A Fresh Look at RF Antenna Design with UWB Connectivity on the Rise

What you’ll learn:

• How UWB technology differs from traditional systems like Bluetooth and Wi-Fi, and why these differences matter for antenna design.
• Tips for creating more efficient antennas, managing signal patterns, and avoiding common pitfalls.
• New and creative antenna design options for UWB, from compact chip designs to 3D-printed structures.

The mainstream commercialization of ultra-wideband (UWB), short-range wireless connectivity continues to accelerate in applications like gaming/extended reality (XR), audio, and IoT, sparking increased interest in the topic of UWB antenna design. As demand rises for wireless connectivity that provides low-latency, high-bandwidth throughput and precise positioning/ranging at extremely low power, here are some of the top-line design considerations to examine.

The chief difference between traditional narrowband RF systems like Bluetooth and 2.4-GHz Wi-Fi compared to UWB is the underlying transmission method (see figure). UWB uses an impulse-style radio, whereas most narrowband radio systems employ a modulated carrier frequency.

Frequency and amplitude modulation requires a reference carrier frequency to be established between the transmitter and the receiver, slowing down the initialization of the link as well as restricting data rates. This approach also requires higher transmit power, sapping device battery performance. UWB, on the other hand, uses impulses to transmit data, enabling UWB systems to quickly initialize a link as well as send data extremely quickly.

While narrowband radios are great for long-range communications, they quickly run into interoperability and congestion problems when used for short-range communications. UWB overcomes these challenges by transmitting very low spectral output power using large portions of the RF spectrum (from 3.1 to 10.6 GHz, varying by country). This approach is ideal for high-performance, short-range connectivity.

Maximizing Antenna Efficiency

The central goal of good antenna design leveraging UWB technology is to build an antenna that can transmit everything given to it. All of the energy that’s provided to the antenna should be transmitted and radiated without loss.

First and foremost, then, the key imperative in antenna design for UWB is to increase the efficiency of that antenna as much as possible and to reduce losses. Efficiency is especially important in UWB antennas due to the much lower transmit power level of UWB devices compared with Bluetooth and Wi-Fi. UWB operates at significantly lower power (-41.3 dBm/MHz) compared to Bluetooth (up to 20 dBm) and Wi-Fi (up to 23 dBm), which are designed for higher power and longer-range communication (limits defined per regulatory bodies).

The ability to reduce losses becomes more challenging as we move from 2.4-GHz narrowband to the higher frequencies where UWB resides. Higher frequency results in a much higher path loss, and care must be taken to avoid adding antenna efficiency loss on top of it. This is a core tenet in UWB antenna design.

Radiation Patterns and Power

Radiation pattern is another important consideration in UWB antenna design, to be optimized as dictated by application requirements.

When implementing a directional UWB antenna, a designer might encounter high gain in specific orientations—good performance can be achieved in some orientations, but performance may be suboptimal in others. For this reason, UWB radio implementations generally benefit from omnidirectional antennas that maximize the output power radiating in all directions equally.

Reducing the maximum peak gain pays dividends when it comes time to certify UWB-based products in the lab. The FCC specification for UWB emissions in commercial products states that the effective isotropic radiated power (EIRP) can’t exceed -41.3 dBm per 1 MHz of bandwidth in the frequency region of 3.1 to 10.6 GHz. This is an exacting specification to accommodate—output power must be reduced to meet the maximum allowable thresholds.

Conventional Bluetooth radios are less constrained during lab certification in that they typically don’t approach the EIRP power limits under test. For UWB radios, every decibel counts when you’re outputting so closely to the allowable limit. Omnidirectional UWB antennas are key to reducing maximum peak gain.

Minimizing Nulls in Radiation Patterns

Radiation pattern considerations are likewise paramount when it comes to controlling radiation nulls. Nulls are significant factors in antenna design and placement, compromising coverage and signal strength. Nulls are encountered at specific angles or directions where the radiated power is at a minimum or zero due to antenna structure and surface current variables. No signal is being transmitted or received in that direction.

In a good UWB antenna design—as with other mainstream RF antenna designs—the nulls in the radiation pattern must be controlled so that they don’t reduce range or signal strength at inopportune orientations in space.

Consider, for example, a UWB antenna designed for a high-performance gaming mouse. The engineer designs the antenna in free space knowing that it will perform much differently once implemented within the mouse enclosure. The performance will change, the nulls will change, the gain will change, and the efficiency will change.

Now consider the placement of the gamer’s hand over the mouse during dynamic gameplay. The human hand is a high absorbing material, especially at UWB frequencies. It completely changes the electromagnetic environment.

Fortunately, these variables can be readily addressed in simulation software. Prototyping becomes a much simpler proposition once simulation is employed to run cost-effective virtual tests accounting for mouse body types, enclosures, materials, and so on.

Polarization is likewise an important factor, again because of the limited power transmitted in UWB antenna implementations. This requires optimal alignment to maximize signal reception and minimize losses.

Polarization mismatch between the transmitter and the receiver can create losses of up to 10 to 15 dB. In some cases, it will kill the link in open space or large conference rooms; for example, where there’s not enough reflection to increase the received energy.

Ecosystem of Innovation

UWB antenna design is a relatively new discipline compared to legacy RF platforms, so it comes with some challenges for the uninitiated. Fortunately, there’s a robust ecosystem of technology vendors that have already done much of the hard work.

For its part, SPARK Microsystems provides customers with a complete solution and works closely with designers to customize and refine their antenna implementations. Many of our customers have all of the antenna design acumen they need in house. In either case, SPARK can assist customers in identifying the best options.

Modularity and flexibility are important to any embedded design, and today’s device designers are afforded multiple architectural options for their UWB antenna designs. A growing variety of UWB “chip antennas” is available on the market today, popular for their modularity and ease of integration. The inherent downside of the chip format concerns the limits it imposes on customization.

PCB-based UWB antenna implementations achieved through patterning on the board itself are perhaps the most common today, and SPARK has seen the success of this approach firsthand. It effectively uses available board real estate and costs relatively little to implement.

Sheet-metal antennas, common today in the Bluetooth domain, are likewise emerging for UWB implementations. They’re inexpensive to produce, readily customizable, and can be fit into positions that are inaccessible to PCB format antennas in some cases. Sheet-metal antennas also accommodate some polarization orientations that might be elusive to PCB implementations, thanks to the 3D topology.

Laser direct structuring (LDS) technology affords yet another promising UWB antenna architecture option. LDS allows for three-dimensional antenna structures to be directly laser printed onto complex surfaces. When implemented on plastic, LDS antennas can enable compact and lightweight designs with low-cost structures.

As UWB connectivity grows in popularity, the multitude of UWB antenna design options is a natural advantage for designers. Some customizations may be required in UWB antenna implementations today. However, the UWB implementations of tomorrow will likely be as quick and easy as commodity Bluetooth antennas—with the added major performance benefits of low latency, high bandwidth throughput, and precise positioning/ranging at extremely low power.