GPS World, February 2009

INNOVATION Antenna Technology 10 5 0 5 10 15 20 25 30 35 40 RHCP LHCP 90 60 30 0 30 60 90 Theta degrees Gain dBi Å FIGURE 3 Co and cross polar gain pattern versus boresight angle of a rover antenna 6 5 4 3 2 1 0 0 5 10 15 20 25 30 35 40 Axial ratio dB Co cross polar ratio dB Å FIGURE 4 Converting axial ratio to co cross polar ratio 40 30 20 10 0 80 60 40 20 0 20 40 60 80 Co cross polar ratio dB 10 8 6 4 2 0 80 60 40 20 0 20 40 60 80 Axial ratio dB Theta degrees Å FIGURE 5 Co cross polar and axial ratios versus boresight angle of a rover style antenna in FIGURE 4 FIGURE 5 shows the ratio of the co and cross polar components and the axial ratio versus boresight or depression angle for a typical GPS antenna The boresight angle is the complement of the elevation angle For high end GNSS antennas such as choke ring and other geodetic quality antennas the typical AR along the boresight should be not greater than about 1 dB AR increases towards lower elevation angles and you should look for an AR of less than 3 to 6 dB at a 10 elevation angle for a high performance antenna Expect to see small 1 dB variations of AR versus azimuth at the low elevation angles Maintaining a good AR over the entire hemisphere and at all frequencies requires a lot of surface area in the antenna and can only be accomplished in high end antennas like base station and rover antennas Multipath Suppression Signals coming from the satellites arrive at the GNSS receivers antenna directly from space but they may also be reflected off the ground buildings or other obstacles and arrive at the antenna multiple times and delayed in time This is termed multipath It degrades positioning accuracy and should be avoided High end receivers are able to suppress multipath to a certain extent but it is good engineering practice to suppress multipath in the antenna as much as possible A multipath signal can come from three basic directions The ground and arrive at the back of the antenna The ground or an object and arrive at the antenna at a low elevation angle An object and arrive at the antenna at a high elevation angle Reflected signals typically contain a large LHCP component The technique to mitigate each of these is different and as an example we will describe suppression of multipath signals due to ground and vertical object reflections Multipath susceptibility of an antenna can be quantified with respect to the antennas gain pattern characteristics by the multipath ratio MPR FIGURE 6 sketches the multipath problem due to ground reflections We can derive this MPR formula for ground reflections MPR E RHCP E RHCP 180 E LHCP 180 The MPR for signals that are ref lected from the ground equals the RHCP antenna gain at a boresight angle divided by the sum of the RHCP and LHCP antenna gains at the supplement of that angle Signals that are reflected from the ground require the antenna to have a good front to back ratio if we want to suppress them because an RHCP antenna has by nature an LHCP response in the anti boresight or backside hemisphere The front to back ratio is nominally the difference in the boresight gain and the gain in the anti boresight direction A good front to back ratio also minimizes ground noise pick up Similarly an MPR formula can be written for signals that reflect against vertical objects FIGURE 7 sketches this And the formula looks like this MPR E RHCP E RHCP E LHCP The MPR for signals that are reflected from vertical objects GPS World February 2009 www gpsworld com 44