Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002 1 Artificial Magnetic Conductor Technology Reduces Size and Weight for Precision GPS Antennas William E. McKinzie III*, Ralph Hurtado, William Klimczak, Etenna Corporation BIOGRAPHY Dr. McKinzie received his master’s and doctorate degrees in Electrical Engineering from UCLA in 1989 and 1992 with thesis work in computational electromagentics. He has been the principal investigator on numerous research contracts and corporate IRAD efforts for electrically small, tunable, and reconfigurable antennas. He has led the development of new artificial electromagnetic materials, both dielectric and magnetic, for antenna applications. In recent years, his interests have focused on the development and application of artificial magnetic conductors and related periodic materials for application to low profile antennas. Dr. Mckinzie is currently the Director of Research at e-tenna Corporation, and he leads e-tenna’s technical effort in developing ground planes for precision GPS antennas. ABSTRACT Methods to reduce mulitpath for precision GPS antennas have included flat conductive groundplanes, parasitic groundplane treatments, and reactive groundplane treatments. Of these, the choke-ring reactive groundplane has been widely used and adopted as a defacto standard. While choke rings do deliver good electrical performance, they have three significant drawbacks; size, weight, and cost. In this paper, we present a relatively thin, lightweight, and inexpensive alternative ground plane, called an Artificial Magnetic Conductor (AMC). This new AMC technology is realized with printed circuit technology, and it simulates a magnetic conductor over a limited range of frequencies. It is a type of high- impedance surface, which has a surface wave bandgap for TE and TM modes. This ground plane is designed to suppress surface currents, thereby reducing edge diffraction, ground bounce effects, and forcing the far field pattern to roll off more rapidly near the horizon. An AMC ground plane can replace a conventional milled choke ring of the same diameter with at least a 6x weight reduction and at least a 5x reduction in thickness. Measured gain patterns show comparable performance to a choke ring in terms of pattern roll-off near the horizon, front-to-back ratio, and cross-pol rejection below the horizon. INTRODUCTION In precision GPS surveying, it is desirable to make measurements with sub-centimeter accuracy levels. While the receiver’s signal processing software can greatly reduce multipath errors [1], augmenting the antenna system with a ground plane can shield the antenna from unwanted multipath signals arriving from low grazing angles down to nadir, increasing the overall accuracy of the system. One common cause of this form of multipath is reflection off the earth, and such signals are known as “ground bounce.” An obvious approach to reducing ground bounce would be to use a conducting ground plane to shield the antenna. However, conducting ground planes suffer from surface waves and edge diffraction, which cause phase distortion, pattern nulling at zenith, and amplitude variations in the azimuth pattern. Choke rings provide excellent electrical performance for GPS antennas, but they are usually very large. A typical choke ring is about 15” in diameter, 2.5” tall, and weighs more than 10 pounds. While size and weight are not issues for most base station applications, for a GPS surveyor carrying one around in the field, size and weight are important factors. Choke rings are also expensive, typically costing thousands of dollars. An artificial magnetic conductor (AMC) is a periodic structure, which simulates the boundary conditions of a magnetic conductor over a limited frequency band [1, 2, 3]. Its defining characteristics include (1) a +/-90° reflection phase band over which a high surface impedance is realized, and (2) TE and TM mode surface wave bandgaps (which may be independent of the reflection phase band). AMC’s are typically made from printed circuit board materials; hence they are relatively lightweight and inexpensive.
Transcript
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
1
Artificial Magnetic Conductor Technology
Reduces Size and Weight for Precision GPS Antennas
William E. McKinzie III*, Ralph Hurtado, William Klimczak, Etenna Corporation
BIOGRAPHY Dr. McKinzie received his master’s and doctorate degrees in Electrical Engineering from UCLA in 1989 and 1992 with thesis work in computational electromagentics. He has been the principal investigator on numerous research contracts and corporate IRAD efforts for electrically small, tunable, and reconfigurable antennas. He has led the development of new artificial electromagnetic materials, both dielectric and magnetic, for antenna applications. In recent years, his interests have focused on the development and application of artificial magnetic conductors and related periodic materials for application to low profile antennas. Dr. Mckinzie is currently the Director of Research at e-tenna Corporation, and he leads e-tenna’s technical effort in developing ground planes for precision GPS antennas. ABSTRACT Methods to reduce mulitpath for precision GPS antennas have included flat conductive groundplanes, parasitic groundplane treatments, and reactive groundplane treatments. Of these, the choke-ring reactive groundplane has been widely used and adopted as a defacto standard. While choke rings do deliver good electrical performance, they have three significant drawbacks; size, weight, and cost. In this paper, we present a relatively thin, lightweight, and inexpensive alternative ground plane, called an Artificial Magnetic Conductor (AMC). This new AMC technology is realized with printed circuit technology, and it simulates a magnetic conductor over a limited range of frequencies. It is a type of high-impedance surface, which has a surface wave bandgap for TE and TM modes. This ground plane is designed to suppress surface currents, thereby reducing edge diffraction, ground bounce effects, and forcing the far field pattern to roll off more rapidly near the horizon. An AMC ground plane can replace a conventional milled choke ring of the same diameter with at least a 6x weight reduction and at least a 5x reduction in thickness. Measured gain patterns show comparable performance to
a choke ring in terms of pattern roll-off near the horizon, front-to-back ratio, and cross-pol rejection below the horizon.
INTRODUCTION In precision GPS surveying, it is desirable to make measurements with sub-centimeter accuracy levels. While the receiver’s signal processing software can greatly reduce multipath errors [1], augmenting the antenna system with a ground plane can shield the antenna from unwanted multipath signals arriving from low grazing angles down to nadir, increasing the overall accuracy of the system. One common cause of this form of multipath is reflection off the earth, and such signals are known as “ground bounce.”
An obvious approach to reducing ground bounce would be to use a conducting ground plane to shield the antenna. However, conducting ground planes suffer from surface waves and edge diffraction, which cause phase distortion, pattern nulling at zenith, and amplitude variations in the azimuth pattern. Choke rings provide excellent electrical performance for GPS antennas, but they are usually very large. A typical choke ring is about 15” in diameter, 2.5” tall, and weighs more than 10 pounds. While size and weight are not issues for most base station applications, for a GPS surveyor carrying one around in the field, size and weight are important factors. Choke rings are also expensive, typically costing thousands of dollars.
An artificial magnetic conductor (AMC) is a periodic structure, which simulates the boundary conditions of a magnetic conductor over a limited frequency band [1, 2, 3]. Its defining characteristics include (1) a +/-90° reflection phase band over which a high surface impedance is realized, and (2) TE and TM mode surface wave bandgaps (which may be independent of the reflection phase band). AMC’s are typically made from printed circuit board materials; hence they are relatively lightweight and inexpensive.
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Two different AMC ground plane designs will be described in this paper. They differ not only by their construction techniques, but also by the bandwidth of their surface wave bandgap. The G100 series is a narrowband structure with a surface wave bandgap covering the L2 GPS band. It is a relatively thin ground plane (8 mm thick including an integrated radome) that is fabricated as a solid printed circuit board. It contains an integrated radome, and all metal surfaces are laminated for environmental protection. It has been designed for use with a Dorne & Margolin C146 GPS antenna, but it can be used with many others. A circular aluminum cup can be provided with bolt holes which match the bolt hole circle of the ground plane. This cup is also threaded to accept a surveyor's pole, and is used to route the RF cable to the antenna. The G200 series is a broadband structure with a surface wave bandgap (both TM and TE modes) covering both L1 and L2 bands. It too has been designed to accept the Dorne and Margolin C146 antenna. It is a thicker ground plane with a nominal thickness of 16 mm. However, it is significantly lighter, with less than half the weight of the G100 ground plane. Also, the gain pattern of the G200 will roll off at the horizon more steeply than the G100. The prototype G200 AMCs do not yet have an integrated radome. AMC DESIGN and MATERIAL MEASUREMENTS Artificial magnetic conductors (AMCs) have several basic physical features, and a simple example is shown in Figure 1. The high-impedance side of the AMC is a capacitive frequency selective surface (FSS). It is commonly comprised of one or two layers of printed patches, depending on the required capacitance per unit square. The FSS is spaced a distance h from a solid metal backplane, also called an RF backplane. The AMC resonant frequency, defined by a 0o reflection phase, is given by )2(1 LCf o π= , where C is FSS capacitance,
and hL oµ= is the permeance of the spacer layer of
height h. The spacer layer contains a forest of vias, or posts, which electrically connect metal patches of the FSS to the solid metal RF backplane. Reflection phase bandwidth, spanning +90o to –90o, is approximately )(2 ohBW λπ= , where
oλ is the free-
space wavelength at resonance. Since the FSS is generally very thin compared to h, we see that AMC phase bandwidth is directly related to the height of the AMC ground plane. The G101 AMC is fabricated as a multi-layer printed circuit board (PCB). Details are shown in Figures 2 and 3.
This periodic structure has a square unit cell with a period of 400 mils. The FSS has two metal layers, separated by a 10 mil layer of RO4350, to achieve 3.2 pF/sq.
The upper 60 mil layer of RO4350 is employed as a radome to minimize detuning of the AMC resonant frequency by the presence of standing rainwater. Not shown is a 10 mil layer of FR4 below the RF backplane used for environmental protection. An edge view of the G101 is shown in Figure 4 for a 15 in diameter ground plane. This design was fabricated in diameters of 10”, 12”, 14”, and 15”. Full-wave simulations are used to predict the G101 reflection phase, which is shown in Figure 5. Its resonance is centered at the L2 band, and the reflection
R04350
FR4
R04350
}- FSS
Solid metal ground plane
Plated through hole
R04350
FR4
R04350
}- FSS
Solid metal ground plane
Plated through hole
Figure 2. Cross-section of a G101 AMC.
h
Capacitive FSS
Low Permittivity
Spacer
Metal BackplaneMetal Vias
h
Capacitive FSS
Low Permittivity
Spacer
Metal BackplaneMetal Vias
Figure 1. Simplified view of an artificial magnetic conductor (AMC).
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phase bandwidth is approximately 10.6%. Measured reflection phase, using a bistatic measurement, is shown in Figure 6. Measured resonance and fractional bandwidth are near 1230 MHz and 11%. Here we include the case where dis tilled water has been applied to the radome surface to simulate heavy rainfall. A reduction in AMC resonance of only 3.2% was observed.
AMC MATERIAL MEASUREMENTS
The surface wave bandgap of the G101 ground plane is approximately equal to the reflection phase bandwidth. The measured TM mode cutoff is approximately 1130 MHz, which is near the +90o reflection phase point of 1150 MHz. The measured TE mode cutoff frequency is approximately 1290 MHz, which is just above the -90o reflection phase point of 1284 MHz. So the surface wave bandgap of the G101 AMC extends approximately from 1130 MHz to 1290 MHz, which includes the L2 GPS band centered at 1227 MHz.
The G101 AMC was developed prior to the G200 series AMC because it required less development effort and cost. Its primary limitation is bandwidth and weight, relative to the G200 series of AMCs. However, the G101 AMC is extremely rugged, has an integrated radome, and it has passed environmental qualification testing.
The G200 series AMCs involves two prototype designs. They differ in mechanical fabrication details, but have essentially the same electrical performance. We designed the G200 series to have sufficient reflection phase bandwidth and a surface wave bandgap to accommodate both L1 and L2 GPS bands.
The G200a AMC was designed as an electrical proof of concept only. It was not intended to be manufactured in quantity. Shown in Figure 7 is a description of the unit cell. It consists of a two-layer capacitive FSS, a dielectric spacer layer, and an RF backplane. The FSS is a periodic array of overlapping copper patches, made from double-clad 32 mil Rogers RO4350 substrate material, arrayed in a square lattice. It is designed to offer about 0.8 pF/sq of capacitance. The spacer layer is 500 mils of Nomex honeycomb. The RF backplane is a 31 mil FR4 substrate, which is simply drilled and plated. An array of vias with the same period as the FSS provides RF connection between the FSS and the ground plane. These vias are 22 AWG wire, inserted and soldered by hand. The footprint is circular, with a 15.13” diameter, which is equal to the diameter of the U305-1 choke ring used in our
163 milsFR4
10 milsR04350
Period = 400 mils
60 milsR04350
163 milsFR4
10 milsR04350
Period = 400 mils
60 milsR04350
Figure 3. G101 unit cell.
10.6% BW130 MHz BW
Ref
lect
ion
Pha
se (
deg)
Frequency (MHz)
10.6% BW130 MHz BW
Ref
lect
ion
Pha
se (
deg)
Frequency (MHz)
Figure 5. Predicted reflection phase of the G101.
Reflection Phase
-180
-135
-90
-45
0
45
90
135
180
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Frequency (GHz)
Pha
se (D
egre
es)
DryWetL2L1
Figure 6. Measured reflection phase for the G101 AMC: resonance at 1230 MHz dry, 1191 MHz wet.
Figure 4. Edge view of the G101 design. Total thickness is 0.257 inches.
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experiments. The G200a AMC weight is 1.4 lbs for this diameter, and the total height is about 0.59”.
An edge view of the G200a design is shown in Figure 8. This is a 10”x16” rectangular sample employed in surface wave and reflection phase tests.
Predicted and measured reflection phases are shown in Figures 9 and 10. A comparison of frequencies for critical phase points is shown in Table 1. Resonant frequencies agree to within 2%. Table 1. Comparison of G200a Reflection Phase Data
TE mode surface wave coupling is measured with the test setup shown in Figure 11. The broadband test horns excite a horizontally polarized wave. Absorber is used to mitigate the space wave coupling. Figure 12 shows the resulting coupling measurement. The TE cutoff is a subtle feature, but it occurs at a frequency where the beating, or significant ripple, begins to occur in the coupling. This beating is evidence of a standing wave reflecting between widely separated edges of the sample.
Period = 0.500 in.,square lattice
Height of spacer layer = 0.500 in.
Lower patch is0.485 in. sq.
Height of FSSdielectric =0.032 in.
Upper patches are0.220 in sq. with0.030x0.140 in. “ears”
Figure 7. G200a unit cell
Figure 8. An edge view of the G200a design.
Figure 9. Predicted reflection phase for the G200a prototype AMC ground plane.
Figure 10. Measured reflection phase for a 10”x16” panel of the G200a proof of concept AMC design.
Figure 11. TE mode surface wave coupling test setup using broadband 1-18 GHz horns.
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TM mode cutoff frequencies are much more dramatic and easier to identify. The measurement is made with essentially the same test fixture, but the horns are reoriented such that the electric field is vertically polarized. A metal sheet, the size of the rectangular AMC, is used to calibrate for zero dB of coupling. Then the AMC under test is inserted and measured. Results are shown in Figure 13. The frequency at which the coupling drops below 0 dB is the TM mode cutoff frequency: 1170 MHz. So the surface wave bandgap of this G200a AMC extends from 1170 MHz to near 1660 MHz, which includes both L1 (1575 MHz) and L2 (1227 MHz) GPS bands. The G200 series of AMC ground planes is more challenging to fabricate due to the nominal .59” thickness of the structure. We explored the cost of several manufacturing methods, and down selected to the use of pressed pin technology as a practical manufacturing method. Figure 14 shows a G200b prototype being fabricated in this manner. The electrical design parameters of the period, spacer layer height, and FSS capacitance are consistent with the hand-assembled
G200a AMC. However, the FSS for the G200b design has a single metal layer etched on 20 mil Rogers RO4350. Each FSS patch has a plated through hole in its center, through which is inserted a pressed pin when the FSS is placed on the bed-of-nails. The G200b design is wave soldered on both sides (FSS and RF backplane) to create a very mechanically robust component. A side view of a 14” diameter G200b is shown in Figure 15. The total height is about .64” including pin protrusion of .030” on each side. Several diameters of G200b AMCs have been fabricated: 10”, 12”, and 14”. Table 2 below summarizes all of the sizes and weights of each AMC ground plane presented in this paper. Also listed for comparison is the Dorne & Margolin U305-1 choke ring. Reflection phase measurements are plotted in Figure 16. In this data set, all four G200 series test articles were measured in the same test setup, and all were measured on
Figure 13. Measured TM mode coupling for the G200a 10”x16” AMC panel. TM cutoff is near 1170 MHz.
Figure 14. Fabrication of a G200b AMC whereby pins are being pressed into the RF backplane.
Figure 15. Side View of a 14” diameter G200b AMC ground plane.
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the same day. Agreement between the curves is quite good in spite of the differences in AMC sizes. Resonant frequencies of each AMC size agree to within 20 MHz of each other (1.5%), except for the smallest size of 10” diameter. The impact of edge diffraction appears as ripple in the phase plot, which cannot be calibrated out of the measurement. This plot confirms that, as engineered materials, the G200a and G200b AMCs are electrically equivalent designs. A photo of the G200 series AMCs is shown in Fig. 17.
Table 2. Summary of Ground Plane Size & Weight Model GPS
Figure 16. Measured reflection phase of four different diameter prototypes of G200 series AMC ground planes.
G200a
15.13” Diameter
1.4 lbs
G200b
12” Diameter
0.9 lbs
Dorne & Margolin
choke ring
15.13” Diameter
10.2 lbs
G200b
14” Diameter
1.3 lbs
G200b 10” Diameter 0.6 lbs
G200a
15.13” Diameter
1.4 lbs
G200b
12” Diameter
0.9 lbs
Dorne & Margolin
choke ring
15.13” Diameter
10.2 lbs
G200b
14” Diameter
1.3 lbs
G200b 10” Diameter 0.6 lbs
Figure 17. G200series AMCs in comparison to a Dorne & Margolin U-305 choke ring.
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PATTERN MEASUREMENTS We have performed gain measurements in e-tenna’s Satimo chamber [4] on the Dorne & Margolin C146 GPS antenna when installed on all nine ground planes listed in Table 2.
Elevation patterns for the G101 ground planes at the L2 band are shown in Figure 20. The choke ring offers the most directive pattern, with a peak gain near +6 dBic. The G101 ground planes yield about +4.5 to +5 dBic peak gain, and have slightly less pattern slope. Elevation patterns are similar for all sizes of the G101 AMCs, with slightly better below horizon rejection for the larger sizes. Elevation patterns for the G101ground planes at L1 band are shown in Figure 21. Note that this frequency, 1575 MHz, is out-of-band for this AMC, and so the ground plane behaves similar to a metal plate, with edge diffraction evident through zenith nulls for the larger diameters.
Figure 18 shows the antenna installed on a 12” diameter G101 AMC ground plane, and Figure 19 shows it installed on a 12” diameter G200b AMC.
12”12”
Figure 18. The 12” diameter G101 AMC ground
plane is installed under a D&M C146 GPS antenna.
Figure 19. The 12” diameter G200b AMC ground plane is installed under a D&M C146 GPS antenna.
G101 Elevation Cuts at L2
-40-35-30-25-20-15-10-505
10
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Theta (Degrees)
RH
CP
Gai
n, (d
Bic
)
G101 15.13"
G101 14"
G101 12"
G101 10"
Choke Ring
Figure 20. RHCP elevation gain patterns at 1227 MHz for the G101 AMCs.
G101 Elevation Cuts at L1
-40-35-30-25-20-15-10
-505
10
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Theta (Degrees)
RH
CP
Gai
n, (d
Bic
)
G101 15.13"G101 14"G101 12"G101 10"Choke Ring
Figure 21. RHCP elevation gain patterns at 1575 MHz for the G101 AMCs.
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
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Elevation patterns for the G200 series ground planes are shown in Figure 22 for the L2 band, at 1575 MHz. It is observed that the pattern roll off and the front-to-back ratio are very similar for all cases except the smallest size of ground plane. Peak gain is +5 dBic or better for all cases. Elevation patterns for the G200 series ground planes are shown in Figure 23 for the L1 band, at 1227 MHz. Peak gains are between 0 and +5 dBic for the AMC ground planes. The AMC patterns are less directive, as the pattern roll off is not as severe as the choke ring pattern. Note that the 12” and 14” diameter patterns are virtually identical, with just a 1 dB difference in peak gain. Elevation patterns using the G200 series can be made to exhibit a faster roll off at L1 by increasing the AMC resonant frequency. This will also increase the TE mode cutoff frequency. This modification can be easily achieved with a slight decrease in the FSS capacitance. These AMC ground planes have also been shown to mitigate cross-pol, or LHCP, that originates from ground bounced multipath signals.
G200 Elevation Cut at L2
-40-35-30-25-20-15-10-505
10
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Theta (Degrees)
RH
CP
Gai
n (d
Bic
)
G200B 14"G200B 12"G200B 10"Choke Ring
Figure 22. RHCP elevation gain patterns at 1227 MHz for the G200 series AMCs.
G200 Elevation Cut at L1
-40-35-30-25-20-15-10-505
10
-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180
Theta (Degrees)
RH
CP
(dB
)
G200B 14"G200B 12"G200B 10"Choke Ring
Figure 23. RHCP elevation gain patterns at 1575 MHz for the G200 series AMCs.
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
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SYSTEM MEASUREMENTS
Numerous different antenna/ground-plane configurations were tested at e-tenna using static and kinematic post-processing and off-site using kinematic post-processing. The system tests conducted at e-tenna were performed by GPSS&S using two identical Topcon Positioning Systems (TPS) Legacy E. One system was used for each antenna. The set-up is shown in Figure 24. A Leica AT-504 was used as the antenna for reference system in a differential GPS mode. The static positions were determined using a Topcon Positioning Systems ‘Pinnacle’ post-processing engine. The results of the tests for the different antenna/ground-plane configurations for a duration of approximately 24 hours are shown in Tables 3 –5 for L1 alone, L2 alone and L1 + L2 combined. The choke ring measurement was repeated to verify the validity of the data. Among the antenna configurations tested off-site were the Dorne & Margolin antenna with the U305-1 choke ring. The data was processed using the GPSurvey v2.35 post-processing engine in a forced kinematic mode. This engine can generate a position solution for each epoch (every 15 seconds for the cases considered in this paper). Solutions were obtained at L1 and L2 independently and also the error positions were combined to form a radial error vector and plotted in Figures 25, 26, 27, and 28 shown below for the Aluminum ground plane and the G100 AMC for the L2 case. Table 6 summarizes the results of this testing.
Figure 24. G200b 12” AMC on e-tenna’s rooftop (foreground) for system testing with Leica AT-504 (background) used as the reference station.
Table 3 Error (in mm) ellipses
L1 Static Calculations only, Confidence region = 95%
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
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.
Figure 25 Aluminum Ground plane Statistical distribution plot of the 95% confidence level position errors at L2
Figure 26 Aluminum ground plane positional scatter plot of the 95% confidence level position errors at L2.
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
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Figure 27 AMC, G100 Statistical distribution plot of the 95% confidence level position errors at L2
Figure 28 AMC, G100 Positional Scatter plot of the 95% confidence level position errors at L2.
Presented at the Institute on Navigation National Technical Meeting, San Diego, CA, Jan 28-30, 2002
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CONCLUSIONS Artificial magnetic conductor (AMC) technology has been successfully demonstrated to mitigate multipath signals in a precision GPS antenna system. We have introduced a narrowband G101 AMC, which covers the L2 band, and a wideband G200 series AMC whose surface wave bandgap and reflection phase bandwidth covers both L1 and L2 bands. Gain patterns of an industry standard Dorne and Margolin C146 GPS antenna using a G200b AMC are similar to patterns using a choke ring. Initial system accuracy experiments indicate that the G200 AMC and choke ring provide very similar accuracy. AMC ground planes can be approximately 5 times thinner, and up to 10 times lighter, than a conventional milled choke ring. Since the 12” diameter G200b AMC weighs less than one pound, it may easily be carried into the field for accurate survey applications. Furthermore, AMC ground planes can be manufactured for a small fraction of the cost of milled choke rings of the type used with the Dorne and Margolin C146 antenna. Tests will soon be performed to measure the phase center variation of the G200b AMC ground plane and D&M C146 antenna pair. In the future, AMC technology may be integrated with printed GPS elements to enable lower cost, lighter weight, and even lower profile precision GPS antennas.
ACKNOWLEDGEMENTS The authors wish to thank Dr. Tareef Al-Mahdawi of Leica Geosystems -GPS for the loan of a precision GPS antenna for use as a reference antenna in our system accuracy measurements.
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[2] Dan Sievenpiper, Lijun Zhang, Romulo F. Jimenez
Broas, Nicolaos G. Alexopoulos, and Eli Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microwave Theory and Techniques, Vol 47, No. 11, November 1999, pp. 2059-2074.
[3] Eli Yablonovitch and Dan Sievenpiper, “Circuit and
Method for Eliminating Surface Currents on Metals,” US Patent No. 6,262,495 issued on July 17, 2001.
[4] P. O. Iverson, Ph. Garreau, and Dennis Burrell,
“Real-Time Spherical Near-Field Handset Antenna
Measurements,” IEEE Antennas and Propagation Magazine, Vol 43, No. 3, June 200, pp. 90-94.
