Antennas are major components used in wireless technologies that range from small antennas embedded in mobile devices to massive antenna arrays found in cellular or satellite base stations. Although antennas exist in many shapes and sizes, virtually all of them belong to one of two main categories: Internal or External. Internal antennas are embedded within a device’s enclosure and are relatively out of reach to the end user. These range from small chip or PCB-etched antennas integrated onto the board, to flexible printed circuit (FPC) antennas that are mounted to the inside of a product’s enclosure. Alternatively, external antennas are mounted on the outside of a device’s enclosure via an RF connector. A typical example would be a “rubber-duck” antenna that mounts to the outside of an internet router. The purpose of this article is to outline the different types of external antennas, their key performance parameters, and their specific design advantages.
Bandwidth: This is defined as the optimal range of frequencies for an antenna to efficiently transmit and receive signals. For example, a Bluetooth antenna performs best in the 100 MHz bandwidth of frequencies at 2.4 - 2.5 GHz. By the inherent nature of antennas, this hypothetical Bluetooth antenna would also be susceptible to frequencies outside it’s intended bandwidth, but it is over this specific range of frequencies (2.4-2.5GHz) that the antenna is expected to perform optimally at a certain efficiency (%) level. However, this is dictated by how precise the antenna element is designed. An antenna’s bandwidth is typically defined in terms of Voltage Standing Wave Ratio (VSWR)–a parameter that measures the amount of power that is reflected back to a radio from an antenna (Figure 1). The less power reflected back to the radio, the more efficiently the antenna is able to perform. The preferred VSWR for a particular bandwidth is usually less than or equal to 3:1. For example, an antenna that claims to operate from 100 to 400 MHz may state that its VSWR is less than 1.5 within this bandwidth efficiency. In this case, it would imply that the antenna is expected to reflect around 4% of the power back to the radio.
Radiation Pattern: This is less of a numerical parameter and more of a graphical 3D representation of the energy distribution surrounding an antenna. While a 3D radiation diagram is a full representation of an antenna’s energy distribution, a 2D diagram is also beneficial in providing an easier way to identify where most of the antenna’s energy is concentrated. The main purpose of a radiation pattern is to visualize how omnidirectional (Figure 2) or directional (Figure 3) an antenna is. An omnidirectional antenna is described as having a radiation pattern that is relatively the same in all directions within a single plane (i.e. horizontal x-y plane). Conversely, a directional antenna has less of a symmetrical radiation pattern, with most of its radiated energy concentrated in a single direction.
Gain: This is the first parameter that is considered when evaluating an antenna’s performance. The gain is always described in context of the antenna’s radiation pattern, as it is defined as the signal strength in the direction of its peak radiation when compared to that of an isotropic source. An isotropic source is meant to serve as a “reference antenna,” although it does not exist physically. The “reference antenna” serves as an excellent comparison source to that of an actual antenna, as the radiation pattern is consistent in all directions. Reference (Figure 4) for the general formula for antenna gain (dBi).
For any antenna in practice, increasing its gain means you are increasing its “directivity”, which in turn increases the power in a desired direction at the expense of the power being radiated in other directions
Besides the obvious differences in size and form factor, external antennas offer a series of advantages for customer design in comparison to internal antennas–one being ease of integration. External antennas are typically designed to be “plug-and-play” solutions that simply mate to a transmitter via a specific connector. Conversely, internal antennas (like surface mounted chips) require additional design effort, as further antenna tuning and optimization is required. An internal antenna’s performance is influenced by the PCB ground plane, as it serves as an extension of the antenna. In this case, the board area and components on the PCB would have to be considered. Additionally, an impedance matching network may have to be implemented before the antenna feed point to account for these factors on the PCB that will detune the antenna. An internal antenna is also susceptible to signal loss caused by the product enclosure. On the other hand, the majority of external antennas are “ground plane independent,” making them an ideal approach for customers seeking a solution that requires fewer design resources and shorter time to integrate to allow for a more rapid time-to-market.
In addition to ease of integration, external antennas offer performance advantages in comparison to internal antennas. Overall, external antennas offer superior range and sensitivity due to the nature of their larger size. This often results in a higher rated gain (dBi) than their internal counterparts. Provided their higher gain, external antennas offer greater directional behavior for applications where signal transmissions are required to be concentrated in a specific direction. Another important factor to consider is that a larger antenna is required to support lower frequencies with longer wavelengths. Because of this, many high gain external antennas maintain a bandwidth that spans in the lower sub-GHz range with acceptable performance.
Due to its smaller size, an internal antenna would not function as well to support lower frequencies. For example, it would be difficult to find an internal antenna that could compete with an external antenna in terms of performance in the 400MHz bandwidth. All these inherent performance advantages (superior range, sensitivity, and ease of integration), combined with the fact that external antennas are outside the enclosure thus providing them with a better signal line-of-sight, makes them more suitable for applications with demanding requirements. However, customers must consider cost when pursuing this option, as additional manufacturing processes and materials are required to produce larger external antennas compared to something like a simple ceramic chip antenna.
These “puck-style” antennas are meant to be mounted flat upon a surface such as a ceiling or roof of an automobile. They can be mounted on a metal or non-metal surface depending on the antenna model. The form factor is a big distinguishable trait, as they are generally a more low-profile design, making them ideal for customers looking for a different aesthetic to a larger profile like a terminal-mount antenna. By design, many puck style antennas are also capable of supporting integrated Low Noise Amplifiers (LNA) to dramatically improve signal reception–particularly for weak incoming GNSS signals. Unlike whip style antennas, puck style antennas are often meant to be horizontally oriented to the ground or sky, as they tend to have more of a 360-degree coverage in the vertical plane. An example would be a Wi-Fi ceiling mounted antenna in the center of a single floor of an office. Signal coverage would have to be downward-facing to ensure proper reception to all the computers, phones, and printers below.
Another advantage of the puck-style antenna form factor is that many different models can support multiple wireless protocols. This is optimal for any base station that wants to consolidate all of the different antennas needed for GNSS, Cellular, and Wi-Fi into a single form factor. A combinational antenna (Figure 7) like this is essentially three different antenna elements housed in a single enclosure, with each varying protocol having its own cable and connector.
The continuing evolution of IoT applications has resulted in an increase in demand for a wide variety of antenna options. Whether an engineer is leaning towards an internal solution to meet low cost, high volume, and size requirements; or towards an external solution for ease of design and guaranteed performance–the antenna will always be the crucial interface for your wireless system. It is recommended that the antenna be finalized early in a project’s design phase to ensure optimal performance. Fully settling on your product’s requirements (such as its PCB design, size, and enclosure) without taking the antenna into consideration will decrease your ability to modify your design in the event that a selected antenna does not fit or is incompatible. In addition to having a full understanding of your application’s requirements, being familiar with the different antenna types, their unique advantages, and performance parameters (gain, bandwidth, VSWR, radiation pattern) will always be instrumental in narrowing down the numerous antenna designs that exist today.
The continuing evolution of IoT applications has introduced a demand for wireless devices to become smaller with each iteration. As devices continue to decrease in size, their antennas must become smaller along with them, thus making internal antennas increasingly more prevalent in today’s market. Compact devices require internal antennas that are capable of meeting stringent size requirements while meeting high-performance standards. However, the smaller the antenna, the more difficult it is to meet these standards. Additionally, internal antennas present a new set of challenges for designers in comparison to their external antenna counterparts, as they are much more sensitive to product design factors such as ground plane size, enclosure material, and components that are in close proximity on the PCB. The purpose of this article is to outline the different types of internal antennas, their key performance parameters, and their specific design advantages.
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In comparison to internal antennas, external antennas are much more performance stable and easy to integrate. This is because external antennas are generally designed to be ground plane independent, meaning that the size of the PCB area to which it is connected does not play as big of a factor when it comes to performance. Conversely, internal antennas tend to have poorer efficiency, Voltage Standing Wave Ratio (VSWR), and gain (dBi) figures overall, especially in the lower frequencies, such as LTE bands 12 and 13 that cover the 700MHz spectrum. Due to all of the design considerations associated with internal antennas, engineers often opt for an external solution if they are not limited to size and form factor constraints for their project. For a more detailed analysis into key antenna performance parameters and comparisons between internal versus external antennas, please refer to our External Antennas: Different Types and Advantages blog.
An embedded PCB antenna is a custom solution that is typically a metal trace printed onto the top layer of the PCB. Other types of PCB antenna designs require the trace to be laminated evenly across multiple layers on a board where vias are needed, to interconnect all the traces on each layer.
There are multiple key advantages with this antenna design approach. First, PCB antennas are the most cost-effective option to implement compared to all other internal antennas due to the fact that you are technically not purchasing a third-party component from a supplier. This is because the manufacturing cost of a PCB antenna is included in the PCB assembly process when a project is in the mass production phase. A second advantage of using a PCB antenna is that it is the lowest profile approach for internal antennas due to it being a part of the PCB’s surface. The only space a PCB antenna occupies is a copper-free surface where there is a sufficient amount of space for the antenna to be mounted (the antenna clearance area). However, keep in mind that because of this, the total area required is always much greater than the size of the metal trace itself which could be a potential waste of valuable PCB real estate.
Despite the affordable cost and low-profile benefits of PCB antennas, there are a couple of key drawbacks that can make them undesirable for certain projects. First, although they are a very simple design in principle, properly integrating them to achieve optimal performance for your specific PCB will require a considerable amount of RF design expertise–there is no “one fits all” approach for PCB antenna designs. Since the PCB antenna is part of the board itself and is entirely ground plane dependent, it makes it very susceptible to any subtle changes to the PCB size. Proximity to other components on the board can also cause undesirable effects to the antenna’s performance. Any slight variation in the PCB design could result in the antenna being detuned from its intended center frequency and cause overall signal reception issues. Furthermore, when comparing the RF performance of a PCB antenna to other internal antennas, their inherently simple design and extreme sensitivity make them the poorest performing option in terms of range. Additionally, since the antenna is integrated during the PCB manufacturing process, customers will have no flexibility in terms of optimizing the antenna’s performance. Consequently, it is common that multiple design iterations of the entire PCB will be required just to optimize the antenna, thus resulting in additional cost and development time for the customer
A chip antenna is made of ceramic and is the smallest, discrete type of internal antenna available. These antennas can be purchased as a separate component, which is one of their main advantages in comparison to that of a custom solution (such as a PCB antenna), as their performance optimization becomes easier and more flexible even after the chip antenna is integrated on the board. Chip antennas offer numerous features that contribute to their ease of optimization, including their impedance matching network and the option for additional ground clearance beneath the antenna to further adjust frequency tuning.
The antenna performance of a system will always require active measurements and tests in advance RF facilities, especially during the certification process. For PCB antennas, any adjustments will result in additional cost and time, as it will require the development of an entirely new PCB iteration. Comparatively, a chip antenna that is performing poorly would require a simple swap of the antenna or an adjustment to its impedance network components to resolve this issue.
Chip antennas are the most ideal solution for small IoT applications that have strict size requirements for the PCB. A typical trace antenna will not be suitable in this scenario because although they are very low profile, they would still require more space on the PCB to account for both the metal trace and the required ground clearance area beneath it. Conversely, chip antennas are better suited for small boards since they typically take up less PCB area, despite being thicker in size. Additionally, there are chip antennas that are designed to work on-ground, or over a copper layer (often called “over-metal”) of the PCB. These on-ground antennas require a more vertical design compared to their off-ground counterparts, fortunately without compromising radiation performance. This makes the chip antenna an even more viable solution for small form factor applications, as there will not be a need to increase PCB size to integrate them–thus saving on manufacturing costs. Chip antennas also boast superior performance when compared to a trace antenna, since they are less susceptible to detuning from other components or environmental factors that are within close proximity.
Although chip antennas have their benefits, they still come with a few disadvantages. One factor being their initial cost, since the customer will have to rely on a specific manufacturer to supply this component for mass production. Another prominent challenge that chip antennas present, is that despite them being easier to optimize, they are still very sensitive and are subject to similar design rules as trace antennas during integration. Finally, considerable RF design expertise would be required to achieve optimal system performance.
A patch antenna, also known as a microstrip antenna, is essentially made up of two main layers: a ground plane at the bottom and a dielectric substrate on top. Upon the substrate layer lies a conductive patch including a specific geometry that serves as the radiating component that connects to the radio via a feedline. These antennas are popular due to their low profile, robust design, and stable performance for high volume applications. They are especially well-suited for very high frequency wireless protocols due to their physical dimensions that have a direct correlation with the wavelength. Additionally, the inherent design of patch antennas allow them to have a good amount of flexibility in terms of manufacturing and utilization.
There are multiple techniques that can be applied for feeding a patch antenna, including coaxial probe feeds, microstrip lines, aperture coupling, and proximity coupling. Each technique provides their own set of advantages in terms of ease of implementation, out-of-band spurious emissions, and bandwidth. Patch antennas are also diverse in terms of their polarization capabilities. Depending on their design, these antennas can be both linear and circular polarized (Dual Polarized). This is particularly useful for GNSS and satellite communication applications that typically rely on right-hand circular (RHCP) or left-hand circular (LHCP) polarizations for their antennas. The polarization of an antenna essentially describes the general direction of the electric field that is oscillating away from the source. Another unique feature of many patch antennas is that they can be integrated with ICs to effectively make them active antennas. The most common implementation is to have Low-Noise Amplifiers (LNA) embedded within these antennas to drastically improve signal reception. For example, this could be used to address weak satellite signals that are commonly found in GNSS applications. Additionally, this could be very useful for designers who typically have to design in the LNA as a separate external component on the antenna’s feedline–which effectively takes up more PCB space, often resulting in the need to increase the board size and ultimately increasing costs.
Although patch antennas have several advantages, they also have a few disadvantages. For example, when compared to other discrete surface mounted antennas (i.e. chip antennas), patch antennas would generally take up more PCB area/size. Additionally, if the patch antenna has an LNA integrated, you would have to take into account the additional power consumption of that active component.
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