Anechoic Chamber

Construction of Large Anechoic Chamber Atsuhiro KAWANO* Wataru KATSUNO*

Abstract In recent years, with the growth in the number of electrical/electronic components installed in vehicles and the degradation of the electromagnetic environment surrounding vehicles, regulatory requirements relating to the vehicle’s electromagnetic compatibility (EMC) are becoming increasingly stringent worldwide. To comply with future regulatory requirements and to accommodate the electromagnetic environment caused by the rapid increase of wireless systems such as mobile phones and digital terrestrial broadcasting, in August 2006 we constructed a large anechoic chamber and other facilities and equipment to improve and upgrade the EMC tests. Key word: EMC

1. Introduction The Commission Directive 2006/28/EC of January 2006, the first major amendment in approximately ten years, demands more stringent methods for testing a vehicle’s immunity against incoming electromagnetic interference and for measuring electromagnetic emission from the vehicle. Because our previous anechoic chamber was too small and the related test facilities and equipment were not expected to meet the requirements of the new directive, we constructed a large anechoic chamber and installed upgraded facilities (Photo 1 and Photo 2). These facilities enable us to carry out efficient and comprehensive tests including homologation test for Directive 2006/28/EC and development test, in addition to the tests for Directive 95/54/EC. For not only more high-frequency, complicated and powerful electrical/electronic components installed in vehicles but also for the EMC environment of the vehicle including the high-frequency radio emission from medical equipment like CT systems and cellular phone, complicated modulation radio emission from cellular phone and internet security system, and intense radio emission from air traffic control radar, we installed the facilities to upgrade accuracy and functions for EMC tests.

Main building housing the large anechoic chamber Entrance

Work shop

Photo 1

Electromagnetic wave testing laboratory (seen from the west)

Another anechoic chamber in function testing laboratory, approximately 1/6 the size of the large chamber

2. Overview of the large anechoic chamber Photo 2

2.1 Test items The large anechoic chamber is applied to two types of testing: immunity tests and emission tests for vehicles. The immunity test checks whether the components of the vehicle relating to driving, turning and stopping function normally when the vehicle is exposed to intense radio waves such as from the antenna of a broadcasting station while driving nearby. We can per*

Electronics Engineering Dept., Development Engineering Office

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Large anechoic chamber

form the vehicle test for electrical disturbance from offvehicle radiation source conforming to ISO standards in the large anechoic chamber. The emission test checks whether the electromagnetic energy emitted from the vehicle causes interference to radio or TV reception in the residential environ-

Construction of Large Anechoic Chamber

Table 1

Shielding performance

Table 2

Room

Frequency range

Component

Attenuation (dB)

Anechoic chamber

14 kHz – 30 MHz

Magnetic field

80 or more

Measuring room

14 kHz – 30 MHz

Electric field

100 or more

Amplifier room

20 MHz – 1 GHz

Plane wave

100 or more

1 – 18 GHz

Plane wave

100 or more

C/D control room

Fig. 1

Field distribution uniformity performance

Applicable standard

Measuring method

IEC61000-4-3

20 MHz – 2 GHz Within 0 – 4 dB at 12 of 16 points tested by the IEC standard

ISO11451-2 (2006/28/EC)

In the frequency from 20 MHz – 2 GHz, at frequency points of more than 80 % of the total, within –6 – 0 dB in 0.5 m horizontally to each side from the base point.

Site attenuation performance

room, the amplifier room and the chassis dynamometer (C/D) control room are also shielded in a similar manner. To minimize the reflection of radio waves within the large anechoic chamber, the ceiling and walls are covered with ferrite hybrid absorbers, while the floor is covered with ferrite tiles. The chamber has site attenuation performance conforming to JASO D002-2004 standard and field uniformity performance conforming to ISO standard (Table 2 and Fig. 1).

Photo 3

RF immunity test

ment. In the large anechoic chamber, we can perform the 10-m test of CISPR (Comité international spécial des perturbations radioélectriques) and the initial step of Directive 2006/28/EC ANNEX V. 2.2 Shielded rooms (Table 1) The large anechoic chamber has enough space to carry out 10-m test. All six sides (the floor, walls and ceiling) of the chamber is covered with steel plates to prevent radio waves from leaking out. The measuring

2.3 Test equipment 2.3.1 Intense electric field generator The generator installed in the large anechoic chamber consists of four antennas and three amplifiers, and the intense electric field of 200 V/m from 100 kHz to 2 GHz (not all frequencies in the range available) is generated onto the vehicle (Photo 3). To help improve the efficiency of testing, the following can be remotely performed from the measuring room: switching between the amplifiers/antennas; adjustment of the antennas’ height and angle; and selection of polarization. 2.3.2 Emission measuring device The emission measuring device installed in the large anechoic chamber consists of a bilog antenna for measuring broadband noise and a pneumatically driven

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Construction of Large Anechoic Chamber

Photo 4

Chassis dynamometer Photo 6

Photo 5

Actuators

antenna mast. This eliminates antenna replacement and polarization selection. By operating the turntable in a coordinated manner with the measuring device, it is possible to continuously measure broadband noise emitted from both sides of the vehicle, thus improving efficiency. 2.3.3 Chassis dynamometer (Photo 4) Using the chassis dynamometer installed in the large anechoic chamber, it is possible to continuously run the vehicle at up to 100 km/h or to carry out test on ABS system under low µ condition. The chassis dynamometer can be operated using either the controller in the large anechoic chamber or remotely on a PC in the measuring room. This improves test efficiency. 2.3.4 Vehicle operation actuators (Photo 5) In the anechoic chamber, an intense electric field is generated onto the vehicle, so actuators are used to operate the vehicle on behalf of humans. Besides the actuators for the accelerator, brake and clutch pedals, we developed actuators for the ignition key switch, column switch and other switches for the new facilities. These improve test efficiency and expand the scope of tested components.

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Actuators for accelerator, brake and clutch pedal

Photo 7

Ignition switch actuator

• Actuators for foot-operated controls (Photo 6) The accelerator pedal actuator can control vehicle speed. A displacement sensor is installed at the end of the actuator cylinder and the vehicle speed data is taken from the chassis dynamometer. Based on these data, the actuator cylinder is extended or retracted to obtain the required vehicle speed. The brake pedal actuator can control the pedal stepping force at a high or low level, and it can operate ABS system. The clutch pedal actuator can release the clutch pedal at 10 different speeds. • Actuators for hand-operated controls The ignition switch actuator can operate the switch through the entire sequence from engine start to stop: OFF → START → IG → ACC → PUSH → STOP (Photo 7). The column switch (straight line) actuator can move a column switch between three positions, i.e. up, neutral and down, or forward, middle and rearward. This can be used, for instance, to operate the turn signal (right → cancel → left) or flash the headlamp main beams. The column switch (rotational) actuator can rotate a switch between two positions such as ON and OFF. It can, for example, turn on and off the headlights. The

Construction of Large Anechoic Chamber

immobilizer to malfunction. Our new facilities are expected to eliminate such malfunctions.

Photo 8

TLS Antenna

actuator can also push a switch at the end of a column switch to inject windshield washer. The air conditioner / audio switch (straight line) actuator can push a switch ON and OFF, such as the switch of the air conditioner, audio or hazard warning lamp. The air conditioner / audio switch (rotational) actuator can rotate a switch between two positions, such as ON and OFF. The power window switch actuator can move the switch to open and close the power windows: open → stop → close. 2.4 Safety systems The large anechoic chamber is equipped with an interlock system that disables radio wave radiation when the shielded door is open, and an emergency button to stop radio wave radiation and chassis dynamometer in emergencies. The chamber is also equipped with safety sensors for fire, high CO concentration, fluid leakage and earthquake, and give optical and acoustic warnings.

3. Technical features of the new TLS antenna 3.1 Overview TLS (Transmission Line System) antennas can generate electromagnetic field of relatively low frequency of 100 kHz – 30 MHz or so-called LF, MF and HF bands. The new TLS antenna consists of a base, a balun, radiation elements, a load and cables that connect these components (Photo 8). 3.2 Background and purpose of the installation In the ISO11451-1 (3rd 2005) on immunity test, its required frequency ranges 0.01 – 18,000 MHz. Of that range, this facility can cover 0.1 – 30 MHz frequency test. The installation is aimed at checking for electrical disturbance from AM radio stations, motors, etc. One example of electrical disturbance with our retailed cars is: an aftermarket product that the customer installed on his car generated noise in a band around 125 kHz, which was also used by the immobilizer, causing the

3.3 Brief description of the TLS antenna The base, which is stowed just below the ceiling of the anechoic chamber, is lowered to the work position by four winches housed behind the ceiling. With the base lowered, the elements are then installed in position for testing. The elements can be moved horizontally by the air pump mounted on the base. The antenna is operated on the controller installed in the anechoic chamber. The winches and air pump are operated through TLS antenna controller. From a PC in the measuring room, GPIB signals are sent to the balun to switch the direction of electromagnetic field (Vertical: E mode; Horizontal: H mode). For safety, the base is equipped with four drop-prevention belts in case the cable breaks. Three height limits for the base (lowest, highest with cables on, and highest) are programmed in the system. As the base is raised or lowered, the program works according to the operation of the rotary encoders, each of which is installed on a winch. These encoders also monitor the inclination of the base. If the data difference between the encoders exceeds the preset level, the limit function intervenes. 3.4 Mechanism of electric field generation Signals are generated by a signal generator, and are then amplified to a maximum of 10 kW by the amplifier in the underground amplifier room. The amplified signals are then sent to the balun where the signals are divided to the core side and the ground side of the coaxial cables. The elements are on the core side while the floor is on the ground side. This creates a potential difference between the elements and the floor, generating vertical electric fields (E mode) (Fig. 2). With the newly installed TLS antenna, it is possible to connect the core side of the coaxial cables to one element and the ground side of the coaxial cables to the other element. In this case, TLS antenna generates horizontal electric fields between the elements while also creating vertical magnetic fields (H mode) (Fig. 3). 3.5 TLS antenna performance It has been verified that the TLS antenna can generate 200 V/m in a 100 kHz – 30 MHz range when the elements are arranged 2 m apart at a height of 2 m (E mode). 3.6 TLS summary • The TLS antenna generates electric fields in low frequency bands. • Vertical (E mode) and horizontal (H mode) electric fields can be generated. • This is the first large-scale electromagnetic field generation facility covering below 20 MHz frequency for vehicles in Mitsubishi Motors Corporation (MMC). • Four anti-drop belts (similar to seat belts) are fitted for safety.

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Construction of Large Anechoic Chamber

Fig. 2

E mode (vertical electric fields)

Fig. 3

H mode (horizontal electric fields)

4. Conclusion The new large anechoic chamber is constructed on many years of experience of all those concerned. We believe that full utilization of this state-of-the-art facility will help us to develop even higher quality, higher performance cars. Finally, we sincerely thank everyone at and outside MMC for helping to construct the anechoic chamber.

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Atsuhiro KAWANO

Wataru KATSUNO