The antenna ground plane is a key component in cellular and global navigation satellite system (GNSS) devices. It plays a major role in the performance and reliability of both the device and all of the applications that run on it. That means it also directly affects the revenue potential for the device and applications because consumers and enterprises will churn if their expectations aren’t met.
In mobile devices such as smartphones, wearables, and Internet of Things (IoT) sensors, the ground plane is the virtual, electrical equivalent of the earthen ground system that fixed antennas have, such as for TV and radio stations. To understand how ground planes work, it helps to start with the concept of gain, which measures an antenna’s directionality as it relates to the strength of the signal that it’s transmitting or receiving.
For example, a GNSS antenna with high gain improves the quality of the signals available to the receiver and thus reduces Time to First Fix (TTFF). (For a deeper dive, see “Understanding Antenna Gain and How It Affects Device Performance, Reliability, and Competitiveness.”)
External vs. Embedded Affects Ground Plane Considerations
Device design is one major factor that affects antenna gain. It’s here that ground planes come into play. For example, if the device uses an embedded antenna, the printed circuit board (PCB) serves as the ground plane. The size of the PCB ground plane, as well as the antenna’s location within that space, both affect gain.
Key Considerations:
- Ground Plane Length
- Keep out areas
- Proximity to metal
Ground Plane Length
- #1 most common issue for antenna performance
- Target: 1/4 free space wavelength
It’s also important to take a holistic view of the device to understand how the module (transmitter and/or receiver) and antenna work together. Module manufacturers specify the peak gain limits for antennas used with their products. With embedded antennas, the ground plane, the components surrounding the antenna, and the device housing all affect gain.
In the case of external antennas, installation location is a major factor. For example, some vehicular applications mount the antenna on a vehicle’s roof or trunk. Those metallic surfaces serve as the ground plane. As with PCB ground planes, the size of that surface and the antenna’s location within it both affect gain and in turn signal quality.
This highlights another important design consideration: If the device uses an external antenna, will it have access to a ground plane? The answer helps narrow down the types of external antennas that are viable for that device.
Take the example of a GNSS antenna on a drone, where a ground plane might not be available because the shell is plastic rather than metallic. A good choice is a quad helix antenna that has an even gain across the hemisphere, such as the Taoglas EAHP.125 (embedded) or the Taoglas Accura Series, TS.125.0111W antenna (external).
If the use case ensures that a ground plane will always be available, a patch or crossed dipole are good choices for an external GNSS antenna, or a terminal mount monopole for a cellular antenna.
Carrier Certification
In the case of cellular, the ground plane plays a major role in the device’s ability to pass carrier certification. Each mobile operator has a set of minimum antenna efficiency requirements — which are measured as total radiated power (TRP) and total isotropic sensitivity (TIS) — for the bands where it holds licenses.
If an embedded antenna’s ground plane isn’t sized properly to meet an operator’s efficiency requirements, then the device won’t be certified for use on its network. Some external antennas don’t require a ground plane. If they use a cable longer than 20 cm, they don’t need to go through carrier certification. (For more information, see “Why Cellular Pre-Certification is Critical and How to Successfully Navigate the Process.”)
A related issue is the type of cellular technology. For example, in the LTE family of technologies, each operator has a specific set of efficiency requirements for Cat 1, another for Cat 1 bis, another for Cat M1, and so on. Each set also can vary by device size. For instance, if a Cat M1 device has a form factor under 107 mm in the longest direction, AT&T has a specific set of TIS and TRP requirements.
Navigating all of these requirements and other variables can be complex, which is why device OEMs frequently turn to Taoglas for guidance. To learn more about how to properly integrate your antenna, speak to Taoglas’ Engineering team by clicking on the button below.
