Use Miniature, High-Performance GNSS Antennas to Minimize Design Compromises

A significant aspect of engineering centers on the art of making component tradeoffs with respect to your determination of system “must haves” and “nice to haves.” These tradeoffs include speed versus power dissipation, footprint versus functionality, and capabilities versus cost.

Making these decisions involves “what if” modeling and simulations along with an experience-earned sense of what will work and to what extent. The resulting component choices largely define the final product’s capabilities, attributes, and limitations.

Fortunately, not every component choice requires a difficult tradeoff or compromise. Consider the Global Navigation Satellite System (GNSS), a catch-all term for GPS, GLONASS, Galileo, BeiDou, and other satellite constellations that provide global positioning, navigation, and timing (PNT) services across the 1.1 to 1.6 gigahertz (GHz) RF spectrum (Figure 1). By using signals received from satellites supported by advanced signal processing and sophisticated algorithms, GNSS can be used for applications ranging from low-precision asset tracking to high-precision 3D location within centimeters.

Figure 1 : There are many GNSS systems in use, each with one or more assignments in the 1.1 to 1.6 GHz segment of the RF spectrum. (Image source: Taoglas)

GNSS antenna designs vary to address the wide range of applications. Some are designed for one or a few GNSS bands across the allotted spectrum, while others cover many or all of them. In addition to selecting their desired center frequency (or frequencies) and bandwidths, designers need to choose the physical and electrical characteristics. Physically, antennas can be external or embedded; electrically they can be active or passive:

• External GNSS antennas are best suited for applications that require the highest precision and clear-sky visibility, ensuring accurate and reliable satellite signal reception in telematics, surveying, and autonomous vehicle systems.
• Embedded antennas are a good choice for applications requiring a high degree of packaging integration. They offer a path to a seamless, highly integrated GNSS end product, such as an asset tracker.
• Active GNSS antennas incorporate an internal low-noise amplifier (LNA) that is powered by an external source to boost the signal. Boosting the signal increases the signal-to-noise ratio (SNR), a key parameter for achieving improved received-signal recovery and reducing subsequent data errors and bit error rate (BER).
• Passive GNSS antennas have no internal amplifier. They are a simpler solution that captures incident RF energy and passes it directly to the RF front-end (RFE), offering a smaller, less costly, unpowered antenna option.

The inclusion of an LNA, even for an embedded antenna, may be necessary to mitigate signal loss caused by long cable runs or challenging environmental factors, such as reflections from buildings or absorption by foliage.

While an active embedded GNSS antenna generally provides improved performance, it can also increase complexity, power consumption, physical size, and BOM cost. A passive antenna is simpler, but can have somewhat reduced RF performance and is more sensitive to placement.

Fortunately, thanks to the breadth and depth of GNSS antenna solutions from Taoglas, designers can find an optimal antenna combination for their application based on the priority parameters. A look at two representative models, one embedded passive and one active external, provides comparative perspective.

Passive and active antennas

The HP2356.A (Figure 2, top) from the Inception Series is a good example of a passive, multi-band GNSS embedded patch antenna designed for optimal positional accuracy and placement. This antenna uses an innovative ceramic patch-within-a-patch antenna design with optimized gain for GPS L1/L2, Galileo, GLONASS, and BeiDou bands. Among the many performance parameters provided for this antenna are efficiency (Figure 2, bottom left) and gain (Figure 2, bottom right), with the graphs showing the frequencies of peak response and those of deliberately reduced response.

Figure 2 : Shown is the HP2356.A multi-band GNSS passive embedded patch antenna (top); among the critical performance parameters are antenna efficiency (bottom left) and gain (bottom right). (Image source: Taoglas)

Both graphs clearly show the passive antenna’s ability to perform in the bands of interest, and the reduced response outside of those bands.

The antenna measures 35 × 35 × 6 mm, and its low-profile design allows designers to integrate a multi-band L1/L2 GNSS patch into devices where it would not have previously been possible due to height constraints. The RF traces from the antenna to the front-end circuitry must maintain a 50 ohm (Ω) impedance. Taoglas recommends centering the antenna on a circuit board ground plane measuring at least 70 × 70 millimeters (mm) to ensure optimal performance.

For an active external device, designers can use the XAHP.50.A.301111 Colosseum X multi-band GNSS antenna (Figure 3, top), which performs well across the full GNSS spectrum. This antenna supports high location accuracy and stable position tracking in urban environments.

The efficiency of the XAHP-50.A.301111 is shown (Figure 3, bottom left), and due to the inclusion of the LNA, the gain (Figure 3, bottom right) is dramatically improved compared to a passive antenna. Key RF specifications across the GNSS band include a gain of 22 and 28 decibels (dB), an out-of-band (OOB) attenuation range of 25 to 50 dB, and a low noise figure (NF) between 2.6 and 4.5 dB.

Figure 3 : The XAHP.50.A.301111 multiband GNSS antenna (top) includes an LNA for boosting received signal strength and thus enhancing SNR and BER; the efficiency (bottom left) and the gain (bottom right) is dramatically improved compared to a passive antenna. (Image source: Taoglas)

This antenna has excellent performance across its full bandwidth, and its design provides even gain across the 3D reception hemisphere. This provides a broad axial ratio, which in turn enhances its multipath rejection. The built-in LNA operates from 1.8 to 5 volts DC (VDC) and draws less than 20 milliamperes (mA); connection to the antenna is via a standard three-meter (m) RG-174 coaxial cable fitted with an SMA(M) straight connector.

The robust, vandal-resistant, permanent-mount ASA enclosure is IP67-rated, just 57 mm high, with a diameter of approximately 94 mm, and is designed for ease of installation. It mounts to the panel via a hollow M20 × 1.5 threaded stud that allows the coaxial cable to feed through. While it can be mounted on any surface, performance can be affected when mounted on metal.

Conclusion

Designers of systems that receive one, several, or all available GNSS constellations do not have to compromise on finding a suitable antenna for their specific needs. Taoglas offers a full range of passive and active embedded and external antennas for all segments of the GNSS band. By choosing one that closely matches the requirements, designers can remove a source of project difficulty and uncertainty and have greater flexibility in defining the remainder of the project.