On this page
The antenna is often the most crucial part in a wireless design. In satellite IoT, connected devices send and receive very low-power signals from satellites at a distance from a few hundreds to a few thousands kilometers.
Antenna requirements for nominal performances
Direct satellite line-of-sight
RF signals are strongly attenuated across concrete or metallic walls, and the budget link for Kinéis communication requires a direct line of sight whenever possible.
Ominidirectional pattern to the horizon
Since our satellites are in low Earth orbit, they appear on all sides of the device, which makes a directive antenna useless.
Besides, satellites most often appear to connected devices at low elevations:
- 60% of the time under 20°
- 90% of the time under 45°
A direct geometrical consequence is that satellites appear further at low elevations:
- distance at zenith: between 500km and 850km depending on the satellites
- distance at horizon: up to 3000km
Considering the above, the best solution for a Kinéis device is a dipole-like omnidirectional radiation pattern.
Right-hand circular polarization (RHCP) would be the best option to match the satellite on-board antenna, with a theoretical 3dB gain, but circularly-polarized antennas are often directive and the wavelength for our frequency ~400MHz would involve a bulky antenna rarely compatible with IoT constraints. Instead, a vertical polarization is recommended.
A typical gain between -3dBi and 0dBi is required for nominal performances. A smaller gain will impact the quality of reception of the signals by the satellites.
What to consider when selecting an antenna?
For an antenna, the efficiency is the ratio between the total radiated power and the injected power.
The highest antenna efficiency will offer the best performances, i.e. the best quality of reception of the signals vs the power injected before the antenna (directly impacting the power consumption). In general, bigger antennas offer better efficiencies than smaller antennas, but multiple parameters can impact the overall performances.
The recommended omnidirectional pattern cannot be achieved for all possible device orientations. As a consequence, the antenna selection must consider the device orientation (often guided by the final installation).
An antenna is very sensitive to its close environment. All material with high conductivity (metal, water etc…) should be avoided around the antenna.
Internal antenna (embedded inside the device) must be tuned considering impedance matching (usually 50 ohms for off-the-shelf antennas), which should be performed in conditions as close as possible to real deployment conditions: in the final device casing, on an representative support etc…
IoT devices often call for small antennas, which is why quarter-wave antennas are common. The efficiency of such an antenna depends on the ground plane dimensions: shrinking the recommended ground plane dimensions decreases the radiation efficiency of the antenna.