Key design considerations for selecting the right RF antenna
When faced with the need to make design decisions, system engineers are challenged with multiple options, any of which can have a major impact on the effectiveness of the wireless performance.There are many considerations when selecting RF antennas for wireless electronic devices. Some design choices may already be constrained, based on the overall system design. Basic choices that a system designer may make include:
- What frequencies (often determined by what wireless protocol – e.g., Wi-Fi, BlueTooth, Cellular, GPS, NFC etc.) are needed?
- What modem / transmitter will be used?
- How much power will be available for the system? (e.g., Impacts antenna choices and wireless performance)
- How much data throughput / speed is needed?
- How much efficiency (Antenna gain) is required?
- Where does the antenna need to be and even potentially what orientation(s) will the device be for wireless operation (e.g., Mobile or fixed?)
- Where will the antenna be located on the PCB? (e.g., Cabled to the PCB but inside or connected to the device? External to the device?)
- How much flexibility exists with the overall system design to modify antenna choices (e.g., location, size, type, number of antennas in the system). For example, if an antenna is positioned too close to metal, an LCD, a battery or another device component, it may not work at all. However, if the antenna is a few millimeters away from those components, it may work fine.
Today, an overwhelming majority of antennas are not visible externally – either they are on the PCB or internally cabled to the device. The days of the external ‘walkie talkie’ antennas are distant memories when with most new wireless devices due to the efficiencies of RF electronics and the pervasiveness of wireless networks around us.
There are, of course, other considerations. Efficiency can vary by antenna type (e.g., monopole, dipole, slot, PIFA, panel, etc.) and today’s wireless devices employ many different antenna designs. That’s important because smartphones, wearable electronics, other wireless devices and the growing demand for broadband connectivity are driving the use of more cellular, Wi-Fi™ and Bluetooth® antennas. The Internet of Things (IoT) and machine-to-machine (M2M) technology are also leading to additional wireless deployments that are expected to increase productivity and efficiency while evaluating the measurement applications, quality control, decision-making and customer relationship management.
Applications for wireless devices are incredibly diverse, ranging from industrial applications to smart home devices and surveillance systems. Wi-Fi antennas are showing up on refrigerators, door bells and various other home appliances.
Choosing an antenna
When evaluating antennas for a wireless device, the first step is always to determine if a standard, off-the-shelf antenna is suitable. Off-the-shelf antennas offer a great advantage in terms of aiding in quick time to market. Being able to quickly identify, prototype, and measure one or more antennas for a system can assure that the right choice is made and quickly – reducing overall product risk. Additionally, because their cost is amortized over large production runs, standard antennas are typically the lowest-cost option. However, today’s device designs are more complex and compact, and device manufacturers are forcing more antennas into smaller and complex geometries surrounded by unfriendly materials and objects. There may be times, however, where a custom device may be the best or only option for the application.
The starting line
After the frequency or frequencies and basic wireless architecture parameters have been determined, device manufacturers typically begin the design process by selecting one or more appropriate standard antennas for evaluation, which are available for virtually every allocated frequency. Based on diverse manufacturing and RF technologies, standard antennas offer high performance and ease of integration for wireless applications in the industrial, consumer, medical and automotive markets, among others. Standard antennas can perform well for less cost and risk than a custom solution and offer shorter lead times for bringing wireless products to market.
Once the antenna is selected, engineered and mounted into the prototype device, ideally there is some testing for performance in an anechoic chamber. If the antenna does not work, then either the design is modified, or another antenna is tried.
Calling on custom
However, if a standard antenna does not work effectively, designers can utilize the design expertise of experienced antenna suppliers. The supplier may be able to customize an existing standard solution, providing the system designer with what they need, cost-effectively, and quicker than going through a ground up new design process, for a full custom antenna. Customizations of existing antennas may range from changing cable lengths or connectors on the antenna cables; to making small changes to the antenna pattern to ‘tune’ the antenna so, that it can match the performance need of the application.
The design process typically begins with the device manufacturer sharing a CAD model of the proposed product with the antenna supplier. Software is used to simulate antenna performance based on the properties of the device, the enclosure and any surrounding metal, measuring parameters such as return loss and efficiency. Based on that information, the supplier builds a prototype antenna to test it and then adjusts the design as needed.
For example, one custom project included a cellular antenna plated on the plastic device enclosure. A small number of prototypes were created and tested, and it was determined that they needed to be several millimeters shorter to compensate for the effect of the plastic enclosure. Using calipers and a sharp knife, the radiating arms were trimmed to the appropriate length based on the wavelength of the signal the device was designed to pick up. The tested, shortened antenna design went into full production.
There are many technologies that are used to produce custom antennas – one of the most common involves plating conductive antenna ‘traces’ on plastic and is called Laser Direct Structuring (LDS). LDS is used to make antennas in many 4G cell phones and other compact devices that could not support any other type of standard internal antennas. Generally, higher volumes are needed to justify the effort to use LDS and other custom antenna technologies.
Test, retest, and re-retest
Once selected and designed into the system, it is very smart to test to see how effective the antenna is. Antenna design is iterative in nature. This is true with both design of the actual antenna element and can be true even when designing in an off-the shelf antenna into a system. Ask RF engineers if changing a particular antenna parameter would improve the design and they will typically say “There’s no way to know. We have to test it.” In other areas of applied science, engineers might be willing to provide an educated guess to a similar question, but with RF, antenna sensitivity to surroundings and typically razor-tight performance margins make actual testing the product the only answer – then testing it again and again, if needed.
Designers never begin a device development project hoping it will require a custom design; standard off-the-shelf RF antenna solutions are always preferred due their lower cost and short lead times. However, when a custom antenna is needed, the design and testing resources of an experienced antenna supplier are vital in creating a solution that works effectively for today’s complex, often small wireless devices.
Finally, it is worth noting that with nearly all RF devices, some type of approval is required with the full system for release to commercialization. All the previously mentioned antenna testing is not only important to ensure adequate device performance and happy end users, but it may be necessary to get FCC approval for the device. In the event that the device is used in a cellular network, it will be necessary for the device to pass the requirements of all carrier RF requirements. Generally, the higher the speed of a device on a 4G network, the more demanding and challenging this certification process is – e.g., a Smartphone is required to go through more extensive testing and validation than a LTE enabled home surveillance base-station. Devices with well-designed antennas that have gone through iterative cycles of antenna performance testing, will fare through finished product RF validation and testing such as carrier or FCC testing.
In summary, the more attention that can be given to making smart antenna choices early in the design process, the better the wireless performance will be.
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Matt McWhinney is a business development manager at Molex. Matt has a degree in electrical engineering, enjoys working with innovative customers and is a technology enthusiast of smart and power electronics.