Antennas & detection zones

Accurate audience measurement begins with defining the zone in which people should be counted. BlueZoo refers to this area as the detection zone. The antenna pattern determines the shape (e.g. sphere or cone) of the detection zone, while sensor calibration establishes its size (i.e. maximum detection distance).

For signage, the detection zone should correspond to the viewshed, the zone from which people can see the signage and recognize the brand. For other applications, such as occupancy measurement, the detection zone generally corresponds to the physical area where people are to be counted.

Selecting the appropriate antenna and properly calibrating the sensor ensures that the detection zone maps precisely to the intended measurement area.

The shape of a detection zone is determined by the antenna pattern.

• Omnidirectional antennas produce an approximately spherical detection zone, allowing measurement of smartphones all around the sensor.
• Directional antennas produce a conical detection zone, concentrating measurement in one specific direction.
  

A detection zone that is too large will capture people beyond the intended measurement area, resulting in overcounting. A zone that is too small will exclude people who should be counted, resulting in undercounting. For signage applications, the alignment between the detection zone and the signage viewshed is essential.

BlueZoo Customer Success engineers can provide antenna patterns for all antennas sold by BlueZoo.

BlueZoo sensors measure visits and visitors across two detection zones:

• Inner detection zone: The primary measurement area, typically corresponding to the intended viewshed or occupancy area.
• Outer detection zone: The area extending beyond the inner zone that provides additional information about people detected farther from the antenna.

The inner perimeter separates the inner and outer zones and is established through sensor calibration. The outer perimeter separates the outer zone and the zone beyond.

High-gain antennas permit detection zones with a detection distance of more than a 1 kilometer. The gain of the antenna defines the maximum detection distance of a sensor. Inner and outer perimeters lie inside this maximum distance.

Antenna gain is described in terms of dBi. A zero (0) dBi antenna has the gain of a theoretical isotropic radiator. BlueZoo’s simple omnidirectional (external) dipole antennas are rated at approximately 2dBi. BlueZoo’s highest gain directional antennas are rated at 14dBi.

The antenna should be selected based on the shape and orientation of the area being measured.

Directional Antennas

Directional antennas, including panel, patch, Yagi, and dish antennas, produce a conical detection zone. They are well suited to applications where measurement needs to be concentrated in a particular direction, such as one-sided digital signage.

BlueZoo’s standard panel antenna has a beamwidth of approximately 90 degrees, although antennas can be designed with narrower or wider beamwidths.

For a single screen, a directional antenna can be positioned to project the detection zone outward into the signage viewshed. For back-to-back screens, two directional antennas can be used, with one directed toward each side.

Omnidirectional Antennas

Omnidirectional antennas, typically a single or a pair of dipole antennas, produce an approximately spherical detection zone and are useful when people need to be measured in all directions around the sensor.

BlueZoo offers sensors with integrated (on the PCB) omnidirectional antennas as well as sensors with external antenna connectors. External configurations can support either omnidirectional or directional antennas. Multi-segment high-gain dipole antennas can also be used when longer detection distances are required.

For back-to-back signage or other applications requiring coverage around the sensor, an omnidirectional antenna can provide a natural alternative to deploying two directional antennas when coupled with suitable modelling.

The natural antenna pattern does not always determine the final shape of a detection zone. Physical structures and materials in the environment can block or attenuate microwave signals and therefore modify the effective measurement zone.

Masonry and other heavy walls can substantially block microwave signals, effectively cropping a spherical or conical detection zone. Concrete floors, walls, and certain types of glass can have similar effects.

These environmental characteristics can sometimes be used to define a detection zone. For example, an omnidirectional antenna positioned against a masonry wall may produce an effective detection zone resembling a hemisphere rather than a complete sphere. Similarly, an obstruction can limit the extent of a directional antenna’s conical detection zone.

Environmental conditions should therefore be considered when planning sensor placement and selecting an antenna. A well-designed detection zone should reflect the area in which people are intended to be measured; whether that is a signage viewshed, a circulation zone, or an entire occupancy space. Proper antenna selection, sensor placement, and calibration help ensure accurate measurement from the start.

The natural range of a sensor can extend far beyond the area that an application needs to measure. A standard 2 dBi omnidirectional (half-wave “whip”) antenna, for example, can detect signals at distances of approximately 300 feet. Most measurement applications require a much smaller, deliberately defined zone.

Without calibration, a sensor will count phones throughout the area from which the sensor can detect their signals, rather than only within the intended detection zone. Calibration adjusts the sensor’s measurement parameters so that the effective detection zones correspond as closely as possible to the desired detection zone.

BlueZoo supports multiple calibration methods, including:

• Distance-based calibration, which uses the distance to the desired detection boundary and a measured power-distance relationship for the antenna.
• Range-detection-based calibration, which measures smartphone signal strength at the desired detection-zone boundary in the actual deployment environment.
• Ground-truth-based calibration, which compares sensor measurements with known counts of people in the space.

Ground-truth-based calibration provides the highest level of validation and also forms the basis for BlueZoo’s accuracy auditing process. For detailed information about calibration methods and the calibration process, see Calibration & Auditing.

Because calibration primarily changes the size of a detection zone rather than its fundamental shape, the antenna should be selected to match the desired detection-zone shape as closely as possible. Calibration can then establish the appropriate measurement boundaries and compensate for environmental factors.