Ground Planes for Vehicle Antennas, GPS Antennas

Ground Planes

Ground planes are a key consideration in the design, mounting, and performance of GPS antennas and other types of vehicle antennas. Though they are not necessary for every GPS antenna, and other vehicle antennas, they can often be a critical enhancement.

In this short article, we explore the use of ground planes in a variety of leading GPS antenna types and the physical parameters that make ground planes effective.

What is a ground plane?

A ground plane is an antenna component that provides a flat, horizontal, electrically conductive surface for an antenna to be mounted on. Ground planes are usually circular and can be integrated into the design of an antenna and its mounting or procured separately. The surface is typically made of metal and fully electrically grounded.

The ground plane performs the function of an artificial ground for the antenna (as if it had been mounted on the Earth) and is needed where there are no suitable ground conditions or the antenna will be mounted at height.

Ground planes, benefit antenna function by directing radio  frequency signals towards the antenna, amplifying them, and enhancing signal reception. Circular round planes provide a reflective surface for radio waves, with the reflected radio waves creating an image of the upper half of the antenna on the plane surface. In this way, the ground plane operates as the other half of the antenna, in a similar fashion to a dipole antenna.

For the ground plane to be effective its radius should, at a minimum, exceed a quarter wavelength of the antenna frequency (as measured from the base of the antenna). The conducting plane is not always circular and in some designs, the ground plane is replicated by using quarter wavelength radials or spokes that are positioned perpendicular to the base of the antenna. The radials provide the same effect as a fully circular ground plane.

The types of functional ground planes vary widely. Many ground plates are metal plates or discs of varying thickness. Magnetic antenna mounts can provide a connection to a metal surface that can function as a ground plane through capitative coupling. Ground radial kits with different numbers of spokes can also be used.

Why do GPS antennas need ground planes?

GPS and other GNSS signals are known to be relatively weak, due to the significant attenuation experienced as they travel from Medium Earth Orbit satellites through the ionosphere and atmosphere of the Earth. They are also affected by optical phenomena (reflection, refraction, etc.) that can generate significant multipath, delaying signal acquisition and a reliable fix.

The benefits of using a GPS antenna ground plane include:

• Improvement of the convergence time for GPS
• Protection from the effects of multipath and other forms of interference
• Broadening of the antenna pattern
• Enhancement of positional accuracy
• Increases in the antenna gain that can be achieved

Ground planes for leading GPS antenna types

GPS antennas may be ground plane dependent or ground plane independent, meaning that they can perform optimally without the requirement of a ground plane.

For ground plane dependent antennas, the function of the antenna will be impaired without the inclusion of a ground plane, usually a metal disc. The size of the GPS antenna ground plane and the centering of the antenna on it can also affect the center frequency of the antenna, shifting it away from the target GNSS frequencies. Key GPS antenna types that require a ground plane include:

• Patch GPS antennas: are one of the most common types of GPS antennas as they can be integrated into a range of devices and antenna designs. They are often embedded in patch antennas that sit on the metal roof of a car, which provides the necessary metal surface. They are typically made from ceramic and metal, mounted on a metal base plate. A ground plane for a patch antenna provides a significant boost to antenna gain and overall performance, especially if the ground plane is large.

• Chip GNSS antennas: are small low-cost omni-directional antennas that experience a significant improvement in performance as the size of their available ground plane increases. The impact that ground planes have on the efficiency of chip antennas means that a sufficiently large ground plane must be factored into the design of the device in which the antenna will be embedded. Without an adequate ground plane, a chip GPS antenna will not function properly.

• Fractal Element Antennas: also rely on ground plane size to deliver adequate performance.

Notable ground plane independent GPS antennas types include Loop antennas and Dipole antennas. The absence of a need for a ground plane makes them very convenient. Helix antennas generally do not need ground planes. The performance of a helix antenna can be improved in the instances where a ground plane is included, but in some cases, the antenna may become more susceptible to interference.