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How to easily design the best antenna for a custom product using PRoCBLE/PSoCBLE
This article introduces the design of antennas in a simple and accessible way, recommending two low-cost PCB antennas that have been tested and proven effective. These PCB antennas are suitable for use with low-power Bluetooth (BLE) solutions. To achieve optimal performance, it's essential to properly match the 2.4GHz RF signal from the BLE module to its antenna. The final section of this application note discusses how to debug the antenna in the final product.
Introduction
An antenna is a crucial component in any wireless system, responsible for transmitting and receiving electromagnetic waves. Designing antennas for cost-effective, high-volume applications and integrating them into handheld devices presents a significant challenge for most original equipment manufacturers (OEMs). The wireless range experienced by end users from an RF product, such as a coin-cell battery-powered device, depends primarily on the antenna design, the plastic housing, and the PCB layout. Even systems using the same chip and power supply can exhibit more than a 50% variation in their RF range due to differences in layout and antenna design practices. This application note outlines best practices, layout guidelines, and antenna debugging techniques, aiming to provide the widest possible bandwidth for a given amount of power.
A well-designed antenna can significantly enhance the working range of wireless products. The greater the energy transmitted by the wireless module, the farther it can travel under a given packet error rate (PER) and receiver sensitivity. Additionally, a well-designed antenna offers less obvious advantages, such as emitting more energy within a specific range, which improves error tolerance caused by interference or noise. Similarly, a good debug antenna and Balun at the receiving end can operate effectively under very small radiation conditions. The best antenna reduces PER and enhances communication quality, resulting in fewer retransmissions and improved battery life.
Antenna Principle
An antenna generally refers to a conductor exposed in space. When the length of the conductor corresponds to a specific ratio or an integral multiple of the signal wavelength, it can function as an antenna. This condition is known as "resonance," where the electrical energy supplied to the antenna is emitted into the surrounding space.
As shown in Figure 2, the conductor's length is λ/2, where λ represents the wavelength of the electrical signal. The signal generator supplies power to the center of the antenna through a transmission line, also known as the antenna feed. Based on this length, voltage and current standing waves form along the entire wire, as depicted in Figure 2. The electrical energy input into the antenna is converted into electromagnetic radiation and radiated into the air at a corresponding frequency. The antenna is powered by an antenna feed with a characteristic impedance of 50 Ω and radiates into a space with a characteristic impedance of 377 Ω. Therefore, there are three key aspects to consider when designing the geometry of the antenna: a. Antenna length, b. Antenna feed, c. Shape and size of the ground plane and return path.
An antenna with a length of λ/2 is called a dipole antenna. However, on printed circuit boards, most conductors used as antennas have a length of only λ/4 but still offer similar performance. By placing a ground plane below the conductor, you can create an image equivalent to a λ/4 length. When combined, these structures function as dipole antennas. This type of antenna is referred to as a quarter-wavelength (λ/4) antenna. Almost all PCB antennas are implemented as quarter-wavelength antennas on the copper ground plane. It's important to note that this signal is now a single-ended feed, and the ground plane serves as the return path.
Antenna Type
As described in the previous section, all conductors with a wavelength of λ/4, when placed on a ground plane and supplied with a suitable voltage, can act as an antenna. Depending on the wavelength, the antenna may be as long as an FM car antenna or as short as a trace on a signal buoy. For 2.4GHz applications, most PCB antennas fall into the following categories:
1. Wire Antenna: This is a piece of wire extending into free space on the PCB. It has a length of λ/4 and is placed on the ground plane. This antenna is powered by a 50Ω impedance transmission line. Typically, this wire antenna provides the best performance and radiation range. The wire can be straight, spiral, or looped. It is a three-dimensional (3D) structure where the antenna is 4-5 mm above the PCB and extends into the space.
2. PCB Antenna: This is a PCB trace on the PCB, which can be drawn as a straight line, inverted F-shaped line, serpentine, or circular line. Unlike a wire antenna, the PCB antenna is not exposed to external space but exists in a two-dimensional (2D) structure on the same PCB layer. When a 3D antenna is placed on the PCB as a 2D PCB trace, certain guidelines must be followed. In general, it requires more PCB space and less efficiency than wire antennas, but at a lower cost and provides acceptable wireless distance for BLE applications.
3. Chip Antenna: This is an antenna with a conductor packaged in a small IC. It becomes advantageous when the antenna is compact. Applications such as USB nano-transceivers use this type of antenna when there is not enough space on the PCB to lay out a PCB antenna. For information on the chip antenna, see the figure below. For a comparison of the dimensions of the various antennas, see Table 4.
Antenna Selection
The choice of antenna depends on its application, available board size, cost, range of radiation, and directionality. Bluetooth low energy (BLE) applications (such as wireless mice) require only a 10-inch radiation range and a data rate of a few kbps. However, for remote control applications that use speech recognition, an indoor antenna is required that has a radiation range of approximately 10-15 inches and a data rate of 64 kbps. For wireless audio applications, a diversity antenna is required. A diversity antenna means that two antennas are placed on the same PCB. This ensures that at least one antenna can always receive some radiation, while the other antenna may be blocked by reflection and multipath weakening. A diversity antenna is required in the case of transmitting real-time audio data and requiring high throughput without losing packets. It can also be used in beacon applications for indoor positioning.
Antenna Parameter
The following sections provide some key parameters for antenna performance.
§ Return Loss: The return loss of the antenna indicates how the antenna is matched to a 50 Ω transmission line (TL), which is shown as the signal feed in Figure 7. Usually, the impedance of this TL is 50Ω, but it can be other values. For industry standards, the commercial antenna and its test equipment have a resistance of 50 Ω, so it is recommended that you use this value. Return loss indicates the amount of incident power reflected by the antenna due to mismatch (Equation 1). An ideal antenna will transmit the full power without any reflection. Equation 1 Return Loss (dB) = 10 log (Pincident/Preflect) If the return loss is infinite, the antenna is considered to be exactly matched with the TL, as shown in Figure 7. S11 is the reciprocal of the return loss and its unit is dB. It is estimated by experience that if the return loss is ≥10 dB (both S11 ≤ –10 dB), it is large enough. Table 1 shows the return loss (dB) and reflected power (%) of the antenna. When the return loss is 10 dB, it means that 90% of the incident power is transmitted to the antenna for transmission.
Figure 7. Return loss
§ Bandwidth: refers to the frequency response of the antenna. It indicates how the antenna and the 50 Ω transmission line match each other over the entire frequency band used, i.e., in the range of 2.40 GHz to 2.48 GHz for BLE applications.
Figure 8. Bandwidth
As shown in Figure 8, the return loss is greater than 10 dB over a bandwidth of 2.33 GHz to 2.55 GHz. Therefore, the bandwidth used is around 200 MHz.
§ Radiation efficiency: Refers to a portion of the non-reflective power consumption (see Figure 7) that is consumed as heat in the antenna. The heat generated is due to the dielectric loss in the FR4 substrate and the loss of conductors in the copper wire. This information is used as the radiation efficiency. When the radiation efficiency is 100%, all non-reflective power consumption is transmitted into the space. For small PCB form factors, heat consumption is minimal.
§ Radiation pattern: This pattern indicates the directionality of the radiation, which means that the radiation in which direction is larger and the radiation in which direction is smaller. This helps to accurately determine the direction of the antenna in the application. A non-directional antenna can perform equivalent emission in all directions on a plane perpendicular to the axis. But most antennas do not achieve this ideal performance. For a detailed description, please refer to the radiation pattern of the PCB antenna shown in FIG. Each data point represents the RF field strength and can be measured by a Receiver Signal Strength Indicator (RSSI) in the receiver. As expected, the resulting contour image is not circular because the antenna is not isotropic.
Figure 9. Radiation pattern
§ Gain: Gain provides information on the direction of radiation being compared to an isotropic antenna that can be transmitted from all directions. The gain unit is dBi, which is the field strength of the radiation when compared to an ideal non-directional antenna.
Cypress PRoC/PSoCBLE Antenna
One limiting factor in designing BLE is the need to integrate the antenna in a compact space and adjust it with up to two external components. The debugging process needs to ensure that the energy of the incoming antenna is kept as much as possible when transmitting within a certain frequency band. This means that the return loss in the required band is greater than 10 dB. When the input impedance of the antenna is 50Ω and the output impedance of the chip is 50Ω, the antenna receives the most energy. When the antenna is used as the receiving end, the above conditions are also satisfied. For the antenna, its adjustment process ensures that the impedance of the antenna is equal to 50Ω. For the chip, the Balun adjustment process ensures that the resistance is close to 50Ω.
The impedance of the integrated balancer in the PRoC/PSoCBLE device is not equal to 50Ω, so it may be necessary to adjust it with two components. For low data rate applications with small RF ranges, Cypress's recommended PCB antennas do not require any components to adjust the antenna. For high data rate applications (such as voice recognition applications via remote controls), it is recommended to use at least four components for the matching network. Two of them are used for balancer adjustment and the other two are used for antenna adjustment. Two of them can be used for the adjustment process, and the remaining two remain inactive. In addition, Cypress PRoC/PSoC offers different applications such as indoor positioning, smart homes, smart appliances, and sensor hubs. These applications may not be limited by space, so better antennas can be designed for these applications for factors such as RF range and RF pattern. Wire antennas are ideal for non-wearing but fixed position applications. Many applications embed Cypress's modules directly into their main PCB for wireless connectivity. These applications require low cost small modules that pass the FCC. At this time, a chip antenna that satisfies these requirements can be used. Although there are many applications using the 2.4 GHz band, most BLE applications use only the dual PCB antennas described below. Cypress recommends the use of two proprietary PCB antennas, a serpentine inverted-F antenna (MIFA) and an inverted-F antenna (IFA), which are characterized and widely simulated antennas for BLE applications. In particular, MIFA can be used in almost all BLE applications.
Cypress's Proprietary PCB Antenna
Cypress recommends the use of two PCB antennas, IFA and MIFA. The low speed and typical radiation range in BLE applications make these two antennas particularly useful. These antennas are inexpensive and easy to design because they are part of the PCB and provide good performance over the 150 to 250 MHz frequency range. It is recommended to use the MIFA antenna in applications that require minimal PCB space, such as wireless mice, keyboards, demos, and more. For IFA antennas, it is recommended to use them in applications where the size of one side of the antenna is much smaller than the size of the other side, such as a heart rate monitor. The MIFA antenna is used in most BLE applications. The information for each antenna is detailed in the following sections.
Serpentine Inverted F Antenna (MIFA)
MIFA is a common antenna that is widely used in various human interface devices (HIDs) because it occupies less PCB space. Cypress has therefore designed a rugged MIFA antenna that delivers superior performance in smaller form factors. The antenna measures 7.2mm x 11.1mm (equivalent to 284 mils x 437 mils), making it ideal for a variety of HID applications such as wireless mice, keyboards, or demos. Figure 10 shows the detailed layout of the recommended MIFA antenna, which includes the top and bottom layers of the dual-layer PCB. This antenna has a trace width of 20 mils. The value of "W" is the main parameter that can be changed, depending on the PCB stack interval, which represents the width of the RF trace (transmission line).
Antenna Feed Considerations
Table 2 shows the "W" value of the thickness between the top and bottom layers of the double-layer FR4 PCB (corresponding dielectric constant is 4.3). The top layer contains the antenna traces; the bottom layer is the next layer that contains the solid RF ground plane. The remaining PCB space on the bottom layer can be used as a signal ground plane (for PRoC/PSoC and other circuits). Figure 11 shows the "W" value of a typical two-layer PCB thickness. Table 2. The "W" value of the FR4 PCB: the thickness between the antenna layer and the ground plane of the adjacent RF.
For PCB traces that feed shorter antennas, such width requirements are looser. Make sure that the width of the antenna trace is the same as the width of the antenna feed contact. In the case shown in Figure 12, the trace width of the antenna feed is not the width specified in Table 2.
Figure 14 shows a complete 3D radiation gain plot for the MIFA at 2.44 GHz. This information is useful when setting up a MIFA antenna for a custom application, helping to get the most radiation in the desired direction. In the above picture: the MIFA is placed on the XY plane with the Z-axis direction perpendicular to it.
For this radiation pattern, it can be known that the largest radiation occurs in a conical space at an angle of 30° to the X axis. This is because the MIFA cannot maintain a positive or vertical direction in the XY plane. Both the vertical part and the tip of the MIFA participate in the radiation and form a slanted radiation pattern.
Antenna Length Considerations
Depending on the thickness of the PCB, the length of the MIFA antenna needs to be adjusted so that the impedance and frequency selection of the antenna radiation can be adjusted. Depending on the board thickness, Cypress offers the following antenna lengths.
Table 3. Length of vertical section and tip (L_Tip/L_leg)
Figure 15 shows two MIFA antennas for two different board thicknesses. Refer to Table 3 when the designer adjusts the length of the MIFA antenna based on the specific board thickness.
Inverted F Antenna (IFA)
For applications where the size of the antenna is limited (e.g., heart rate monitor), it is recommended that you use this IFA. Figure 16 shows the detailed layout of the recommended IFA, which includes the top and bottom layers in a two-layer PCB. Its trace width is about 24mm. For FR4 PCBs with a thickness of 1.6 mm, the IFA is designed to be 4 mm x 20.5 mm (157.5 mils x 807 mils). Compared to MIFA, IFA has a larger aspect ratio (ratio of width to height).
Top layer (antenna layer)
Bottom layer (RF ground plane)
The width "W" of the feed trace is affected by the PCB stack in the product. Table 4 provides the corresponding "W" value (corresponding dielectric constant of 4.3) for the FR4 substrate based on the different PCB thickness between the top layer (antenna layer) and the bottom layer (adjacent RF ground plane). The top layer contains the antenna traces; the bottom layer is the next layer that contains the solid RF ground plane. The remaining PCB space on the bottom layer can be used as a signal ground plane (for PRoC/PSoC and other circuits). Figure 17 links the concept of "PCB thickness" for a typical two-layer PCB to the "F" value. Table 4. The "F" value of the FR4 PCB: the thickness between the antenna layer and the ground plane of the adjacent RF.
For short traces less than 3 mm, the antenna feed thickness can be adjusted. The thickness of the antenna feed can be the same as the thickness of the antenna trace, see Figure 12.
The frequency of the IFA over the bandwidth of 220 MHz (S11 ≤ -10 dB) is about 2.44 GHz, as shown in FIG.
Figure 19 shows a qualitative radiation pattern of IFA on the XY plane. This information is useful when setting up an IFA antenna for a customer application, helping to get the most radiation in the desired direction. For ease of observation, only the direction of the qualitative radiation is shown. For detailed radiation patterns on all XY, YZ, and ZX planes, contact Cypress Technical Support.
Chip Antenna
For niche applications with very small PCB sizes (such as Bluetooth transceivers), chip antennas are a good solution (Figure 20). They are off-the-shelf antennas that take up minimal PCB space and provide better performance. But the chip antenna adds a bill of materials (BOM) and requires assembly costs. Because it requires ordering and assembling external components. Typically, the price of a chip antenna is about 10-50 cents, depending on size and performance.
Another important factor to consider when using a chip antenna is that it is affected by the radiated ground area. Therefore, the manufacturer's recommendation for the grounding area must be followed. Unlike PCB antennas, chip antennas cannot be adjusted by changing the length of the antenna. In addition, a matching network is required to adjust the antenna, so more bills of material are added. Cypress only recommends the use of chip antennas in specific applications where minimal PCB space is required, such as the Nano Bluetooth transceiver. For such applications, Cypress recommends using a 2450AT18B100E chip antenna with Johansson technology, which measures 63milx 126mil. For most applications, PCB antennas such as MIFA or IFA are recommended. These small form factor (small footprints) antennas are not only cheap, but also provide excellent performance. Figure 21 and Figure 22 show the 2450AT42B100E chip antenna layout guide with Johansson technology. Its size is 118milx196mil. For more detailed guidance on these antennas, please refer to their relevant web sites.
The layout also shows a 50Ω feed transmission line and its matching components. The width of the feed transmission line depends on the thickness of the board. The exact board thickness is specified in Table 4.
The performance of the chip antenna is determined by the ground plane. In general, they require more grounding area and more space. As shown in the figure above, for the antenna of the 2450AT42B100E, the minimum grounding distance is 0.8mm. When the spacing is 2-3 mm, the observed s11 will be more pronounced. Chip antennas are not necessarily strictly isotropic. There are certain priority directions for radiation. Depending on the Gnd spacing and plastic fittings, the direction of maximum radiation is different. See Figure 23 for the common radiation directions for Johansson's chip antenna (2450AT42B100E).
Cypress only recommends the use of chip antennas in specific applications that require minimal PCB space, such as Nano Bluetooth transceivers or ultra-small modules. For these applications, Cypress also recommends using the 2450AT42B100E chip antenna with Johansson technology because it is larger in size, better in RF performance, and requires less Gnd spacing than the 2450AT18B100E. The contents of the Hansen technical antenna are for reference only. For more information on 2.4GHz chip antennas, please contact your supplier, such as Murata, Vishay, etc. For most applications, PCB antennas such as MIFA or IFA are recommended. These small (small footprint) antennas are inexpensive, but offer excellent performance.
Wire Antenna
The wire antenna is a conventional old antenna. The antenna is formed by fixing an aluminum wire or a quarter-wave clip on the PCB. The wire or paper clip is mounted on the PCB in a spiral shape and then at a distance from the PCB 5. The position of -6mm is parallel to this layer. No need to introduce them, because they are exposed to the air as 3D antennas, their RF performance is very good. This type of antenna has the best signal range and the most isotropic radiation pattern. The wire antenna's wireless coverage can exceed 100 feet. This type of antenna is not recommended for BLE applications that require a small form factor antenna because it takes up a lot of space and vertical height. But if there is enough space, this antenna can achieve the best RF range, directionality, and radiation pattern.
Wire antennas have the best RF performance. The antenna gain and radiation performance of the wire antenna are the best compared to other antennas. Refer to Figure 25 for a qualitative radiation pattern of the wire antenna.
Figure 25. Qualitative radiation pattern of the wire antenna
Comparison of various antennas
Please refer to Table 5 to quickly select the right antenna for your application.
• It consists of a strip conductor and a ground plane with a dielectric in between. If the dielectric constant of the dielectric, the width of the line, and its distance from the ground plane are controllable, its characteristic impedance is also controllable and its accuracy will be within ±5%.
The impact of the environment on antenna performance
The antennas typically used in consumer products are very sensitive to the size of the PCB RF ground plane and the plastic housing of the product. The antenna can be modeled as an LC resonator, and as L (inductance) or C (capacitance) increases, the resonant frequency of the LC resonator decreases. Larger RF ground planes and plastic enclosures increase the effective capacitance, which reduces the resonant frequency.
Impact of the ground plane
Cypress has extensively studied the size of the RF ground plane and the effect of the nearby plastic enclosure on the resonant frequency of the antenna. Through experiments and measurements, Cypress can determine the sensitivity of the antenna and provide a simple, powerful, and effective solution for debugging the antenna. To evaluate the antenna's sensitivity to the RF ground plane size, you can experiment by installing an antenna on a PCB of various possible sizes. Figure 26 shows an example where the MIFA is placed on a PCB with a different ground plane size. The size of the PCB ranges from 20 mm x 20 mm to 50 mm x 50 mm. It can be seen from the curve that the larger the area of the RF ground plane, the lower the resonance frequency and the better the ground plane, and therefore the smaller the return loss. This is a key condition in a good PCB layout. The better the ground plane provided to a quarter-wave antenna, the better its relationship to theoretical performance. This is a key concept in antenna design that addresses the difficulty of not providing enough space to ground a small module antenna.
Plastic casing effect
Similarly, to determine the effect of the plastic housing of the product on the antenna, a wireless mouse is used for the experiment, as shown in Figure 27. Place the Cypress MIFA in the plastic case of the wireless mouse and measure the resonant frequency of the antenna.
Through Figures 26 and 27, the following main contents can be understood:
§ When the antenna is placed close to the plastic case, the resonant frequency is reduced. § The resonant frequency varies from 100MHz to 200MHz. The antenna must be re-tuned to obtain the desired frequency band.
In summary, the size of the ground plane and the plastic casing are increased to reduce the resonant frequency of the antenna to a range of 100 MHz to 200 MHz. Product Shell and Grounding Layer Guide
§ Make sure that there are no components, fixing screws or ground planes near the antenna tip or antenna length range. § The battery or audio cable must not pass through the same side of the antenna or antenna on the PCB. § The metal casing cannot be completely covered by the antenna. If the product housing is metal or a protective cover, do not completely cover the antenna. § The direction of the antenna should be in the direction of the final product so that the antenna has the greatest amount of radiation in the desired direction. § There should be enough space: the larger the ground plane, the higher the S11 parameter value (return loss) of the MIFA, IFA, chip antenna, and wire antenna. § There should be no ground plane directly below the antenna. Please refer to Figure 14. This setting applies to all antennas. § There must be sufficient space (gap) from the antenna to the ground plane, and the width of the ground plane should be minimal. Please refer to FIG. 10, FIG. 15, and FIG.
Antenna Debugging
The antenna commissioning process ensures that the return loss of the antenna (from the direction of the chip output) is greater than 10 dB in the desired frequency band. Similarly, for the chip (Balun), the same procedure is performed to ensure that the impedance of Balun is 50Ω in the accept mode. At this time, antenna debugging and Balun debugging are called antenna debugging.
The 0 Ω reference point is connected to a network analyzer with a port network. During antenna debugging, the connection to the chip can be disconnected by removing the Balun matching component. During the Balun debugging, the connection with the antenna matching component is disconnected. The following sections detail how to use the network analyzer to debug the antenna. Although only the debugger for a Pioneer kit wireless mouse is shown here, this program is suitable for all antenna debugging.
Debugging Process
As mentioned earlier, the effects of the outer casing and ground plane demodulate the required frequency band of the antenna and affect the return loss. Therefore, the antenna debugging process consists of two steps: first, the PCB blank board is debugged into the required frequency band; then, after the ID is determined, the debugging is performed by contacting the plastic case with the human body. Use a network analyzer to check antenna debugging. In the first step, the network analyzer is calibrated first, then the antenna is debugged by adjusting the matching network components and verifying the debug in the Smith chart.
Will be used during debugging:
§ Agilent 8714ES Network Analyzer (calibrated) § Pioneer kit mouse (e.g., DUT) § Semi-rigid cable with electrical delay time of 350ps § Quality RF component catalog (Johanson kit P/N: L402DC)
The main steps of the debugging process are:
1. Prepare the ID 2. Set up and calibrate the network analyzer 3. PCB empty board debugging. As shown in the figure, the return loss of the markers 1, 2 and 3 is greater than 15 db. 4. Use plastic and human contact to adjust and debug
Prepare ID
This step is very important because the placement of the coaxial cable will result in a s11 change of 3 dB. Try to make the ground connection of the coaxial cable shield close to the transmission line return path. Please do the following:
1. Open the plastic case, remove the battery or disconnect the power supply. 2. Bring the coaxial cable close to the RF output pin of the chip. Disconnect the chip. Otherwise, not just the antenna, even Balun will connect to the coaxial cable. See Figure 29. 3. Make sure that there is a bare ground plane near the coaxial cable head. Ground the shield or casing of the cable. When grounding the shield/case, minimize the distance between it and the ground. The smaller the distance, the higher the debugging accuracy. The difference in return loss measurements can be 3 dB depending on where the coaxial cable is grounded. 4. Connect a 10pF capacitor from the first pad of the 50Ω reference point to the antenna tip. Always connect a capacitor between the coaxial cable and the antenna. This can block the DC power of the network analyzer.
Figure 29. Connection point of the coaxial cable
Set up and calibrate the network analyzer
1. Calibrate using the 3.5mm calibration kit. Next, after setting the network analyzer's calibration kit to 3.5mm, press the cal button on the Agilent 8714ES. You can also use other calibration kits, such as the N-type calibration kit.
2. Press the frequency button to set the start frequency and stop frequency to 2 GHz and 3 GHz respectively, and set the format to the Smith chart. 3. Press the marker button to set the frequency of each marker to 2.402 GHz, 2.44 GHz, and 2.48 GHz. 4. Press the cal button to select S11 on the network analyzer and set it to User 1 Port Calibration. 5. When connecting to "open" loading, please connect "open" to load and press measurestandard. 6. Connect the "Short" load and press measurestandard. 7. Connect to the "broadband" load and press measurestandard. The network analyzer then calculates the coefficients and displays the 50 Ω load as a reference point on the Smith chart that is clearly labeled 50,0. 8. Debug the coaxial cable and set the electrical delay by pressing the 'scale' button and setting the electrical delay correctly.
Debug PCB empty board
To debug a PCB blank, first determine the impedance of the antenna and then reduce the return loss to less than 10 dB in the desired frequency band based on the matching network components.
1. Connect an 8.2pF capacitor in series with the antenna. The impedance of this capacitor is 0 Ω in the desired frequency band. This impedance is the antenna impedance. The impedance of the antenna is equal to (100.36–j34.82) as shown by the red circle in the Smith chart.
2. After determining the impedance of the antenna, use the topology to make the antenna impedance 50Ω by performing impedance conversion. Most of the matching networks of Cypress MIFA or IFA (shown in Figure 31) consist of two components.
The matching network components can be simulated using standard open source tools such as Bern V3.10 from the Bern Institute. By connecting a 0.45pF shunt capacitor and a 3.6nH series inductance to the antenna, the antenna impedance can be converted to 50Ω, enabling the virtual portion to be removed in the desired frequency band. Since the exact value is not available, we have to choose a 0.5pF shunt capacitor and a 3.6nH series inductor.
Shown below is the final schematic of the matching network. ZL represents the impedance of the antenna when the impedance is 0 ohms. Zin refers to the impedance observed by the network analyzer when the output impedance is 50 ohms.
Simulation software helps to understand component values. However, the actual component values differ greatly from the simulated values. This happens because at 2.4 GHz, the inter-line inductance of the capacitor, the parasitic loading of the pad, and the ground return path create an additional parasitic loop that completely changes the Smith chart. For this application, a 0.7pF capacitor and a 1.2pF series capacitor are required to achieve resonance.
Below is a brief description of the operation. The antenna impedance is derived from an 8.2 pF capacitor with an assumed impedance of 0 ohms. In addition, the figure also shows the parasitic capacitance of the inductance between the traces at a frequency of 2.4Ghz. The ground return path is next to the antenna. However, due to the use of matching components, the ground return path will have additional parasitic capacitance. The inductance of the antenna will be large, so add several capacitors to adjust the inductance. This is a typical problem encountered when adjusting the antenna. There are obvious differences between theory and practice. The user can add a capacitor, but note that if you add an inductor, the Smith graph will move in a certain direction. Figure 35 shows the final Smith chart using the actual components.
Figure 35. Smith chart with actual components
The figure also shows that the flags 1, 2 and 3 with frequencies of 2402 MHz, 2440 MHz and 2480 MHz are close to the (50, 0) point on the Smith chart. The display is a good match. The following chart shows the callback loss of the component values. Return loss greater than 15db is in line with our application.
Figure 36. Return loss when using actual components
As shown in the figure, the return loss of the markers 1, 2 and 3 is greater than 15 db.
Use plastic and human contact to adjust and debug
The plastic housing of the PCB changed the antenna adjustment. All antennas are affected by near-field or far-field objects. If it is a narrowband antenna, it is very much affected by near-field objects. The plastic housing and the nearby moving battery cable can completely distort the antenna and its return loss will be less than 10 db in the preferred frequency range of 2.402G to 2.482G. Therefore, after debugging the PCB blank board, it is necessary to keep the PCB in the plastic case and use the mouse to re-detect and debug. This is complicated, especially when the coaxial cable is taken out of the plastic assembly. The coaxial cable can be led out by drilling a small hole in the ID. Finally, use a plastic case or place one hand on top of the plastic case (as when the user uses the mouse) to check the results. The observed return loss has the least impact.
Figure 37 SmithChart when using a plastic device, chart connected to ID
To sum up
This application note provides an overview of how to easily design the best antenna for a custom product using PRoCBLE/PSoCBLE. Antenna layout guidelines are also provided for different antenna types.