Dual Millimeter Wave Reconfigurable Dielectric Lens Antenna R. A. Santos(1), G. L. Fré(1), F. B. Mejia(2) and D. H. Spadoti(1) 1
Institute of Systems Engineering and Information Technology - IESTI, Federal University of Itajubá - UNIFEI, Itajubá - MG, Brazil (e-mail:
[email protected]) 2 National Institute of Telecommunications - INATEL, Santa Rita do Sapucaí - MG, Brazil
Abstract—The objective of this paper is to present a directive hemispherical dielectric lens antenna, with reconfigurable frequency, operating in the millimeter wave ranges. Numerical results obtained from Ansys HFSS® software attest the structure functionality. The dielectric lens antenna is capable to reconfigure the operating band from 44 GHz to 75 GHz, with gain in the direction of maximum irradiation reaching 24.5 dBi. Index Terms—Lens Reconfigurable antennas.
I.
antennas,
Printed
antennas,
more directive as it collimates the radiated energy. Therefore, this feature could intensify frequency reconfigurable radiators. The associated lens may suppress rejected frequencies and improves the reconfigured frequency. Thus, the main objective of this paper consists in modelling a frequency reconfigurable printed antenna (FRPA), successfully associated with dielectric hemispherical lens, for operations at millimeter waves spectral region. The whole system scheme is shown in Fig. 1.
INTRODUCTION
The evolution of wireless communication services [1], [2], associated with device-to-device communications [3], [4], have challenged researcher around the globe to find new solutions to future of telecommunications. Among those challenges, there is also the spectral scarcity for commercial license frequency ranges [5]. The possibility of meeting the increasing data rates from current technology are quite reduced. For this reason, the spectral region, called by millimeter waves, has been object of several researches in order contribute to solve these issues. Authors such as L. Kong et al. [6] and T. S. Rappaport et al. [7], [8], have proposed models for future communication systems designed for millimeter waves. Exploring this spectral region is convenient for communication purposes, once it is almost unused. Besides the advantages of operating at millimeter waves, there are some challenges to be overtaking. The path loss associated with high frequency, It's a major challenge. Therefore, great amounts of power would be necessary for short communications links. However, there are frequencies in the band millimeter waves whose path loss not so extreme, as at 40 GHz and 70 GHz. Nevertheless, the link range will never be compared to UHF communications links. Thus, improving wireless communication distances at these frequencies makes necessary a conception of high frequency and high directivity antennas [9], [10]. Those features have pointed the use of dielectric lenses (Fig. 1) as a possible solution. There are on literature several antennas (radiators) capable of change operating frequency [11], [12]. Thus, a single structure can operate in both most convenient spectral regions for millimeter waves, achieving the more effective use of spectral range. For high frequency operations, combining radiators with lens is an interesting solution to dealing with enormous path loss. The lens, such as parabolic reflectors, makes the system
Fig. 1. Dual Millimeter Wave Reconfigurable Dielectric Lens Antenna.
II. FEEDER:FREQUENCY ANTENNA (FRPA)
RECONFIGURABLE
PRINTED
The lens illuminator must present, in real time, a frequency dynamic reconfiguration. This feature could by be obtained with several design techniques for frequency reconfigurable radiators [13–16]. From a rectangular microstrip antenna (MSA), Sanket Kalamkar et al. [17] describes a simple method which consists in inserting a diagonal cut along irradiator element length, L, as shown in Fig. 2. A reconfigurable frequency feature is achieved by setting a mechanism to connect both parts of the split radiator. With both part connected, there is an electric length greater than the separated condition. The MSAs, whose design techniques are widespread [18] has its operation in TM010 fundamental mode, with a resonance frequency, , that is inverse proportion to its length. An approximated value for, , is obtained by:
original one. Thus, activating the switch, the resonance frequency actually becomes lower than the reference design.
Fig. 2. Frequency reconfigurable printed antenna.
f ≅
3 × 108 1 2 εrd L
(1)
in which, is the relative electric permittivity of the substrate dielectric plate. Hence, the resonance frequency, , varies with, , modifications. Furthermore, that modification must be done as long as the antenna operation, which leading in a dynamic real-time reconfigurable capability. According with the concepts proposed by T. S. Rappaport et al. [8], millimeter waves are interesting spectral region for this application. Mainly, because of convenient size of devices for this frequencies (between 40 and 70 GHz). In this paper, a 60 GHz MSA was taken as basis for design of a FRPA (Frequency Reconfigurable Printed Antenna). The dielectric substrate chosen is the commercial RT/Duroid® 5880 with 2.2 and thickness 0.254. This dielectric plate is widely employing for high frequency systems, as it shows low dielectric loss [19]. Following the methods proposed in [18], the final dimensions for this MSA is: L = 1.543 mm and W = 1.976 mm. The feeder transmission line width, wℓ = 0.5 mm, being, Lc = 0.514 mm and Wc = 0.083 mm, respectively, the length and width of its lateral cuts. The reflection coefficient was analyzed in order to evaluate the performance of MSA. This parameter is related with impedance matching. There is a maximum acceptable reflection coefficient, by convention it is 10 dB, which ensures that more than 90% of generated energy is effectively delivered to radiating structure. Thus, the system bandwidth is defined for any frequency range in which remains acceptable. As shown in Fig. 3, this MSA operates from 58.39 GHz to 61.73 GHz, representing a bandwidth of 5.56% of central frequency. In order to convert the MSA in FRPA, a slit with width 3 /5 is inserted, splitting the resonator in two elements, as seen in Fig. 2. The slit position is set to transfer the resonance point from around 60 GHz to 70 GHz. As observed in Fig. 3, this modification results in a reduction of L, and places the bandwidth between 70 and 77.5 GHz. Hence, a cupper switch is placed inside the slit to perform the dynamic reconfiguration. The switch width must be enough to interconnect both metal surfaces, so a width of was considered. It is observed that inserted switch modifies the original resonance point to 45 GHz, establishing its bandwidth between 44.5 GHz and 45.5 GHz. It is explained by Fig. 2, the switch is placed at center of MSA, and the path of electric current rises, indicating an electric length larger than the
Fig. 3. Frequency reconfigurable printed antenna reflection coefficient.
The rectangular coordinates radiation pattern of FRPA are seen in Fig. 4, for xz-plane in different frequencies. The frequency transfers cause modifications in radiation behavior. For 0 , referring the frequency of 45 GHz, the structure reaches a gain 6.2 dBi with active switch, otherwise 15.9 dBi otherwise. Also, for 73 GHz, 3.5 dBi with deactivated switch, and 8.17 dBi otherwise. Thus, the reconfiguration is numerically validated for FRPA according both parameter and G0. The desired features for illuminate a hemispherical dielectric lens was achieved.
Fig. 4. Frequency reconfigurable printed antenna radiation pattern.
III.
FREQUENCY RECONFIGURABLE LENS ANTENNA FOR MILIMMETRIC WAVES
In dielectric lens design, two dimensions are particular relevant, as seem in Fig. 1: the lens radius, , and the distance from the flat lens surface to the illuminator, . The radius could be given in terms of wavelength, which depends on the material electric properties. For this desing, the lens model has considered High Impact Polystyrene (HIPS), with ℓ 2.6, as dielectric material. So, the radius was established as three free space wavelengths for 60 GHz ( 3λ). Operating at 45 GHz, the radius is about 2.25λ and for 73 GHz the R is 3.65λ. The expression who defines the focal distanceis [20]: 1 F ≅ − 1 R ε −1 r
(2)
Thus, the whole design of a dual millimeter wave reconfigurable dielectric lens antenna is concluded by finding the focal distance. The nominal frequency operation is 60 GHz, resulting in ≅ 3.164 mm. In order to numerically validates the entire system operation, the commercial software Ansys HFSS® was employed. It utilizes the finite element method to evaluate parameters such as and radiation pattern. This simulation gives the influence of lens upon FRPA impedance matching. Always that an electromagnetic wave crosses the interface that separates different media, a portion of wave is reflected in the interface. This reflected wave modify the original impedance matching, as seen in Table 1 and Fig; 5. Besides the small changes in the resonance frequency for both switching condition, the presence of the lens does not significantly affect the system operation.
Fig. 6. Dual Millimeter Wave Reconfigurable Dielectric Lens Antenna radiation pattern.
IV.
Fig. 5. Dual Millimeter Wave Reconfigurable Dielectric Lens Antenna reflection coefficient.
This paper demonstrated a frequency reconfigurable hemispherical dielectric lens antenna for millimeter waves. A theoretical approach confirmed the possibility of dynamic frequency reconfiguring the rectangular MSA. It was shown that the reconfigurable radiator associated with the hemispherical dielectric lens, improve the entire realized gain for both operation bands. Moreover, it intensified the suppression effect to rejected band, intensifying the switching effect. It was numerically demonstrated the whole structure operation at frequency ranges around 44 and 75 GHz, with the maximum gain of 24.5 dBi. The resulting has shown a radiating structure with great potential for further wireless communication technologies
Table 1. Comparation of frequency range operations. FRPA (GHz)
Operation mode
Band 1
CONCLUSIONS
ACKNOWLEDGMENT FRLA (GHz)
Band 2
Band1
Band 2
Key OFF
70.0-77.5
-
70.6-78,3
-
Key ON
-
44.5-45.5
-
43.7-44.3
An analysis of radiation pattern is presented in Fig. 6. This is the gain quantified in xz-plane at frequencies that have provided better impedance matching condition. For the lowest frequency, the realized gain is about 19.6 dBi, and the for the higher frequency approximately 24.5 dBi. This happens because the shorter is the wavelength the larger is the lens electric radius. Similar to parabolic reflectors, the gain provided by a dielectric lens is direct proportional to its electric length, hence the gain increases for high frequency operations. The lens effect intensifies the frequency reconfiguration function. It provides at least 20 dB of suppression in rejected frequency in direction of maximum irradiation.
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