA antenna. So the determination of is one of the key factors for the antenna design, which can be primarily chosen as quarter operating wavelength.
The length of the longest dipole can be calculated from (4) according to the lowest operating frequency. Then the length of the other elements can be calculated from (2); meanwhile, the parameters and can be optimized by the full-wave simulation software HFSS. The width of the other elements and the spacing between the elements can be obtained from (2) and (3). After optimization, the detailed dimensions of the proposed antenna are = 0.9 mm, = 0.7 mm, = 0.5 mm, = 0.4 mm, = 2.63 mm, = 1.63 mm, = 0.88 mm, = 0.63 mm, = 1.7 mm, = 1 mm, = 1 mm, = 0.6 mm, = 0.8 mm, and = 0.13 mm.
3. Experimental Results
The CPW PLPDA antenna at 25 GHz–40 GHz is designed, fabricated, and tested. The measured and simulated input return losses of the proposed antenna are presented by Figure 4. The is better than 10 dB from 28 GHz to 38.5 GHz. Note that over a narrow band (around 35 GHz), return loss values in excess of 15 dB can be obtained. However, compared with the simulated result, the measured center frequency of the proposed antenna is shifted toward the lower frequency about 1.5 GHz, which is mainly introduced by the relationship between the frequency and the dielectric permittivity without consideration in the simulation.
The radiation patterns and gain are examined in the microwave chamber. The measured and simulated antenna gain of the proposed antenna is shown in Figure 5. It can be seen that the CPW PLPDA antenna can achieve the maximum gain of 4 dBi at 39 GHz. The low gain, varying from 1.5 dBi to 4 dBi over the entire operating frequency band, is resulted from the large substrate losses of the Fr-4. The measured antenna gain of the CPW PLPDA antenna is less than that of the simulated results, which is mainly provided by the losses of the connectors and the increasing substrate loss of the antenna at millimeter-waves.
The radiation patterns of the 28 GHz–40 GHz CPW PLPDA antenna are measured over the entire band, which present the similar characteristics. So for brevity, only the 35 GHz radiation patterns are shown in Figure 6, which shows that the front-to-back ratio is larger than 15 dB. However, the main beam is shifted from the bore sight, which is caused by the nature asymmetrical feature between the central line and the ground of the CPW.
The proposed PLPDA antenna fed by CPW provides a viable choice for millimeter-wave broadband printed antennas in modern wireless communication system. The new CPW feed is more appropriate than that introduced before at millimeter-wave frequency, as it avoids the requirement for air-bridge line, vias, and any additional balun design in direct CPW-to-PLPDA connections. Therefore, the proposed antenna can be achieved on only one metallic layer of a single layer substrate, which can reduce the loss, cost, and complexity for the fabrication. The prototype for the proposed uniplanar CPW PLPDA antenna has been fabricated and measured. Radiation pattern and gain of the CPW PLPDA antenna are studied and presented. The advantages such as low cost, broadband, compactness, and simplicity are verified by the experiment. The design concept can easily be scaled for applications with different bandwidth and directivity requirements by adjusting the parameters of , , , , and , which can also improve the VSWR. So the proposed antenna can be used for wideband integrated multimode radio communication such as a digital cordless system (DCS), personal communication system (PCS), WLAN, UWB systems, and some systems required to work at Ka band.
This work was supported in part by NSFC under Grant 61201069, in part by Research Fund for the Doctoral Program of Higher Education of China under Grant 20120076120035, and in part by Open Foundation of State Key Laboratory of Millimeter-Waves under Grant 201308.
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