A Miniaturized Pattern Reconfigurable Antenna for Automotive Applications Jerzy Kowalewski 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—This paper presents a realization approach of a pattern reconfigurable antenna. Based on the results of the previous research using wave propagation simulation, the radiation patterns optimized for automotive urban scenarios are chosen. The patterns are determined by a special antenna synthesis method. The antenna in this work generates two switchable patterns obtained from this synthesis. The first one is in and against the driving direction and the second is directed orthogonal to the driving direction to the left and right hand sides of the vehicle. The pattern switching is realized by switching the phase between the parallel fed radiating elements. An easy method of phase switching with means of a tapered line balun and pi-n diodes is proposed. The antenna covers the 2.45 GHz ISM band, and therefore can be easily used for measurements in an unlicensed band. As a proof of concept a prototype of the antenna utilizing p-i-n diodes as switching elements has been fabricated and measured. The maximal gain achieved is about 6.5 dBi. The measurement results correspond well with the simulation results in terms of S-parameter and radiation. Index Terms—reconfigurable antenna, p-i-n diode.
I. I NTRODUCTION Due to common use of wireless systems and developments in microwave technology a growing demand for higher data rates is observed. Since the mobile end users are interested in continuous connection to the network, also during their car travels, this trend is also true for automotive infotainment systems. As an answer to the growing demands the concept of car connectivity is being developed. Various mobile communication standards are foreseen for intelligent transportation systems (ITS) like entertainment or traffic management [1]. Since the vehicles are moving in constantly changing environments, the systems can be strongly affected by temporal fading and thus disruption of connection. The environment of the car is multipath-rich therefore so-called MIMO (multipleinput-multiple-output) systems can be used to battle this problem and even increase the capacity [2]. On the other hand MIMO systems utilize front-ends at both transmitter and receiver side. Hereby, multiple directivity patterns at transmitter and receiver can be realized. However multiple front-ends generate additional cost. The study presented in [3] shows, that in most cases only two to three sub-channels with adequate signal-to-noise-ratio (SNR) exist, if the mobile user stays in similar environments. Therefore, it is possible to construct a MIMO system with a reduced number of antennas that excite specific radiation patterns. In such case the conventional omnidirectional antennas can be replaced with reconfigurable
antennas, that realize radiation patterns for these sub-channels and switch to the best pattern depending on the environment. According to the research presented in [3], patterns for the two sub-channels with highest eigenvalues focus in the driving and the opposite direction and second, and orthogonal to the sides of the car. Therefore realizing these particular patterns leads to decreased outage probability. So far, antennas with omnidirectional patterns were proposed for automotive communication applications [4]. However, as discussed in the previous paragraph, this is not an optimal solution. The pattern reconfiguration is a solution offering more flexibility and better directivity. Activation of parasitic elements as the ESPAR (electronically steerable parasitic array radiator) antenna presented in [5] offers enough flexibility in terms of pattern reconfiguration. However when mounted on a car roof, this antenna is bulky and has a complex construction. Hence it is not suitable for automotive applications. The antenna presented in [6] has smaller dimensions, yet its flexibility in terms of pattern generation is limited. This work presents a pattern reconfigurable antenna fulfilling the requirements for an automotive roof antenna and realizes patterns optimized for urban scenarios. Pattern switching is realized by a phase shift between two parallel fed elements. II. A NTENNA D ESIGN A. Antenna Requirements As discussed in the introduction, the presented antenna has to switch between the two patterns, which have been found to be optimal for urban scenarios (in the ± driving direction and the other orthogonal to it). Another important requirement is antenna matching that should cover a broad band of about 200 MHz. The center frequency is placed at 2.5 GHz. The chosen frequency of operation enables measurements of the antenna in an unlicensed band at 2.45 GHz. However, the antenna partially covers the LTE (Long Term Evolution) band and can easily be scaled to cover the whole band. The last requirement is small dimension, thus the antenna can be installed inside a standard automotive antenna housing. B. Antenna Principle The antenna consists of two radiating elements separated by about 38 at 2.5 GHz (see Fig. 1). This distance was chosen instead of /2 due to the requirement on compact construction. Instead of simple monopoles, inverted-L antennas were chosen
S1 off 1
¼λ diodes
2
RF choke
SMA connector
y
S2 on
x
z
S1 on
(a) Front side view
50
2
x
z
4.8
6
S2 off
diode
17
SMA connector (b) Back side view
Fig. 1. Layout of the antenna structure with DC feed line. The dimensions are given in mm.
as radiating elements. Therefore the height of the antenna is reduced to 15 mm. However another antenna type could be chosen as well for these element radiators. The structure is printed on a 0.8 mm thick Rogers RT5880 substrate. Substrate length and height are 50 mm and 17 mm respectively, and is mounted vertically on a 50x50 cm2 metallic ground plane (see Fig. 1). The antenna is fed in the middle with a coaxial line connected to the horizontal microstrip line [see Fig. 1(a)]. Thus an additional T-junction is omitted. The microstrip line enables parallel feeding of the radiators. In order to obtain two switchable patterns the radiating elements should either be fed in phase or in opposite phase (180 phase shift). As a solution to generate the phase shift between the elements a linear tapered microstrip balun is used [see Fig. 1(b)]. Such baluns are very often used for ultra wide-band antennas [7]. The balun in this work was reduced in width in comparison with other designs known from literature. Furthermore it was optimized to generate an optimal phase shift between the two radiating elements what is crucial for the generation of the pattern in the ± driving direction (along the x-axis). In order to do so, the antennas were replaced by ports and the phase for both states was measured. The simulation proved that at the left end of the line was either 180 or 0 phase shift depending on the state. To enable switching between the states, there is a metallic pad on each side of the substrate (see Fig. 2) connected with a via. The pads are connected with contrariwise polarised p-i-n diodes to the microstrip line at the front and tapered ground at the back side. The p-i-n diode used in this work is a BAP64-02 from NXP. Prior to the antenna design, the diode was measured and an equivalent model was developed based on the measurement results. The forward case equivalent circuit consist of a 2 ⌦ resistor and
Fig. 2.
S3 off
-VDD
front
3/8 λ
y
S3 on
+VDD
State 2
17
State 1
50
back
Switch configuration for the two states on the front and back side.
State 1 State 2
Switch 1 (front up) off on
Switch 2 (front low) on off
Switch 3 (back) on off
VD
Port DC
+10 V 10 V
+1 V 0V
TABLE I C ONFIGURATION OF SWITCHING ELEMENTS AND CONTROL VOLTAGE
a 1 nH inductance in series and the reverse case of a 10 k⌦ resistor and a 0.13 pF capacity in parallel. As discussed in the previous paragraph, there are two different lines connected to the left monopole one on the front and one on the back side of the substrate (see Fig. 1). In case if state 1 is active, the upper front switch (S1 ) is off and the switch at the back side (S3 ) is on. At the same time the lower front switch (S2 ) should be on (see Fig. 2) to ensure good matching and proper phase at the left antenna. The left antenna is fed by the tapered line at the backside of the structure (see Fig. 1). Thus the radiating elements are fed with signals that are opposite in phase. In this state the two radiators interfere constructively along the x-axis and destructive interference along the z-axis occurs [see Fig. 3(a)]. Here the coordinate system is defined as follows: the azimuth angle as the tangential component going from 0 to 360 and the elevation angle ✓ as the radial component of the plot ranging from 0 in zenith to 180 . In order to activate state 2 the upper front switch (S1 ) is on and the switch at the back side (S3 ) is off, and the lower front switch (S2 ) should be off. Thus the radiating elements are fed with signals that are in phase. In this state the two radiators interfere constructively along the z-axis and destructive interference along the x-axis occurs [see Fig. 3(b)]. To minimize the influence of DC lines on the antenna properties, the DC supply line is placed in the lower left corner of the structure [see Fig. 1(a)] and decoupled from the high frequency signal with a choke. The resistor of 10 k⌦ is used as a choke. It acts as open for RF signal and does not disturb the DC supply because the current floating through the p-i-n diodes is relatively low (1 mA). The DC pin is placed under the ground plane to further minimize the influence of the DC network on antenna parameters. Apart from the DC signal fed
0
S11 in dB
-5
-10 State 1 Meas. State 2 Meas. State 1 Sim. State 2 Sim.
-15
-20
2
2.2
2.4
2.6
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3
Frequency in GHz
(a) Radiation pattern for state 1
Fig. 4. Simulated and measured S-Parameters of the proposed antenna for two states.
y SMA connector
z
x
(a) Front side view
y
(b) Radiation pattern for state 2
x
Fig. 3. Simulated radiation patterns of antenna with implemented switch model at 2.45 GHz.
by the DC pin, voltage is also fed by the SMA connector with aid of a coaxial bias-T. The control voltage configuration for both states is presented in Table I. C. Simulation results The presented antenna was simulated with aid of CST Microwave Studio. The simulated matching of the antenna for both reconfigurable states is shown in Fig. 4. As it can be seen the measurement corresponds well with the simulation and the requirements on center frequency and bandwidth are met. The center frequency, however is slightly shifted by about 50 MHz, but it is still around 2.5 GHz and the 6 dB bandwidth is broader than 200 MHz. The simulation results for the state 1 show that the antenna radiates along the x-axis [see Fig. 3(a)]. The maximum gain achieved by the antenna is 6.8 dBi, the 3 dB-beamwidth in azimuth is 116 and the beamwidth in elevation is 35 . In this case some asymmetry can be observed. The gain in the positive x direction (driving direction) is about 1.2 dB higher than in the opposite direction. In this case the left antenna element (see. Fig. 1) is fed with the opposite phase, thus connected to the line at the backside of the structure (see Sec. II-B).
z
y SMA connector
z
x
(b) Back side view Fig. 5.
Fabricated prototype of the antenna utilizing p-i-n diodes.
Since the currents in the tapered line are not as strong as those in the microstrip line and additional losses due to the diode appear, the radiation in the negative x direction is weaker. The simulation for state 2 gives a maximum gain of 7.1 dBi with a 3 dB-beamwidth of 108 in azimuth and of 30 in elevation [see Fig. 3(b)]. In this case only a slight asymmetry of about 0.2 dB between ± z direction can be observed. It can be easily explained by the substrate placed behind the antenna in the negative z direction and thus higher radiation in this direction. The requirements for the antenna’s reconfigurable patterns were discussed in section II-A. As stated in the introduction the optimal directivity patterns for a car’s roof-top antenna are concentrated in the ± driving direction and the other orthogonal to it towards the sides of the car [3]. The directivity patterns realized by the antenna presented in this work for both reconfigurable states (see Fig. 3) fulfill these requirements and correspond very well with the patterns for the two best subchannels presented in [3].
(a) Radiation pattern for state 1
Fig. 6(a)]. The measured values are in general about 1.5 dB lower than expected from the simulation. The difference between simulation and measurement is caused mainly by the additional phase shift caused by the diode and inaccuracies in the manual fabrication. Therefore the phase shift caused by the diode should be investigated more carefully in the future work. Additional losses are due to the measurement inaccuracies. However all the measured curves correspond well with the simulated ones. The maximum gain for the state 2 in the direction orthogonal to the driving direction ( =90 ) is about 6.5 dBi [see Fig. 6(b)]. In this case the difference between measured and simulated values is lower than for the state 1. The gain in the opposite direction is about 5 dBi. For this state is the radiation in the driving direction ( =0 ) about 5 dB lower than in the =90 direction and thereby slightly better decoupled than in simulation [see Fig. 6(b)]. However in this case decoupling between orthogonal directions (along x- and z-axis) is worse than for the state 1. It could be improved by increasing the distance between the radiators. Nevertheless the dimensions of the antenna would increase in this case. IV. C ONCLUSION
(b) Radiation pattern for state 2 Fig. 6. Simulated and measured gain of the presented antenna at 2.45 GHz (elevation cuts).
This work presents a compact pattern reconfigurable antenna. The antenna is able to switch between two different radiation patterns. The generated radiation patterns realize the patterns given by channel simulations and are optimized for automotive urban propagation scenarios. The approach to generate the wanted patterns with means of parallel fed radiators and phase shifter based on a tapered line balun is presented. A prototype of an antenna was fabricated and measured. The maximal achieved gain is about 6.5 dBi. The measurement results correspond well with the simulation results. R EFERENCES
III. M EASUREMENT AND DISCUSSION Finally, the prototype of the designed antenna was fabricated. The antenna was printed on Rogers RT5880, and the p-i-n diodes, the RF choke and the SMA connector were soldered manually to it. The antenna is vertically mounted through two slots in metal plated FR4 material with a size of 15x10 cm2 (see Fig. 5). Such construction enables easy attachment of the antenna to a bigger metal plate for needs of measurements. During the measurements the bias-T and DC line (DC pin beneath the ground plane) were connected to a laboratory power supply and fed with appropriate signals (see Table I). The radiation patterns and gain of the fabricated model were measured in an anechoic chamber. The maximum gain for the state 1 in the driving direction ( =0 ) is about 5 dBi [see Fig. 6(a)]. As expected from the simulation, the gain in the opposite direction is about 3 dBi and thus lower than in the driving direction. The radiation in the orthogonal direction ( =90 ) is about 17 dB lower than in the =0 direction and thereby better decoupled than in simulation [see
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