Global navigation satellite system (GNSS) antennas can be grouped into two types: directional and omnidirectional. They’re important concepts because they play a major role in the reliability and performance of the positioning and timing applications that use GNSS.
One way to understand these concepts is in terms of gain, which measures an antenna’s directionality as it relates to the strength of the signal that it’s receiving. The antenna’s job is to give the receiver as much signal as possible to work with. The more effective it is at pulling in signals weakened by distance or by physical obstructions such as skyscrapers and foliage, the easier it is for the receiver to do its job. High gain improves the signal-to-noise ratio and reduces Time to First Fix (TTFF).
GNSS satellites transmit signals using right-hand circular polarization (RHCP). The device’s antenna should have the same polarization to avoid mismatch. GNSS signals are already relatively weak by the time they reach the Earth, and polarization mismatch can result in at least half of the remaining signal being lost. An antenna’s axial ratio is a measurement of its ability to reject LHCP signals.
Why Installation Location is a Major Factor
Patches, quad helices, and cross dipoles are three common types of directional antennas. (Technically they’re semi-directional because they receive signals from a broad direction of angles in a specific direction rather than a single, narrow direction.) Directional antennas should be used when their installed orientation is well known or reasonably well known. An example is a GNSS antenna installed on an EV charger to identify its location and to time stamp each credit card transaction. The manufacturer will specify exactly how the charger should be installed, which in turn determines where the antenna will be mounted on the charger.
Omnidirectional antennas — also known as “linear” — should be used when their installed orientation will vary significantly, such as in smartphones, tablets, and other handheld devices. The Taoglas TFX125.A flexible transparent, FXP611 flexible PCB loop, and GGBLA.01 ceramic loop are examples of how omnidirectional antennas come in a variety of designs.
The antenna’s installation location is a top consideration when choosing between directional and omnidirectional designs. Ideally the GNSS antenna should be installed in a location where it’s facing up to have a clear view of the sky. For example, in vehicular applications, an obvious location would be on the roof.
Vehicular applications highlight another important consideration: access to a ground plane. The metallic roof serves as the ground plane and thus helps narrow down the choice of GNSS antenna to patches and crossed dipoles. In other applications, no ground plane will be available, which means a quad helix antenna is the ideal choice.
An example of a crossed-dipole antenna is the Taoglas EAHP.50, an embedded antenna for demanding applications such as autonomous driving, precision agriculture, tracking high-value assets, and timing accuracy synchronization.
If there is no ground plane available and the orientation is mostly up but subject to some variability (like a drone/UAV), a quad helix antenna is a good choice. One example is the Taoglas Colosseum QHA.50.A.301111, which has an even gain across the hemisphere and achieves an excellent axial ratio, making it resilient to multipath rejection.
But sometimes the antenna needs to be inside the vehicle to keep it hidden and/or protected from damage or tampering. In those cases, the ideal location is inside the windshield at the top. The Taoglas TFX125.A multi-band GNSS polymer antenna includes an adhesive backing to facilitate these kinds of installations.
Taoglas TFX125.A multi-band GNSS polymer antenna
If it’s difficult or impossible to ensure that the antenna will always face up at the sky, an omnidirectional GNSS antenna is the way to go because it’s designed to pull signals from any direction.
Sometimes an omnidirectional antenna is worth considering even when the installation will ensure that it’s always facing up at the sky. That’s because an omnidirectional antenna can receive signals from every direction, which increases the likelihood that the receiver will have enough to work with.
One exception is when the device is likely to spend a significant amount of time in multipath environments, such as an autonomous vehicle driving through a downtown. Some of the GNSS signals will bounce off the tall buildings, leading to delays in their arrival at the antenna and thus creating errors in positioning calculations. These problems can be mitigated by choosing a GNSS antenna that is designed to reject multipath signals using high directionality or other techniques. (For more information, see “Multipath Analysis Using Code-Minus-Carrier Technique in GNSS Antennas.”
All of this is a lot to consider, which is why companies frequently turn to Taoglas early on in the design process. This helps avoid the need for expensive device redesigns to facilitate antenna integration or the need for a custom GNSS antenna — both of which can delay the product’s time to market and time to revenue.
