GPS World, February 2016

FIGURE 2 Antenna array and digitizing front end in the anechoic chamber during broadcast tests with a diameter of 20 meters as depicted in FIGURE 2 The array was mounted on a surveyors tripod and placed at a known position on a rotatable pillar at the center of the chamber The chamber contains two sleds Sled A and B which can be precisely positioned along an arc through the zenith at positions between 115 either side of the vertical These antennas include 10 to 60 GHz vertically and horizontally polarized standard gain horn antennas Because the characteristics of the antenna array itself are central to the ultimate performance of beamforming or null steering techniques a thorough characterization of the gain and phase properties of each of the seven antenna elements was conducted To do so a network analyzer was used to observe the gain and phase response of the antenna under test from a range of observation angles The array was operated in transmit mode broadcasting a signal sourced from Port A of the network analyzer which was received by an antenna mounted on one of the movable sleds and fed to Port B of the network analyzer The network analyzer was configured to broadcast a series of 201 equally spaced tones spanning 20 MHz centered at 157542 MHz at a power of 7 dBm from the antenna array A mechanical RF multiplexer was used to implement a time division multiplexing of this broadcast measurement signal across each of the seven elements such that the series of tones were transmitted once per antenna 64 GPS WORLD WWW GPSWORLD COM FEBRUARY 2016 element By performing the scan for each antenna element for a range of positions of Sled A and repeating this for different rotations of the central pillar a precise frequency response could be calculated for a large set of points across the entire upper hemisphere of the antenna The scan was computed on signals received by both the horizontal and vertical elements on Sled A such that both the RHCP and LHCP response could be computed The vertical cuts of this gain pattern were measured with resolution of 2 while the horizontal cuts were measured with a resolution of 5 The average gain response calculated across the 20 MHz band for each of the seven elements is depicted in FIGURE 3 The elevation cut of the peripheral element is taken such that the 90 direction of the cut aligns with a radial line pointing away from the center of the array The azimuth cuts are oriented such that the 0 direction aligns with a radial line extending from the center of element number 1 to the center of element number 2 It is interesting to note that the gain pattern exhibited by each element is sensitive to its position on the ground plane and its position relative to other elements Because of the rotational symmetry of the array the gain patterns of all of the peripheral elements are similar differing only in orientation each one exhibiting a deflection of the maximum gain towards the center of the array The central element is circularly symmetric with a single lobe in the direction of the zenith while gain of the peripheral elements is deflected inwards having lower gain away from the center of the array and an increased gain for high elevation angles from the center of the array The difference in gain pattern across elements is stark and should perhaps influence the choice of elements to be used when forming a beam or null in a given direction One or other of the signals should be scaled to compensate for this gain difference MEASURING SIGNAL REJECTION Before exploring factors that influence signal rejection this section details the figure of merit which might quantify the achievable performance of the array We examined the nulling performance of the system in terms of its rejection capability assessed as the relative