A choke ring antenna is a type of omnidirectional antenna that is used for GNSS applications. Its design consists of a central antenna element, which is surrounded by several concentric conductive rings. These antennas are known for their ability to reject multipath signals (a reflection of satellite signals from nearby objects) from a source. This includes signals reflected from ground and water surfaces, buildings, topography, vegetation, and other sources. Choke ring antennas have specially designed filters, inserted in each groove, which reduces multipath signals in both the L1 and L2 bands within the same antenna configuration.
Since the path that a signal takes from a transmitter to a receiver can be used to measure the distance between the two, this makes it highly suitable for GPS applications in surveying and geological measurement (forestry applications). The reduction of multipath can greatly increase the accuracy of GPS recordings, especially in environments where signal attenuation is difficult. As a result, centimeter-level accuracy might theoretically be obtained when choke ring antennas are connected to mapping-grade GPS receivers.
Construction
Choke ring antennas are generally machined from a single billet of aluminum and consist of several concentric rings (3 to 5) of equal depth around a central ring that holds an antenna receiver. The choke rings are usually a quarter wavelength deep, in order to create a high impedance surface that prevents propagation of surface waves near the antenna and excitation of undesired modes. The net effect is a very smooth controlled pattern with low susceptibility to multipath. The entire choke ring is usually 300-450 mm in diameter and 60 mm in depth and weighs approximately 7 kg. These dimensions result in unhandy field measurement studies and thus limit its application in forestry. Choke rings function to eliminate the electromagnetic energy reflected from the Earths surface. The design of a choke ring is relatively standard, although some variations occur in the vertical arrangement of the rings, from flat to conical, the latter of which is used to assist in increasing signal quality from low elevations.
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Due to the way this antenna is constructed, it is often enclosed in a protective cover when used for outdoor applications.
Limitations
An inherent disadvantage of the typical choke ring design is reduced antenna gain at low elevation angles (below 20°). The reduced antenna gain results in signal strength reduction followed by poor positional accuracy.
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