Over-the-Air (OTA) performance is crucial for the end product. If a small form factor device integrates antennas with noticeable performance requirements, for example, Global Navigation Satellite System (GNSS) or Long-Term Evolution (LTE), the antenna related design should be prioritized.
The key requirement in antenna design is that all antennas fulfill their performance targets in the actual use cases of the end product. The efficiency target for the LTE, DECT NR+, and GNSS antennas should be a minimum of 50%. For the best possible GNSS performance, the recommended efficiency is a minimum of 75%. For the best possible LTE-NTN performance, the recommended efficiency is a minimum of 75%.
For more information on antenna design and integration, see nRF91 Series Antenna and RF Interface Guidelines.
Troubleshooting
This section describes typical performance related issues in the end product's antenna design and ways to reduce them.
- Harmonic performance worse than expected
- The performance given in Harmonics performance represents a 50 Ω load case. Real-life antenna designs often deviate from 50 Ω, which can negatively impact harmonic performance. In several customer studies where poor radiated harmonic performance was observed, the root cause was traced to a non-linear active component that was located close to the LTE antenna and increased the harmonic level. In this scenario, the nRF9151 TX signal couples into a nearby component where it becomes distorted, generates harmonics, and increases the overall harmonic levels.
- The placement of the antenna on the PCB and ground plane prevents from achieving good enough antenna radiation performance.
- Follow the antenna vendors recommendations for placing the antenna on the PCB and ground plane.
- RF path routing loss between antenna and nRF9151 is too high.
- Verify 50 Ω impedance-controlled routing between the antenna and nRF9151.
- Mechanics, casing, battery, or wires deteriorate antenna performance.
- Co-design the mechanics to match the PCB and antenna design.
- Sharing a small PCB between several antennas, for example, GNSS and LTE, causes mutual loading between the antennas.
- Verify that the antennas do not cause mutual loading, for example, with passive antenna measurements in an anechoic chamber. Terminating an unused antenna, for example the LTE antenna during GNSS, can have a noticeable impact on the mutual loading effect between the antennas. To minimize the load effect, the LTE antenna is terminated internally to 50 Ω. Alternatively, the LTE antenna can be terminated through the AUX port (pin 37) to 50 Ω load during GNSS. For more information, see nRF91x1 Cellular AT Commands.
- Active components on the PCB cause wideband noise coupling to the antennas.
- Verify that during reception, especially GNSS, active components on the PCB do not radiate noise to the GNSS antenna. For example, buck regulators and LEDs are typical sources of radiating wideband noise. The RSSI scan testing method can be used. For more information, see nRF9151 Hardware Verification Guidelines.
- The orientation of the end product in a typical use case is less than optimal for the antenna radiation pattern.
- Consider in the design phase what the orientation of the end product is going to be in a typical use case.
- Nearby objects deteriorate antenna performance.
- Instruct the end user to use the device in the manner it is designed for. For example, inform the user not to expect the best possible radio performance when the device is used inside a building, on a metal surface, or on their body.
- GNSS signal environment is more challenging than assumed in the design phase.
- Instruct the end user to use the device in the manner it is designed for. The GNSS signal type must be Line-of-Sight (LOS), which is rarely available in places like city centers with tall buildings nearby. The issue is even more severe in the polar regions because the GNSS system is designed to perform the best in the equatorial regions.