GPS World, February 2012

ADVERTORIAL New Topcon antennas support expanding GNSS frequency bandwidth new systems T he need for geodetic GNSS equipment to support the expanding GNSS frequency bandwidth grows with modernization of existing satellite constellations and the expectation of new systems coming online This requires antennas that support existing and new signals while capable of providing the best signal possible by rejecting multipath and indirect signals Addressing these considerations are the goals for Topcon antenna development Designs are to be achieved without decreasing the proven multipath rejection capabilities of a Choke Ring style antenna Topcon now offers two new geodetic antennas for use with existing and next generation GNSS constellations These antennas use Topcons new TA 5 full spectrum GNSS antenna element in two ground plane designs the first being Topcons improved Choke Ring style while the second uses Topcons new innovative pin based convex impedance ground plane TA 5 Full Spectrum GNSS Element Topcons new antennas employ the newly designed TA 5 full spectrum GNSS antenna element The TA 5 antenna element utilizes an array of vertical convex dipoles Fig 1 shows the main components of this antenna element It has an antenna radome 1 a cup with dipoles 2 and power summarizing unit 3 The latter is capacitively coupled with the dipoles Such an array of dipoles results in a very smooth behavior of the reactance versus frequency and in turn broadband functionality This antenna element possesses a bandwidth which is larger than the entire GNSS band from 1160 up to 1615 MHz Topcon CR G5 Choke Ring Antenna The new Topcon CR G5 antenna incorporates the Topcon TA 5 antenna element into Topcons improved choke ring ground plane design The choke ring style ground plane is one generally accepted for minimizing multipath and adverse effects from indirect signals The purpose of the choke groove structures used by the antenna is to decrease the antenna gain for the directions below the horizon This reduces multipath error significantly Choke grooves form a so called impedance structure This impedance surface exists where the relationship between the electric and magnetic fields is of another type compared to regular conductors or isolators This impedance surface passes Fig 1 TA 5 antenna element Fig 2 Cross sectional view of the choke grooves structure through the choke groove openings Fig 2 schematically shows a cross sectional view of the choke grooves structure with the impedance surface shown by a dashed line The surface impedance of the choke groove structure exhibits variations over the GNSS frequency band Topcon PN A5 Pin Design Convex Impedance Ground Plane Another design creating an impedance surface is a straight pins structure shown schematically with Fig 3 Within the design process it has been established that the desired property of the impedance surface formed by the pins structure demonstrates 30 percent less frequency derivative compared to a choke groove structure This allows for more consistent antenna functionality over the expanded GNSS frequency band If the surface is made convex rather than flat then this same scenario holds true Fig 4 Only the grazing directions are now below the horizon This improves sensitivity to low elevation satellites An important consideration requires the radius of the curvature of the surface be properly chosen as not to increase the antenna sensitivity for multipath signals coming from underneath Topcons new PN A5 antenna GPS World February 2012 www gpsworld com 32