OEM7 Installation and Operation User Manual v1 183APPENDIX D Importance of Antenna SelectionAn antenna behaves both as a spatial and frequency filter, therefore, selecting the right GNSSantenna is critical for optimizing performance. An antenna must match the receiver’s cap-abilities and specifications, as well as meet size, weight, environmental and mechanical spe-cifications for the intended application.Factors to consider when choosing a GNSS antenna include:1. Constellation and signalsEach GNSS constellation and SBAS has its own signal frequencies and bandwidths. Anantenna must cover the signal frequencies transmitted by the constellation and bandwidthsupported by the GNSS receiver.2. Antenna gainGain is a key performance indicator of a GNSS antenna. Gain can be defined as the relativemeasure of an antenna's ability to direct or concentrate radio frequency energy in a par-ticular direction or pattern. A minimum gain is required to achieve a minimum carrier-to-power-noise ratio (C/No) to track GNSS satellites. The antenna gain is directly related to theoverall C/No of the navigation GNSS receivers. Hence, antenna gain helps define the track-ing ability of the system.3. Element GainThe element gain defines how efficient the antenna element is at receiving the signals. Inany signal chain, you are only as good as the weakest link, so an antenna element with lowelement gain might be compensated by an increased low noise amplifier gain. However, thesignal-to-noise ratio or C/No is still degraded.4. Antenna beamwidth and gain roll-offGain roll-off is a factor of beamwidth, and specifies how much the gain changes over theelevation angle of the antenna. From the antenna’s point of view, the satellites rise from thehorizon towards zenith and fall back to the horizon. The variation in gain between zenith (dir-ectly overhead) and the horizon is known as the gain roll-off. Different antenna technologieshave different gain roll-off characteristics.5. Phase center stabilityThe phase center of the antenna is the point where the signals transmitted from satellitesare collected. When a receiver reports a location fix, that location is essentially the phasecenter of the antenna.The electrical phase center of any antenna will vary with the position of the transmitting sig-nal it is receiving by as much as a few millimetres. As GNSS satellites move across the sky,the electrical phase center of the signal received will typically move with the satellite pos-ition unless the antenna has been carefully designed to minimize Phase Center Offset (PSO)and Phase Center Variation (PCV).The PCO with respect to the Antenna Reference Point (ARP) is the difference between themechanical center of antenna rotation and electrical phase center location. The PCO is alsofrequency dependent which means that there can be a different offset for each signal fre-quency. The PCV identifies how much the phase center moves with respect to the satelliteelevation angles.
