Due to consumer desire for high-tech functions in a single packaging, compact low power gadgets have long been preferred, particularly in automotive and smart wearables. Additionally, PCB antennas’ high strength and network compatibility make them strongly connected and lasting, which is why they are used in the majority of consumer electronics products. By converting the electric signals into electromagnetic waves, high frequency PCB antennas allow devices to communicate across great distances.The key benefit of PCB antennas is that they minimize the device’s physical footprint and maintenance expenses. An antenna’s size should be compact and minimal, and to maximize efficiency, numerous microstrip patches are connected together in order to achieve the desired gain from the minimal size. The operating frequency’s wavelength has a direct impact on the size of the patches. PCB antennas exist in a variety of sizes and forms, and since they have so many uses, designing them is an important consideration. There are several parameters involved in the methods used to design a PCB antenna that must be taken into account. There are several PCB antenna types, including:
- Loop antenna
- Patch antenna
- Inverted -F antenna
In many systems, antenna design is crucial for achieving successful low power and short-range transceiver transmission. During the design phase, the patch size is calculated using the wavelength of the target operating frequency.
Equation 1: Calculation of width and length
W, L, and R are the microstrip patch antenna’s width, length, and dielectric constant, respectively, from the equation above. If the frequency and the dielectric constant of the material being used are known, the calculation may be done using the method above to determine the dimensions of a microstrip patch antenna. Designers must decide to enter the signal’s velocity of propagation if the material’s dielectric constant is unknown.
Calculating Antenna parameters
Fig 1- Substrate dimensions Source- http://www.emtalk.com/images/calculator/mpa_model.jpg
The first step before calculating Antenna parameters is to select the substrate material and the length and width of the material required along with the operating frequency of the antenna. The PCB should be made out of FR4 material because of its high dielectric strength and relative permeability of 4.4
Equation 2: Height of the substrate
The aforementioned equation, where hs is the substrate’s height, f is the frequency (in GHz), c is the velocity (in m/s), and r is the substrate’s dielectric constant, is used to calculate the height of the substrate. When expanding an antenna, one of the most important factors is the substrate’s height. Throughout the designing process, the chosen substrate’s height is maintained constant. The height is kept at 1.5mm for materials like FR4, Glass Epoxy, Bakelite, etc.
Equation 3: Trace width
The width of the trace is calculated using the aforementioned equation, where wt is the width of the trace. Improper trace widths can result in greater signal reflections, therefore signal traces, power traces, and impedance traces need to be properly designed and calculated. The lowest trace width is 6 mil, or 0.625 mm, and the maximum trace width is 10-12 mil, or 0.254–0.3 mm, for tighter tolerances.
Equation 4: Length of the trace
The length of the trace is calculated using the aforementioned equation, where Lt is the physical length, ff is the effective permittivity, and L is the trace length. As the trace gets longer or smaller, trace resistance rises. Therefore, it is imperative to keep higher current traces as short as feasible.
Equation 5: Width to depth ratio
The above equation is used to determine the ratio of microstrip width to depth ratio, where d is the trace width, w is the width of the substrate and A is the effective area. The ideal microstrip width to depth ratio for a 50 ohms FR4 material is about 2:1.
Designing the antenna
Antennas are highly sensitive to their environment, thus when an antenna is embedded into a PCB, the design and layout should be taken into account in accordance with the requirements since this may have a significant impact on the performance of the wireless device. The performance of the antenna can be impacted by even the smallest variables, such as the material, layer count, layer thickness, etc. There are several steps to comprehend when it comes to designing a PCB antenna. The following are a few of them.
Positioning the antenna
Antennas operate in a variety of ways, and the places in which they must be installed depend on their radiation output. For instance, along the PCB’s long or short sides, or in one of its corners. One of the best places for an antenna to be placed is in a PCB corner. This is so because the antenna can have clearance in up to five spatial directions from a PCB’s corner point, while the feed for the antenna is in the sixth direction. Since there are numerous antenna designs available, PCB designers can choose the antenna that is best suited for their application and layout.
Fig 2- Omnidirectional spacing of an antenna
Source: https://img.mwrf.com/files/base/ebm/mwrf/image/2020/04/Figure_1_Antenna_with_directions_clear.5e8e1df21085f.png?auto=format&w=1300&h=730&fit=max
Keep-Out Area
Designers must make sure that no components are positioned in the near field directly surrounding the antenna since this could cause signal interference and degrade the circuit’s performance. Additionally, the vicinity of the antenna must be free from metallic things, such as mounting screws. The ground plane, against which the antenna radiates, is connected to the frequency at which the antenna functions. The ground plane of the antenna must therefore be given the correct size and spacing.
Fig 3- Keep-Out Area – Source: Author
Ground planes
The size of the ground plane on a PCB is crucial to take into account since improperly constructed wires that are used to communicate with different devices and batteries that supply power to the device may change. The size of the ground planes must be carefully considered by designers to minimize the impact of cables and batteries on the antenna. Some PCB antennas are ground-plane dependent, which means that in order to balance the antenna currents and lower layers of the PCB, which can impair the antenna’s performance, the PCB itself functions as the ground portion of the antenna. Designers must make sure no battery is placed close to the antenna in such circumstances.
Fig 4- Ground plane – Source: https://img.mwrf.com/files/base/ebm/mwrf/image/2020/04/Figure_3_Clearout_area_with_additional_clearance_below_the_ground.5e8e1e2dc303f.png?auto=format&w=1300&h=730&fit=max
Proximity to other PCB components
The embedded antenna must be kept far away from any other circuit elements that can interfere with the antenna radiation during design. The antenna’s distance from other components changes depending on the component’s height and width. High switching speeds in various connectors, such as USB, HDMI, Ethernet cables, and batteries and LCDs, raise the possibility of signal interference while the device is in operation. The component will ideally be at a safe distance if positioned below this line, as shown in Fig. 4, if a line is drawn at an angle of eight degrees from the base of the antenna. Detune effects between the antennas as they alter each other’s radiation may occur if other antennas operate nearby in comparable frequency bands. Antennas need to be segregated by at least -10dB for 1GHz and -20dB for 20 GHz to prevent this. Increasing the distance between the antennas or rotating them to be positioned 90 degrees or 180 degrees apart from one another can accomplish this.
Fig 5- PCB Component’s Proximity – Source-https://img.mwrf.com/files/base/ebm/mwrf/image/2020/04/Figure_5_Distances_H_T_A_B_used_to_estimate_impedance_of_antenna_trace.5e8e1e6d22f7e.png?auto=format&w=1300&h=730&fit=max
Designing the transmission line
The transmission line of an antenna is the RF trace that transports the radiofrequency energy in PCB antennas that send the signal to the receiving. In order to prevent signal reflection back to the reception and a deteriorated signal-to-noise ratio (SNR), which could lead to desensitization in the radio receiver, the transmission line must be constructed at 50. The transmission line is the RF trace that transports the radiofrequency energy in PCB antennas to send the signal to reception. The transmission line must be meticulously designed. The transmission line must be straight in the first place since corners and bends increase the likelihood of losses. Vias must be evenly distributed along both sides of the trace in order to maximize performance by isolating noise from surrounding traces, as shown in Fig 6. This will help to decrease noise and signal losses in the antenna’s performance.The antenna should be tuned for use at a characteristic impedance of 50 using RF matching components and the width of the transmission lines. Because thinner transmission lines may be more susceptible to losses, the transmission line’s dimensions can have a significant impact on the performance of the signal. The antenna can transmit signals more effectively and efficiently with less losses if the transmission lines are as short as they can be.
Fig 6- Design of transmission line – Source: https://img.mwrf.com/files/base/ebm/mwrf/image/2020/04/Figure_5_Distances_H_T_A_B_used_to_estimate_impedance_of_antenna_trace.5e8e1e6d22f7e.png?auto=format&w=1300&h=730&fit=max
Techniques to Achieve Better Performance
There are several ways to increase the effectiveness and get better outcomes with PCB antennas aside from designing. Here are a few of them:
- By using matching networks to tune the antenna, performance-related aspects can be taken into account.
- The antenna must be put in the best possible location, which is at the PCB edge, and the ground plane should be suitable to prevent signal crosstalk.
- Because antenna signals cannot pass through metal, the exterior antenna case should not be done as this could interfere with the transmission.
- Antenna performance can be harmed by placing them close to plastic surfaces. The dielectric constant of plastic is typically higher than that of air, which negatively impacts transmitted signals by dampening the RF signal and causing losses. This will result in a larger dielectric constant, a longer electrical length for the antenna, and a decrease in the antenna’s radiating frequency.
- It is strongly advised to use high-quality FR4 circuit boards to prevent problems with RF performance.
Fig 7- Achieving better outputs – Source: https://media.istockphoto.com/photos/telecommunication-tower-with-mesh-dots-glittering-particles-for-picture-id1210455314?s=612×612
Conclusion
The performance of an antenna can be impacted by a number of factors, making PCB antenna design a complex procedure. Prior to creating an antenna, the first step is to choose a material (ideally FR4) with precise dimensions. PCB traces also play a significant role since transmission lines use these traces to convey RF signals. Trace widths must be carefully planned because an inappropriate width may result in problems like increased signal reflection. When it comes to tracing length, a trace’s resistance rises with length. Therefore, it is optimal to design greater current-carrying traces to be as brief as possible. The location of the PCB must be done while creating a PCB antenna in order to give the antenna higher radiation clearance. When placing components close to the antenna field area, designers must exercise caution because this causes signal interference. Additionally, since their faster switching speeds increase the likelihood of signal interference, components like LCDs, HDMI cables, USB cables, etc. must be maintained far away from other circuit components. Because different connections to the antenna—from wires to batteries—have a negative impact on the currents and lower layers of the PCB, the ground plane design must be appropriately designed. To prevent signal deterioration, transmission lines must be built at a 50-degree angle and should be straight, devoid of bends or corners.
