Wideband, Multiband, Tunable, and Smart Antenna Systems for Mobile and UWB Wireless Applications 2014
View this Special IssueResearch Article  Open Access
A Multiband Printed LogPeriodic Dipole Array for Wireless Communications
Abstract
A multiband printed Logperiodic dipole array (LPDA) antenna for wireless communications is presented. The antenna has been designed starting from Carrel’s theory, optimized using CST Microwave Studio 2012, and then realized. The comparison between simulated and measured results shows that the proposed antenna can be used for wireless communications both in the S (2.4–3 GHz) and in the C (5.2–5.8 GHz) frequency bands, with very good input matching and a satisfactory endfire radiation pattern. Moreover, it has a compact size, is very easy to realize, and presents an excellent outofband rejection, without the use of stopband filters, thus avoiding interference out of its operating frequency band.
1. Introduction
The increasing demands of wireless, and short range, high data rate transmissions, pushed to propose new wireless protocols using different bands of the frequency spectrum, in order to support high data rate wireless communications. This rapid development of shortrange radio links in the mobile communications and wireless industry (especially WiFi and wireless local area network (WLAN)) calls for antennas able to operate in different frequency bands simultaneously (multiband antennas), offering wideband operations covering the whole WLAN services. The most common desirable requirement consists of providing multiband operations, and the frequency bands required to a single antenna are 2.4–2.484 GHz for Bluetooth applications, 2.4 GHz and 5 GHz for WiFi applications (following HiperLan protocol), and 2.4 GHz, 5.2 GHz, 5.4 and 5.8 GHz for WLAN applications (following WLAN IEEE 802.11 standards). The demand for antennas offering high performance, compact size, and low cost, besides an easy integration into frontend circuits, suggests the use of printed technologies [1–3]. As a matter of fact, planar antennas are widely used because of their low profiles, easy design, and fabrication. In the design of the planar antennas, the microstripfed [4–6] and coplanar waveguide (CPW) fed [7–10] are the most popular feeding structures adopted in the recent literature. Multifrequency antennas are becoming very important since their use allows the reduction of the numbers of antennas and the meeting of the applications of many different wireless communication systems simultaneously, such as Wireless Local Area Network (WLAN) and IEEE 802.16 Worldwide Interoperability for Microwave Access (WiMAX). A variety of structures for designing multiband WLAN planar antennas have been proposed in recent years [4–12], based on known antenna concepts, but showing either a multiband or a tunable behaviour. Among the available current wireless communication standards, WiFi is nowadays rapidly gaining more and more supporters. The WiFi standard is based on the wellestablished protocols IEEE 802.11a, 802.11b, and 802.11 g and the emerging 802.11n [13]. The considered operating frequencies are within the industrial, scientific, and medical (ISM) free window: 2.412 and 2.484 GHz (2.45GHz center frequency and 72MHz bandwidth) for the 802.11b and 802.11g protocols, 5.170, and 5.805 GHz (5.5GHz center frequency and 635MHz bandwidth) for the 802.11a protocol, while the 802.11n protocol employs both frequency bands simultaneously [13, 14].
In this work, a multiband printed logperiodic dipole array, working both in the S and in the C frequency bands (from 2.4 to 3 GHz and from 5.2 to 5.8 GHz), which can be used as a multiband antenna for wireless communications, is presented, satisfying the requirements of several wireless communication standards, such as HiperLan, IEEE 802.11 and Bluetooth [13]. The proposed antenna is very easy to realize, is very compact, and presents an excellent outofband rejection, without the use of stopband filters. The designed antenna meets also the requirements of meteorological radars, whose operating frequency bands are 2.7–3.0 GHz for the Sband and 5.4–5.8 GHz for the Cband [1]. Therefore, it can be effectively used also as a feed for reflector antennas in weather radar applications.
2. Antenna Design
In this section the design of a high gain feed for wireless communications is presented. As pointed out in Section 1, the S and Cband are the most widely used in wireless communications, and therefore the design of a high gain printed log periodic feed able to work both in S and Cband is discussed here. The concept of logperiodic printed antennas is applied separately to two different groups of dipoles designed to operate each one in a specific frequency band. The two groups of dipoles have been then connected together, obtaining the configuration shown in Figure 1. The distance between the two groups of dipoles has subsequently been optimized, aiming for the best input matching of the whole antenna. The final distance is relatively small, and comparable to the spacing between two adjacent dipoles of the LPDA, resulting in a very compact multiband antenna.
(a)
(b)
This solution allows the obtaining of a log periodic antenna with a reduced size, operating only in the ranges 2.4–3.0 GHz and 5.2–5.8 GHz, instead of a complete printed LPDA array working between 2.4 and 5.8 GHz.
2.1. SBand and CBand LPDA Design
The two LPDAs operating, respectively, only in the Sband and only in the Cband have been designed separately, and their geometrical parameters are shown in Table 1. For each group of dipoles, the number of dipoles and the scaling factor must be defined [15–17]. Even though both parameters can be different for each group of dipoles, as a design rule we choose to use the same values for both groups.

The number of dipoles of each group is determined by the design specifications (i.e., the requirements on the frequency bandwidth and the directivity). In our case, for the proposed printed LPDA feed, we require an average directivity of 9 dBi, and therefore, following Carrel [15], we set the log period, , and the spacing factor, , of both groups of dipoles to the values and .
The chosen dielectric substrate is the ARLON AD 450, a material developed for high power applications [18], with low losses (dielectric loss tangent ) and a dielectric permittivity . The substrate thickness and metallization are, respectively, mm and mm. The metal thickness has been chosen to be twice the typical metal thickness of LPDAs (0.035 mm), so as to increase the power level capability of the antenna.
The characteristic impedance of the printed feeding lines (paired strips) of the two groups of dipoles has been selected in order to obtain easy matching with the employed UT056 coaxial cables [19]: by choosing Ω, we obtain mm.
The number of elements of each group of dipoles is computed by using the expressions given by Carrel [15–17]; starting from the required bandwidths, we get number of dipoles and aperture angle .
The lengths and and the widths and of the longest dipole of each group (Sband and Cband) have been evaluated using the cutandtry procedure described in [16, 17], obtaining mm, mm, mm, and mm.
The lengths and widths of the other dipoles of each group are computed by using the wellknown expressions for LPDAs [16, 17, 20]: In Figure 2 the simulated reflection coefficient for the group of dipoles designed to work in the Sband is reported, and the input matching is less than −10 dB in the required frequency band 2.4–3 GHz. Figure 3 shows the simulated reflection coefficient for the group of dipoles designed to work in the Cband, and, also in this case, the input matching is less than −10 dB in the required frequency band (5.2–5.8 GHz).
Figures 2 and 3 show also the simulated realized gain for the group of dipoles working in the S and Cband, respectively. In both cases, the realized gain rapidly decreases out of the working bandwidth, while the radiation pattern deteriorates in the same way, showing a higher SLL and a bad fronttoback ratio with respect to the values assumed within the antenna working bandwidth.
2.2. Complete Antenna Design
The two groups of dipoles designed in Section 2.1 have been connected together, obtaining the configuration shown in Figure 1. The geometrical parameters of the dipoles are reported in Table 1.
The value of the distance between the two groups of dipoles has been chosen so to obtain the best input matching of the whole antenna. The distance has been optimized using CST Microwave Studio, and the optimal value is equal to only 8.3 mm. This distance is relatively small, and comparable to the spacing between two adjacent dipoles of the LPDA, resulting in a very compact multiband antenna.
The starting length of the final termination of the paired strips has been chosen to be equal to one half of the freespace wavelength at the highest operating frequency and then optimized aiming at the best antenna input matching, obtaining the value of mm.
The feeding network selected for the designed antenna consists of a coaxial cable. The outer conductor of the coaxial cable is soldered to the bottom layer of the LPDA, and the inner conductor is connected to the top layer of the antenna using a viahole inside the substrate. An additional floating mirror coaxial cable, soldered in the top layer of the array, able to improve the antenna performances, has been used (as indicated in Figure 1). In fact, the insertion of an additional mirror coaxial cable gives to the antenna significantly better radiation performances, and allows the stabilizing of the phase center, without affecting the antenna input matching [16]. On the other hand, the LPDA could also be fed using a fully planar network, namely a coplanar waveguide [17], solution which allows a more simple realization, and with a low cost, a compact size, and an easier connection with the SMA connector, but handling only low power levels, due to the dielectric breakdown of the air between the metallic strips.
3. Results
The LPDA, designed in Section 2, has been manufactured (see Figure 4) and fully characterized. All the numerical results take into account also the additional floating mirror coaxial cable, positioned in the top layer of the array.
(a)
(b)
In Figure 5 the comparison between the simulated and experimental reflection coefficient for the complete antenna is shown, and the input matching is very satisfactory, being less than −10 dB both in the S and in the Cband (2.4–3 GHz, 5.2–5.8 GHz). The simulated and measured data are in very good agreement, and the outofband rejection is very good, especially considering that no stopband filters have been used in the antenna design.
Figure 6 reports the frequency behaviour of the realized gain G_{R} for the antenna shown in Figure 1 (both evaluated by CST and measured). The antenna gain has an average value equal to 8.75 dB in the Sband and equal to 9.35 dB in the Cband. On the other hand, it rapidly drops to less than 3 dB out of the working frequency band, confirming the very good outofband rejection of the proposed antenna.
In Figure 7 the simulated  and Plane antenna radiation patterns are shown. The crosspolar component is not shown, since it is always below −35 dB with respect to the copolar component of the radiated field. The radiation pattern shows an endfire behavior within the design frequency band (2.4–3 GHz and 5.2–5.8 GHz), with a SLL below −27 dB and an F/B ratio above 28 dB, while it deteriorates very rapidly outofband, with both a bad SLL and fronttoback ratio. The symmetry of the inband radiated field is very good both in the  and in the planes, thanks to the additional mirror coaxial cable, soldered in the top layer of the LPDA. Therefore, the proposed LPDA can be successfully used as a multiband antenna for wireless communications.
(a)
(b)
4. Conclusion
A multiband printed Logperiodic dipole array (LPDA) antenna for wireless communications, covering both the S (2.4–3 GHz) and the C (5.2–5.8 GHz) frequency bands has been presented. The antenna is fed using two coaxial cables, which provide the required broadband input matching, and improve the radiation pattern when compared with an antenna fed with a single coaxial cable. The simulated and measured results are in very good agreement, showing a very good input matching, an endfire radiation pattern, and an excellent rejection out of its operating frequency band, without the use of stopband filters, avoiding undesired interference. The antenna realized gain is above 8.75 dB within the working band, decreasing to less than 3 dB in the outofband range.
Conflict of Interests
The authors declare that there is no conflict of interests.
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Copyright
Copyright © 2014 Giovanni Andrea Casula and Paolo Maxia. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.