A Trident like Antenna with Reconfigurable Patterns for Automotive ...

2017 International Workshop on Antenna Technology: Small Antennas, Innovative Structures, and Applications (iWAT)

A Trident like Antenna with Reconfigurable Patterns for Automotive Applications Jerzy Kowalewski 1 , Sebastian Peukert 1 , Tobias Mahler 1 , Jonathan Mayer 1 , and Thomas Zwick 1 Institut

1

f¨ur Hochfrequenztechnik und Elektronik (IHE), Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76127 Karlsruhe, Germany, [email protected]

Abstract—Within the scope of this work, a reconfigurable antenna for automotive applications has been developed and constructed. The concept of planar parasitic array has been chosen due to its size. Furthermore, as an improvement to this concept, the parasitic elements were also used as active radiators. The antenna realizes four reconfigurable patterns that cover the angular regions in and against the driving direction as well as those orthogonal to the driving direction. Additionally a DC feeding network using metamaterials to decrease its influence on antenna characteristics is proposed. The measurement results match the simulated results and maximal gain of about 8.5 dBi is obtained.

is presented in [4]. The reconfiguration flexibility and size make it interesting for automotive case. However, since the antenna is horizontally polarized it cannot be applied for given application without vast modification. This work presents design and evaluation of a pattern reconfigurable antenna. Presented antenna fulfills the requirements for an automotive roof antenna and realizes patterns optimized for urban scenarios. Furthermore a DC feeding network using metamaterials decreasing the influence of DC lines on antenna characteristics is proposed.

Index Terms—reconfigurable antenna, p-i-n diode.

I. I NTRODUCTION Nowadays, more and more data is being transmitted in wireless communication systems. In order to be able to transfer this ever increasing amount of data channel capacity should be improved. To do so MIMO (multiple-input-multiple-output) systems can be used in multipath-rich environments. The quality of the channels established in MIMO communication in turn depends on the signal-to-noise ratio (SNR). Thus pattern reconfiguration plays an increasingly important role in increasing the channel capacity and improving the outage probability. In the automotive industry, wireless communication systems are also gaining more importance. It can be observed that many car models already have access to the mobile internet (Long Term Evolution, LTE) and due to the development in the direction of autonomous driving, or driving assistance systems, communication between the automobiles and the base stations is becoming more and more important. In the future fifth generation of the mobile communication (5G) a significant increase of the channel capacity in comparison to the current LTE standard and networking of cars as well as other devices is planed. Most of the modern cars use antennas with omnidirectional patterns for communication applications [1]. However, as discussed in the previous paragraph, this is not an optimal solution. A solution offering more flexibility and better directivity as well as low costare pattern rsconfigurable antennas. One of the possible concepts is ESPAR (electronically steerable parasitic array radiator) presented in [2]. This antenna offers enough flexibility in terms of pattern reconfiguration. However this antenna is bulky and has a complex construction and thus cannot be applied for automotive applications. The antenna presented in [3] has smaller dimensions, yet its flexibility in terms of pattern generation is limited. An interesting concept

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II. A NTENNA D ESIGN Antenna developed in this work has to switch between the radiation patterns covering the ± driving direction and the direction orthogonal to it (to the sides of the car). Another requirement is a bandwidth of at least 200 MHz, which is sufficient enough for mobile communication. The center frequency is placed at 2.45 GHz for the sake of measurement in unlicensed band. The antenna presented in this work uses three radiators. The outer radiators are separated by the distance of 38 at 2.45 GHz (see Fig. 1) and the third is placed exactly in the middle of the distance between them. All the radiators are monopoles, however the outer ones are bent for better radiation. The desired radiation pattern can be obtained by connecting particular element or elements, with aid of p-i-n diodes, to signal line while shorting other to the ground. In following paragraph a design and functionality of antenna is discussed. A. Concept The antenna is fed via a coaxial connector which is connected to coplanar waveguide (CPW) placed on the same substrate as radiating elements. The width of the inner conductor of the CPW is determined by the outer dimensions of the coaxial connector. The Rogers RT5880 substrate used for antenna, which has a primitivity of "r = 2.2, has a thickness of 0.787 mm. The metal thickness is 17 µm. For a central conductor width of 3 mm a gap width of 0.1 mm is chosen to obtain a 50 ⌦ transmission line. The section of CPW is 6 mm long and the central conductor is 1.25 mm longer in order to enable connection of outer radiators. This section can be better seen in zoom picture in Fig. 1(a). This way a junction from one to three lines is established. The p-i-n diodes D1, D2 and D3 are used for activating desired branch of the junction (see


51

31

3/8 λ

31

vias D1 D3 D2

D5

y x

D4

6

SMA connector

z

(a) State 1

(b) State 2

(c) State 3

(d) State 4

(a) Front side R vias

R

R

R

y z

8.75 SMA connector

Fig. 2.

x

Simulated radiation patterns of the presented antenna at 2.45 GHz.

(b) Back side Fig. 1. Layout of the antenna structure with DC feed line (front and back view are top and bottom respectively). The dimensions are given in mm.

State State State State

1 2 3 4

D1 on off on off

D2 off on on off

D3 on on off on

D4 on off off off

D5 off on off off

TABLE I C ONFIGURATION OF SWITCHING ELEMENTS AND CONTROL VOLTAGE

Fig. 1(a)). The lines on the left and right are trimmed in the vicinity of the junction in order to improve matching. CPW ground and the horizontal parts of outer radiators establish horizontal slot lines which are used for feeding of these outer radiators. Further two diodes D4 and D5 are placed in the mentioned horizontal slot lines. Activating D4 or D5 enables switching the outer element to reflector mode. The p-i-n diode used in this work is a BAP64-02 from NXP. The forward case equivalent circuit consist of a 2 ⌦ resistor and a 1 nH inductance in series and the reverse case of a 10 k⌦ resistor and a 0.13 pF capacity in parallel. B. Reconfigurable States In order to be able to realize all reconfigurable states, a particular arrangement of the p-i-n diodes must be chosen. In this case, attention must be paid to the possible switching between the states, so that only the chosen elements are activated when the voltage is applied. Figure 1(a) shows the arrangement and the polarity for all diodes. However many possible reconfigurable states result from this diode arrangement, only the radiation patterns in the ± driving direction and orthogonal, as discussed in requirements, are considered. For state 1, a directional pattern against driving direction (see Fig. 2(a)) is achieved by simultaneous radiation from the middle monopole and the left outer element. At the same time the right outer element is shorted to the ground via

diode D4. The switch configuration for this state is given in table I. The radiation in the driving direction (see Fig. 2(b)) is obtained if the middle monopole and the right outer element are fed. In this case the left element is shorted to the ground via the diode D5. For state 3 two beams in direction of the sides of the car ((see Fig. 2(c)) is obtained by simultaneous feeding of both outer elements, while switching off the middle monopole. Finally for state 4 an omnidirectional pattern is obtained ((see Fig. 2(d)) by switching on the middle monopole and switching off outer elements (see Table I). The outer elements are in this case not shorted to the ground. A possibility of switching the diodes D4 and D5 on in state 4 was considered, however the simulation shows that in such case the gain would be reduced. Therefore the D4 and D5 are left open in this state. For state 1 the maximal gain obtained is 8.31 dBi and the half power beamwidth (HPBW) covers the angular range between = 115 and 245 in azimuth, and ⇥ = 42 and 78 in elevation (see Fig. 2(a)). For state 2 the same gain is obtained and the angular range between = 295 and 65 in azimuth, and ⇥ = 42 and 78 in elevation is covered for HPBW (see Fig. 2(b)). Considering state 3, 6.5 dBi maximal gain is realized and angular areas between = 22 and 168 , and = 202 and 338 in azimuth are covered for HPBW. For state 4 gain of 5.3 dBi is observed. C. DC Biasing An important issue in design of reconfigurable antennas is DC biasing. Without it the switching is impossible, however very often including the DC lines in the antenna design deteriorates antenna performance. Especially if the lines are placed parallel to radiated field. Therefore the biasing structure has to be designed with care. In first step the DC lines for outer elements were designed. The distribution of the surface currents on the elements were observed and vias to the back side were placed in region with lowest current amplitude. Using simple metallic strip leading to DC pin underneath the ground plane with a resistor as DC choke lead to worse


(a) Antenna prototype

Fig. 4. Measured S-parameters of fabricated prototype for four reconfigurable states.

(b) Measurement setup Fig. 3. Fabricated prototype of the antenna and measurement setup in the unechoic chamber.

antenna performance. To avoid these effects it is proposed to use metamaterials for DC biasing. Due to their frequencyselective properties, metamaterials can be used as a filter. In this case it feeds the DC signal while being invisible for electromagnetic wave. A metamaterial cell used is based on electric-LC (ELC) resonator presented in [5]. The structure optimized for design frequency is placed at the back side of the substrate (see Fig. 1(b)). Its height and width are 8.75 mm and 9 mm respectively, and the line width is 0.3 mm. To feed middle monopole with DC signal a layout as in Figure 1(b) is used. A vertical line placed on the back side directly in the center of the monopole is implemented. One resistor is used at the top of the line to block RF signal from monopole. Another resistor is placed in the interruption of the line at the level of the diode 3, to avoid an unwanted coupling between the CPW and the monopole. Finally an arc section leads the line underneath the ground plane. Using this structure introduces only a small reduction of the maximal gain by 0.15 dB. All the resistors used have value of 5 k⌦ so that the diodes can be biased in forward case with 2 mA if 10 V is used. However a smaller resistance value could be used as well, thus the bias voltage would decrease. III. M EASUREMENT AND D ISCUSSION In the next step a prototype of the presented antenna was built. The structure was etched on 0.787 mm Rogers 5880 substrate and p-i-n diodes and resistors were soldered to it. The antenna was then mounted on a metal plated FR-4 board used a ground plane. Two slots were milled into FR-4 board to enable mounting of the antenna and ensure antennas mechanical stability. During the measurement in the unechoic chamber a bigger ground plane with a size of 50x60 cm2 is used (see

Fig. 3(b)). The measurement results show that antenna functions very well in terms of matching (see Fig. 4). It can be observed that matching better than -10 dB is obtained between 2.345 GHz and 2.67 GHz for all reconfigurable states, thus the design requirements are fulfilled. It can be observed that for state 4 where only central monopole is active the matching is obtained at slightly lower frequency than for states 3 where both outer elements are active. Whereas the matching for states 1 and 2 shows combination of matching for central monopole and outer element. In the next step radiation patterns and gain of fabricated prototype were measured in institutes unechoic chamber. As expected from the simulation a wide directional beam in direction opposite to driving direction is obtained for state 1 (see Fig. 5(a)). The form of the pattern corresponds very well with the simulation result (see Fig. 2(a)) and maximal gain of 8.4 dBi is obtained, which is higher than simulated value by about 0.1 dBi. The result for state 2, the same as for state 1, shows very good agreement with the simulation (see Fig. 5(b)). The maximal gain obtained for this state is 8.4 dBi, the same as in the simulation. For state 3 a maximal gain of 6.5 dBi is achieved, and also in this case the pattern resembles the simulated result (see Fig. 5(c)). Finally the result for state 4 shows a nearly omnidirectional pattern with a ripple over the azimuth direction (see Fig. 5(d)). A maximal gain of 5.9 dBi is obtained in this case. If one analyses the measured gain over frequency a ripple on this curve of about 1 dB can be observed. Therefore the realized gain can differ by 0.5 dB from the measured gain, in case of measurement at 2.45 GHz this value could be deduced since a maximum of ripple appears at this frequency. The reason for this ripple is the combination of antenna matching and poor output matching of amplifier used before measured antenna (transmitting antenna). Thus a standing wave between antenna and amplifier is generated. IV. C ONCLUSION This work presents a reconfigurable antenna realizing four switchable patterns. The realized patterns cover the directions important for automotive mobile communication scenarios. The reconfigurable patterns are generated with means of switchable radiators that can be activated with p-i-n diodes.


(a) State 1

(b) State 2

(c) State 3 Fig. 5.

(d) State 4

Measured radiation patterns of the presented antenna at 2.45 GHz.

Furthermore the elements can be also used as reflectors. A prototype of presented antenna was fabricated and measured. The maximal achieved gain for directional states 1 and 2 is about 8.4 dBi and for state 3 covering two directions at the same time a gain of 6.5 dBi is achieved. The measurement results correspond very well with the simulation results. R EFERENCES [1] Yazdandoost K.Y., Miura R., ”Compact printed multiband antenna for M2M applications,” in Antennas and Propagation (EuCAP), 2014 8th European Conference on, pp.2521-2524, Apr. 2014. [2] H.-T. Liu, Gao S., T.-H. Loh, ”Electrically Small and Low Cost Smart Antenna for Wireless Communication,” in Antennas and Propagation, IEEE Transactions on, vol.60, no.3, pp.1540-1549, Mar. 2012. [3] Piazza D., Kirsch N.J., Forenza A., Heath R.W., Dandekar K.R., ”Design and Evaluation of a Reconfigurable Antenna Array for MIMO Systems,” in Antennas and Propagation, IEEE Transactions on, vol.56, no.3, pp.869-881, Mar. 2008. [4] X. Cai, A. G. Wang, N. Ma and W. Leng, ”A Novel Planar Parasitic Array Antenna With Reconfigurable Azimuth Pattern,” in IEEE Antennas and Wireless Propagation Letters, vol. 11, pp. 1186-1189, 2012. [5] D. Schurig, J. Mock, and D. Smith, ”Electric-field-coupled resonators for negative permittivity metamaterials”, Appl. Phys. Lett. 88, 041109 (2006).