23-04-2012, 03:39 PM
antenna
[attachment=20635]
INTRODUCTION
The consistency of the phase center offsets of gps antennas is important when trying to achieve geodetic measurements which are accurate at the millimeter level .The absolute stability of the phase center is modeled and results published on the geo++ and ngs websites.Inpractice,there is some variation between the model and the actual response of an individual antenna.The choke ring antenna have been known to provide most consistent performance from the point of phase center repeatability of phase center variation between randomly sampled production units of a given type of choke ring antenna design.These variations are very repeatable and predictable,usually with in 1mm.The extensive use of this type of antenna in the igs global network makes it a detector standard in the community.An inherent disadvantage of the typical choke ring design is reduced antenna gain at low elevation angles.The reduced antenna gain is translated to lower number and signal strength of tracted satellites;therefore lower quality of the signal,poorer VDOP and position accurarcy.This paper outlines a design effort to mitigate the inherent disadvantage at insufficient gain near horizon while preserving its arttributes,such as stable phase center ,pattern symmetry for amplitude,phase and group delay.
Configuration Using Choke Ring:
The antenna is fixed on the choke ring. Both are fixed on a stand. The antenna is a passive antenna. The use of an amplifier is required, because the signal coming from the antenna is too weak. The single frequency amplifier was used with Phoenix receiver while the dual frequency amplifier was used with the two dual frequency receivers Novatel & Sep-tentrio. Some problems were encountered during the use o Novatel receiver with Dell Inspiron Laptop. The acquisition program crashed many times. This problem seems to be linked with the use of the PCMCIA card offering a serial port to the laptop, which seems not to be robust. This problem was solved by using another laptop with integrated serial ports.
WORKING OF CHOKE RING ANTENNA
A choke ring ground plane consists of several concentric thin walls, or rings, around the center where the antenna element is located. The area between the rings creates "grooves”.Theprinciple of the operation of choke ring ground planes is as follows.The signal that is received by the antenna composed of two components of "direct" signal and "reflected" signal. The grooves have no effect on the direct signal other than decreasing the antenna gain at low elevation angles. For high elevation angles the ground plane works almost like a flat ground plane.
Radiation Pattern Of Omnidirectional Antenna:
The radiation pattern of a simple omnidirectional antenna, a vertical half-wave dipole antenna.In this graph the antenna is at the center of the "donut". Radial distance from the center represents the power radiated in that direction. The power radiated is maximum in horizontal directions, dropping to zero directly above and below the antenna.
. CONCLUSION
A novel conical choke ring ground plane was proposed for GPS antennas used in precise geodetic applications The proposed choke ring configuration allows better reception of low elevation angle GPS satellites and improved multipath rejection. The proposed ground plane does not degrade other performance characteristics including effect of antenna.
[attachment=20635]
INTRODUCTION
The consistency of the phase center offsets of gps antennas is important when trying to achieve geodetic measurements which are accurate at the millimeter level .The absolute stability of the phase center is modeled and results published on the geo++ and ngs websites.Inpractice,there is some variation between the model and the actual response of an individual antenna.The choke ring antenna have been known to provide most consistent performance from the point of phase center repeatability of phase center variation between randomly sampled production units of a given type of choke ring antenna design.These variations are very repeatable and predictable,usually with in 1mm.The extensive use of this type of antenna in the igs global network makes it a detector standard in the community.An inherent disadvantage of the typical choke ring design is reduced antenna gain at low elevation angles.The reduced antenna gain is translated to lower number and signal strength of tracted satellites;therefore lower quality of the signal,poorer VDOP and position accurarcy.This paper outlines a design effort to mitigate the inherent disadvantage at insufficient gain near horizon while preserving its arttributes,such as stable phase center ,pattern symmetry for amplitude,phase and group delay.
Configuration Using Choke Ring:
The antenna is fixed on the choke ring. Both are fixed on a stand. The antenna is a passive antenna. The use of an amplifier is required, because the signal coming from the antenna is too weak. The single frequency amplifier was used with Phoenix receiver while the dual frequency amplifier was used with the two dual frequency receivers Novatel & Sep-tentrio. Some problems were encountered during the use o Novatel receiver with Dell Inspiron Laptop. The acquisition program crashed many times. This problem seems to be linked with the use of the PCMCIA card offering a serial port to the laptop, which seems not to be robust. This problem was solved by using another laptop with integrated serial ports.
WORKING OF CHOKE RING ANTENNA
A choke ring ground plane consists of several concentric thin walls, or rings, around the center where the antenna element is located. The area between the rings creates "grooves”.Theprinciple of the operation of choke ring ground planes is as follows.The signal that is received by the antenna composed of two components of "direct" signal and "reflected" signal. The grooves have no effect on the direct signal other than decreasing the antenna gain at low elevation angles. For high elevation angles the ground plane works almost like a flat ground plane.
Radiation Pattern Of Omnidirectional Antenna:
The radiation pattern of a simple omnidirectional antenna, a vertical half-wave dipole antenna.In this graph the antenna is at the center of the "donut". Radial distance from the center represents the power radiated in that direction. The power radiated is maximum in horizontal directions, dropping to zero directly above and below the antenna.
. CONCLUSION
A novel conical choke ring ground plane was proposed for GPS antennas used in precise geodetic applications The proposed choke ring configuration allows better reception of low elevation angle GPS satellites and improved multipath rejection. The proposed ground plane does not degrade other performance characteristics including effect of antenna.