Bench Talk

(Source: Symmetry Electronics)

From small antennas embedded in mobile devices to massive antenna arrays found in cellular or satellite base stations, antennas are major components used in wireless designs. Available in a variety of shapes and sizes, antennas are either embedded inside a device (internal) or anchored onto the exterior (external). Often used in compact Internet of Things (IoT) applications, internal antennas must adhere to stringent size and performance standards despite a series of inherent design challenges. Conversely, external antennas are much more stable and easier to integrate because of their form factor and larger size.

When working with wireless designs, it is recommended that the proper antenna is vetted early in the design stage to avoid performance issues and costly redesigns. To accomplish this, design engineers must determine product requirements such as the PCB design, size, and enclosure. Additionally, performance parameters should also be considered, including:

• Bandwidth
• Voltage Standing Wave Ratio (VSWR)
• Radiation pattern
• Gain

In the following, we’ll provide design engineers with a comprehensive guide to external antennas, outlining their pros and cons, key performance parameters, and the available different types.

Anchored to the outside of a device’s enclosure via an RF connector, external antennas are simple to integrate and typically require few design resources. Unlike internal antennas, external antennas are often plug-and-play solutions that are ground-plane independent, meaning they do not heavily rely on the size of the PCB area to which they are connected. This allows for shorter development time and a quicker time to market. Due to their larger size, external antennas offer superior range and sensitivity compared to their internal counterparts. Additionally, being anchored to the exterior of the device allows them to deliver excellent line-of-sight. External antennas often withhold a higher rated gain (dBi), enabling exceptional directional behavior for applications where signal transmissions must be concentrated in a specific direction.

Furthermore, larger antennas are optimal in supporting lower frequencies with longer wavelengths. Because of this, many high-gain external antennas maintain a bandwidth that spans in the lower sub-GHz range while still maintaining acceptable performance. Despite the many benefits that external antennas offer, the costs must be considered, as additional manufacturing processes and materials are required to produce larger antennas. Additionally, for design engineers working with small IoT applications, external antennas are not typically suitable because of their size and form factor.

Measuring Antenna Bandwidth and VSWR

Aside from determining the proper PCB design, size, and enclosure for a project, a series of performance parameters must also be considered to ensure proper functionality. Bandwidth should be assessed to determine the optimal range of frequencies for an antenna to transmit and receive signals efficiently. For example, consider a specific Bluetooth^® antenna expected to perform at its best in the 100MHz bandwidth of frequencies at 2.4GHz-2.5GHz. By the inherent nature of antennas, this Bluetooth antenna would also be susceptible to frequencies outside its intended bandwidth, but it is over this specific range of frequencies (2.4GHz-2.5GHz) that the antenna is expected to perform optimally at a certain efficiency (percentage) level. However, this could vary depending on how precise the antenna element is designed. Often the defining factor of an antenna’s bandwidth, the Voltage Standing Wave Ratio (VSWR), should be measured to determine the amount of power that will reflect 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 100MHz-400MHz might 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 percent of the power to the radio.

Figure 1: Antenna bandwidth defined by VSWR. (Source: Mobile Mark)

Less of a numerical parameter and more of a graphical 3D representation, the radiation pattern should be evaluated to determine the energy distribution surrounding an antenna. Although a 3D radiation diagram offers a full representation of an antenna’s energy distribution, a 2D diagram is still beneficial in providing a simple way to identify where most of the antenna’s energy will be 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). Meanwhile, a directional antenna has less of a symmetrical radiation pattern, with most of its radiated energy concentrated in a single direction.

Figure 2: Example of the vertical and horizontal radiation pattern of an omnidirectional antenna. (Source: ResearchGate)

Figure 3: Example of the vertical and horizontal radiation pattern of a directional antenna. (Source: EVDO)

Evaluating an Antenna’s Performance with Gain

The first parameter considered when evaluating an antenna’s performance is the gain. The gain is always described in the context of the antenna’s radiation pattern. Gain is defined as the signal strength in the direction of its peak radiation 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 an actual antenna, as its 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 increases the power in the desired direction at the expense of the power radiated in other directions.

Figure 4: Gain (dBi) formula. (Source: Abracon)

External antennas come in various shapes and sizes. One common example is the rubber duck antenna that mounts outside of an internet router. Belonging to either the omnidirectional or directional subgroups, the different types of external antennas available hold diverse characteristics to best support specific use cases.

Omnidirectional Antennas

Antennas such as terminal-mount, whip, rubber-duck, and outdoor-dipole akin to what you would find on wireless access points (WAPs) are, for the most part, omnidirectional antennas. The typical construction would consist of the antenna element enclosed within a rubber or plastic sheath with an exposed RF connector. These omnidirectional antennas are always ground-plane independent, making simple coupling to the transmitter the only requirement for integration. Because of the non-directional nature of these antennas, they are meant to be vertically oriented to the ground, as they tend to radiate extensively in their horizontal (x-y) plane. Any wireless applications that require point-to-multipoint communication would benefit most from this type of radiation pattern. For example, any office environment where a router is needed to transmit and receive signals from many client devices such as computers, phones, or any end-node modules.

Other Omnidirectional Antennas

Puck-style, magnetic-mount, and screw-mount 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. They are generally a more low-profile design, making them ideal for customers looking for a different aesthetic to a larger profile such as a terminal-mount antenna. By design, many puck-style antennas can support integrated Low Noise Amplifiers (LNA) to improve signal reception dramatically—particularly for weak incoming Global Navigation Satellite System (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 such as this is essentially three different antenna elements housed in a single enclosure, with each varying protocol having its own cable and connector.  

Directional Antennas

Panel, Dish, and Yagi antennas focus on applications that depend upon long-range point-to-point or point-to-multipoint communication. Because of these antennas’ very focused radiation patterns, you can always expect to see a high-rated gain (dBi) in their datasheets (typically above 9dBi). Provided the high-peak gain that these antennas provide in a single direction, they are ideal for any demanding long-range application where an end-node device or a collection of devices are concentrated in a specific area. For example, a pair of office buildings sharing the same wireless network would have an outdoor-rated Yagi or panel antenna on each side with both antennas oriented toward one another to form a point-to-point communication link.

The continuing evolution of IoT applications has increased demand for a wide variety of antenna options. Whether an engineer is leaning toward an internal solution to meet low-cost, high-volume, and size requirements; or toward 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 considering the antenna 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.

A Comprehensive Guide to External Antennas blog was written by Augustine Nguyen and was first published on www.semiconductorstore.com.