Is there any guideline how to place the EZ-BLE module on a host board with different types of enclosure so that the module RF is least affected?
Yes. Guideline for orientation, angular position, polarization, metal obstruction and metal plane clearance, non metallic clearance, shield design and assembly are provided in the following sections.
The Cypress CYBLE-022001-00 is a fully certified and qualified module supporting Bluetooth Low Energy (BLE) wireless communication. The CYBLE-022001-00 is a turnkey solution and includes onboard crystal oscillators, chip antenna, passive components, and a Cypress PRoC™ BLE device. CYBLE-022001-00 supports a number of peripheral functions (ADC, timers, counters, PWM) and serial communication protocols (I2C, UART, SPI) through its programmable architecture. The CYBLE-022001-00 module includes a royalty-free BLE stack compatible with Bluetooth 4.1 and provides up to 16 GPIOs in a small 10 × 10 × 1.80 mm package. The module is already certified for EMC compliance for various regions such as FCC, CE, KC, MIC, and IC. The end-customer need not have to do certification again, and can refer to the module compliance and Bluetooth certification for quick time to market.
The module can be soldered in a host platform to provide seamless BLE connectivity. The extreme small size and various connectivity interface makes the module highly desirable in industrial and consumer applications.
Because of the miniaturized version of the antenna and the module, special care needs to be taken for the placement of the module and antenna enclosure for optimal RF performance. This document describes in detail the requirements on placement of the module on a host board. This document also details the effect of metallic or nonmetallic enclosure and metal obstructions in the vicinity of the EZ BLE module.
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 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. Ground Clearance region defines the bandwidth and the efficiency of the antenna.
The EZ BLE module uses the Johansson 2450AT18B100 chip antenna. The datasheet of the antenna required a ground clearance of 6 .5 mm × 6.5 mm when placed as shown in Figure 2.
In Figure 2, the chip antenna is placed at the edge of a board. The yellow area does not have any ground in any layer. The module placem ent in 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.
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 in 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.
Figure 3 shows an example of four positions of the module in 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 in PCB is not required as the antenna is facing outwards. However, when placed in the middle of the host board, the clearance area must be provided for proper operation.
Out of all the four placements, the 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.
For the best RF reception, the orientation of the receive antenna and the transmission antenna should be having the same polarization. The polarization of the antenna is the direction of electric field in the emanating electromagnetic radiation.
An electromagnetic radiation travels as a wave with the electric field and magnetic field varying with time and space as shown in Figure 4. The electric field and the magnetic field directions are orthogonal to each other. By definition, the direction of the electric field is considered the direction of polarization.
Figure 4 shows a vertically polarized wave. For most PCB antennas and chip antennas, the plane of the antenna in the PCB determines the polarization. If the antenna is in the horizontal plane, the polarization out of such an antenna is horizontal. Figure 5 shows two orientations of the antenna that have two different polarizations.
In Figure 5, the horizontal polarization gives more signal strength for 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 taking into account the use case orientation at the receive side.
The transmitted power from the module varies with orientation of the module as well as the angular position in a particular plane. The radiation pattern of the module is plotted with 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 6 shows the radiation field for the antenna for all possible orientations and polarizations. Horizontal polarization is considered parallel to the surface of earth. Vertical polarization is perpendicular to the surface of earth. Therefore, the electric field in a plane parallel to the surface of earth is considered horizontally polarized.
The shaded region in Figure 6, Figure 7, and Figure 8 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.
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.
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.
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 formed yet. The electric and magnetic fields are not orthogonal to each other. This region is very near 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 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 decreases. 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 a metal, the effect is much more pronounced. Figure 10 shows the regions for a dipole antenna.
For a module based on the 2.4-GHz chip antenna, the near field extends up to 4 mm.
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 11 shows a module antenna in a plastic enclosure. The clearance can be as low as approximately 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 around 5 mm is recommended.
For module placement inside a plastic enclosure, we recommend to check the tuning after the module is placed. Contact Cypress if it is detuned significantly.
Effect of Metallic Objects
The antenna is sensitive to the presence of metallic objects in 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 completely get 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, Cypress recommends an 8-mm gap from the module PCB plane to any metal enclosure.
For our application, the following rule must be obeyed in placing any metal near the antenna. No metallic object should be placed in the region shown below. This region is the near-field region and the effect of metallic objects on the antenna is unpredictable.
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.
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.
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.
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 15 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. Cypres s can provide firmware that programs the chip in continuous transmit mode for determining the radiation pattern. Contact Cypress technical support.
Recommendation on Shield
The main function of the shield is to stop the leakage of the radiations and prevent any radiation affecting the chip. The module has passed precompliance testing without a shield. It is only for FCC modular requirement a shield is provided.
It is up to the customer to keep the shield or do away with this. The presence of shield will change the radiation pattern an d tuning slightly. The entire radiation patterns given in the app note are for unshielded module.
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 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 covered by a metallic enclosure completely. If the product has a metallic casing or a shield, the casing should not cover the antenna. No metal is allowed in the antenna 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 taken into account to orient the module in a way that maximum radiation occurs.
- There should not be any ground directly below the antenna.
Cypress do not recommend tuning the antenna on the module by you 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.