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AIROC™ Bluetooth® module placement

AIROC™ Bluetooth® module placement

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Introduction to AIROC™ Bluetooth® module placement


This document describes the requirements for the placement of the module on a host board, as well as the effect of metallic or non-metallic enclosures and metal obstructions near the AIROC™ Bluetooth® module.

Guidelines for orientation, angular position, polarization, metal obstruction and metal plane clearance, non-metallic clearance, shield design, and assembly are provided in the following sections.

Because of the miniaturized version of the antenna and the module, take extra care for the placement of the module and antenna enclosures for optimal RF performance.

 

Read about:

1. Antenna ground clearance

2. Module placement on host board

3. Antenna polarization

4. Radiation pattern

5. Antenna performance to enclosure

 - 1.1 Antenna near field and far field

- 1.2 Effect of nonmetallic enclosure

- 1.3 Effect of Metallic Objects

- 1.4 Recommendations for Placement over a Large Metal Plane

- 1.5 Recommendation of Slot in Metallic Enclosure

- 1.6 ID-Specific Module Placement in Metallic Enclosure

6. Guidelines for enclosure and ground plane

7. Antenna tuning

 

1. Antenna ground clearance


A monopole antenna requires that no ground plane is below the antenna. The ground plane below it will not allow the field to propagate. This is called the ground clearance requirement. However, after some distance, a ground must be there for the monopole antenna. Defining this region is a very significant step for any antenna design problem. The ground clearance region defines the bandwidth and efficiency of the antenna.

The AIROC™ Bluetooth® module CYBLE-022001-01 uses the Johansson 2450AT18B100 chip antenna. The datasheet of the antenna required a ground clearance of 6.5 mm*6.5 mm on the module when placed, as shown in Figure 1.

Antenna Clearence.png

 Figure 1. Antenna Clearance

 

In Figure 1, the chip antenna is placed at the edge of a board. The red block area does not have any ground in any layer. The module placement on a host board needs to ensure that no traces or ground layer of the host board can come inside this region. Any ground plane below a monopole antenna will kill radiation and adversely affect the efficiency.

2. Module placement on host board


The module will be soldered on the host board, and a clearance must be provided for the antenna where no routing or ground is allowed in any layer. Here is an example of a module placed on a host board. Placing the module at the edge is recommended as it gives the best RF performance and does not require any clearance surrounding the antenna.

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Figure 2. Module Placement in a Host Board

Figure 2 shows an example of four positions of the module on a host board, such as “a”, “b”, “c”, and “d”. The white area around the module is the clearance area. For this antenna, we require a clearance of 4 mm in every direction. When placed at the edge of the board, the ground clearance area on the PCB is not required as the antenna is facing outward. However, when placed in the middle of the host board, a clearance area must be provided for proper operation.

Out of all the four placements, option “a” is the best. It does not require any clearance area as the antenna faces out. The antenna tip is exposed to free space. In option “c,” even though the module is placed at the edge, the antenna tip is not exposed to free space but towards the board. Option “b” not only wastes PCB real estate but also provides diminished RF performance compared to position “a”, as we can have traces facing the antenna tip.

Another Chip Antenna Module Placement on a Host Board.pngFigure 3 Another Chip Antenna Module Placement on a Host Board

 

Figure 3 shows another chip antenna module placement on a host board. A B C are acceptable, and the antenna performance will be A<B<C.

In solution C, there is no ground copper within the orange area from the TOP layer to the BOTTOM layer, but power trace, signal trace, and ground trace are allowed (ground plane is not allowed). Error! Reference source not found.Figure 4 is a solution C host board design example.

Host board example.png

 Figure 4 Host board example

3. Antenna polarization


An electromagnetic radiation travels as a wave with the electric field and magnetic field varying with time and space, as shown in
Figure 5. The electric field and magnetic field directions are orthogonal to each other. By definition, the direction of the electric field is considered the direction of polarization.

Electromagnetic wave.png

Figure 5 Electromagnetic wave

Figure 5 shows a vertical polarized wave. For most PCB and chip antennas, the plane of the antenna on the PCB determines the polarization. If the antenna is in the horizontal plane, the polarization of such an antenna is horizontal.

For the best RF reception, the orientation of the receiving antenna and the transmission antenna should have the same polarization. The polarization of the antenna is the direction of the electric field in the emanating electromagnetic radiation. Figure 6 shows two orientations of the antenna that have two different polarizations.

Effect of antenna polarization.png

Figure 6 Effect of antenna polarization

 

In Figure 6, XY is the horizontal plane, and Z is the vertical direction. RX in the horizontal polarization gives more signal strength to the receive antenna. This is because the transmitting antenna at the TX is in the XY plane and has horizontal polarization. The end customer must fix the orientation of the module, considering use case orientation on the receiving side.

For two modules, Figure 7 placement will get the better signal strength.

Module direction.pngFigure 7 Module direction

4. Radiation pattern


The transmitted power from the module varies with the orientation of the module as well as its angular position in a particular plane. The radiation pattern of the module is plotted with the rotating the module around the three axes, and the receiver antenna is held at two different polarizations.

The module is rotated in 15-degree angular steps. Figure 8 shows the radiation field for the antenna for all possible orientations and polarizations. Horizontal polarization is considered parallel to the surface of the earth. Vertical polarization is perpendicular to the surface of the earth. Therefore, the electric field in a plane parallel to the surface of the earth is considered horizontally polarized.

The shaded region in Figure 8, Figure 9, and Figure 10 is the region of minimum radiation. These regions should be avoided if the RX and TX positions are known beforehand. Note that horizontal polarization is more sensitive to angular positions compared to vertical polarization.

Radiation field – 1.png

                                                                                                    Figure 8 Radiation field – 1

 


Radiation field – 2.png                                                                                                  Figure 9 Radiation field – 2

 

Radiation field – 3.png

                                                                                                          Figure 10 Radiation field – 3

 

Combining the three rotation plots, a three-dimensional field was constructed that shows the field intensity. As expected, along the dipole axis, the radiation was the least. In addition, there are certain regions in the non-axial position that showed radiation minima. The vertical polarization did not show much angular variation like horizontal polarization of receiving antenna.

3D Radiation field.png                                                                                                     Figure 11 3D Radiation field

5. Antenna performance to enclosure


Antennas used in consumer products are sensitive to PCB RF ground size, the product’s plastic casing, and the metallic enclosure.

 

1.1 Antenna near field and far field

Every antenna has two regions around it: the near field and the far field. A near field is the region where the radiated field has not yet formed. The electric and magnetic fields are not orthogonal to each other. This region is close to the antenna. The near-field region has two regions: the reactive near-field region and the radiating near-field region. The transition to a far-field region happens in the radiating near-field region.

The radiation field is formed after the transition to the Fraunhofer region. In this region, the relative angular variation of the field does not depend on the distance. This means that if we plot the angular radiation field at a distance from the antenna in the far-field region, their shapes remain the same. Only with distance, the field strength decrease. However, the shape of the radiation pattern remains the same with respect to the angular variation. This region is called the far-field region. An object in a far field does not affect the radiation pattern much. However, any obstruction in the near field can completely change the radiation pattern. If it is metal, the effect is more pronounced. Figure 12 shows the regions for a dipole antenna.

 

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                                                                                                      Figure 12. Near and Far field

For a module based on the 2.4-GHz chip antenna, the near field extends up to 4 mm.

1.2 Effect of nonmetallic enclosure

Any plastic enclosure changes the resonating frequency of the antenna and gets it detuned. The antenna can be modeled as an LC resonator whose resonant frequency decreases when either L (inductance) or C (capacitance) increases. A larger RF ground plane and plastic casing increase the effective capacitance and thus reduce the resonant frequency. Refer to application note AN91445 for the effect of enclosure.

Figure 13 shows a module antenna in a plastic enclosure. The clearance can be as low as 2 mm. However, this affects the tuning of the antenna. This can be taken care of by tuning the antenna. For a minimal effect on the antenna, tuning a clearance of around 5 mm is recommended.

For module placement inside a plastic enclosure, check the communication distance after placing the module

 

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                                                                                                 Figure 13. Plastic Enclosure

1.3 Effect of Metallic Objects

The antenna is sensitive to the presence of metallic objects in its vicinity. A metallic object shorts the electric field and thus changes the radiation field. Depending on the size of the obstruction with a metallic object or metallic slot, electromagnetic waves go through different diffraction patterns or are completely shielded by the metallic object.

Metallic objects in the near field can have a drastic impact on the radiation pattern. The module thickness with the antenna is 2 mm, and the near-field extends up to 4 mm from the antenna. Therefore, any obstruction must be at least 6 mm away from the PCB plane to avoid problems with the RF performance. In practice, Infineon recommends an 8 mm gap from the module PCB plane to any metal enclosure.

For the application, you must follow the following rule when placing any metal near the antenna: No metallic object should be placed in the region, as shown in Figure 14. This region is the near-field region, and the effect of metallic objects on the antenna is unpredictable.

 

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                                                                     Figure 14. Clearance from Small Metal Obstruction

1.4 Recommendations for Placement over a Large Metal Plane

The other effect of metal is the formation of an image antenna.  The best practice in this case is to orient the metal orthogonal to the antenna to ensure minimum effect. If the length or width of the plane approaches the size of the module, it is considered a large metal in the vicinity of the antenna. Out of the two placement options, option (a) should be avoided.

It is recommended not to have any large metallic objects parallel to the antenna running nearby. This has drastic effect as the image antenna is mostly of opposite polarity. The interference caused by such an antenna is mostly destructive.

If it is not possible to avoid a large metallic object running parallel to the module plane, at least maintain a distance ‘h’ of 30 mm. This will ensure that the interference due to the image antenna will not be completely destructive. The radiation will be strongly directional below the 30-mm limit and the efficiency drastically drops at ‘h’ below 8 mm. At an ‘h’ around 2 mm, the radiation efficiency can go below 20%.

For industrial designs which go with magnetic coating sticking to a metal surface, this must be taken into account.

 

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                                                                                  Figure 15. Clearance from Large Metal Plane

1.5 Recommendation of Slot in Metallic Enclosure

Sometimes, the module will be placed in a metallic enclosure as demanded by the industrial design. In such cases, the best place to put a slot or opening is to find the radiation pattern and position the slot near the maximum radiation. If the slot is placed near a shadow, there is no effect as hardly any radiation comes out of a shadow region.

 

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                                                                         Figure 16. Position of Slot in Metallic Enclosure

The size of the metallic slot should be as large as possible. If the slot is limited, the antenna should be positioned in a way that the beam width will be covered by the slot position.

1.6 ID-Specific Module Placement in Metallic Enclosure

Because of the many ways that an OEM can design the metallic enclosure, we cannot recommend a fixed position of slot or module placement.   Figure 17 shows the module placed in an industrial design (ID) that has a metal base that runs parallel to the module both at the top and at the bottom. If not placed properly, this can have adverse effect on the radiation. A correct clearance must be maintained in the directions shown by the arrows.

The best way for determining the position of the slot is to do a radiation pattern measurement and determine the best position for that particular ID.  Infineon can provide firmware that programs the chip in continuous transmit mode for determining the radiation pattern. Contact Infineon technical support.

 

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                                                                  Figure 17. Example of an ID with Module and Metallic Mase

6. Guidelines for enclosure and ground plane

  • Ensure that there is no component, mounting screw, or ground plane near the tip of the antenna or the length of the antenna.
  • No battery cable, microphone cable, or any trace should cross the antenna trace on the PCB on the same side of the antenna.
  • The antenna should not be completely covered by a metallic enclosure. If the product has a metallic casing or a shield, the casing should not cover the antenna.  No metal is allowed in the antenna’s near field.
  • Ensure the paint on the plastic enclosure is nonmetallic near the antenna for best performance.
  • The orientation of the antenna should be in line with the final product orientation so that the radiation is maximized in the desired direction. The polarization of the receive antenna and the position of the receive antenna should be considered to orient the module in such a way that maximum radiation occurs.
  • There should not be any ground directly below the antenna.

7. Antenna tuning

Infineon does not recommend tuning the antenna on the module because that will necessitate another round of FCC and compliance verification. Change the antenna and matching network at your own risk. You must ensure regulatory compliance after any such change.

 

Version: *B

Translation - Japanese: EZ-BLE™モジュール配置 – KBA97095 - Community Translated (JA)

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