EHF Rotman Lens Fed Linear Array Multibeam Planar Near

EHF Rotman Lens Fed Linear Array Multibeam Planar Near-Field Range Measurements
Mike Maybell
Planet Earth Communications LLC
1983 San Luis Ave. #31
Mountain View, CA
94043-2900
John Demas
Nearfield Systems Inc.
19730 Magellan Drive
Torrance, CA 90502
ABSTRACT
Realized gain measurements of a 44 beam 44 element
linear array over a 43.5 to 45.5 GHz design frequency
range are presented. The prototype array1 is designed as a
single column of a 50 column multibeam 2200 element
planar active receive array for geostationary satellite
communications payload. The 2200 element planar array
is designed to form 1760 simultaneous narrow 0.4 degree
beams, 1463 of which intercept the earth. The multibeam
single prototype column realized gain was tested at the
Nearfield Systems Inc.'s (NSI) facility using a 12’ x 12’
Planar Near-Field Range. Two different linear array
configurations were tested. Each configuration utilized
the same WR-19 waveguide fed 44 beam, 44 element
Rotman lens and integrated RF distribution network
(RFD). An active receive array utilizing only the center 8
array elements of the Rotman lens feed was tested first.
This was followed by a 44 array element passive array
test demonstrating the narrow 0.4 degree half power
beamwidth. Summary and specific examples of the NFR
test results will be presented. These will be compared
with that predicted using the previously measured lens
array factor gain (AFG) and embedded element realized
gain. The AFG was measured using a HP8510C
automatic network analyzer.
Keywords: Antenna measurements; Commercial
products Measurement errors; Measurement systems;
Near-field; Near-field scanners; Phased arrays; Planar
near field; Range evaluation; Sampling; Scanners
1.0 Introduction
The 44 beam 44 element integrated Rotman lens/RFD to
be measured is illustrated as the Level 1 active 1x44
column array in Figure 1. The full EHF uplink array is
designed for TSAT spiral applications. Beamformers for
SHF/EHF satellite downlink and uplink payloads create
1
FA9453-05-C-0033 Air Force Research Laboratory
simultaneous high gain pencil beams feeding 2200
element rectangular planar arrays from geostationary
orbit. Beamformers use column and row 2D Rotman lens
stacks feeding elements in an equilateral triangular lattice
to minimize component count. Radiating active array
elements form an equilateral triangular beam lattice using
the 2D stacks covering the entire 17.4º earth disc with
1760 “pixel” beams. At each lens stack beam port, a 0.4°
HPBW “pixel” beam is formed with frequency
independent beam pointing angle due to Rotman lens true
time delay. Pixel beams are combined, steered and shaped
within the RF Beam switch/combiner resulting in 64
simultaneous independent communication beams.
Computed performance exceeds 80 dBWi EIRP and 18
dB/K minimum G/Ts with constant communication beam
pointing angles over the full bands. This performance
exceeds that currently planned by a considerable amount
and can be easily scaled, resulting in reduced size weight
and prime power.
Demonstration of an active EHF receive array column is
completed. The WR-19 waveguide assembly depicted in
Figure 2 and Figure 3 achieved all predicted performance
parameters. As a result, the beamformer technology
readiness level is near TRL 4. The Figure 1, Phase III
2200 element Active Uplink Array design goals are listed
in Table 1.
Two different linear array configurations were tested.
Each configuration utilized the same WR-19 waveguide
fed 44 beam, 44 element Rotman lens and integrated RF
distribution network (RFD). An active receive array
utilizing only the center 8 array elements of the Rotman
lens feed, as illustrated in Figure 2, was tested first. This
was followed by a 44 array element passive array (Figure
3) test demonstrating the narrow 0.4 degree half power
beamwidth.
RF SW M
atrix/Com
biner
(1463x6
4)
Level 2
Row lens (44)
60”
Level 1
Column
Lens
(50)
Level 1
Column
Active
1x44
Array
HPFL/
Pyramidal Extension
Horn
LNA
0.086” Semi-Rigid Coax
WR-19/2.4mm
End Launch
Transition
Rotman lens/RFD
36”
Active
Receive
Planar
Array
(44x50)
WR-19 Shim
35”
Figure 1 – EHF Active Uplink Array; 64 Beams 2200
Elements
Figure 2 – EHF Active Uplink Array 8 Element RF
Chain and Lens/RFD
Table 1 - EHF Uplink 2200 Element Active Planar
Array Design Goals
Receive Active Array Design Goals
Parameter
Receive Array Size
Aperture Length
Aperture Width
Aperture Payload Depth
Column Spacing
Number Array Elements
Array Beam Performance
Operating Frequency (min.)
Operating Frequency (max.)
Peak Gain
Half Power Beamwidth
Number Pixel Earth Beams
Number Simultaneous Comm. Beams
FOV Radius (Geo)
Element Aperture Efficiency
Element FOV Relative Gain (min.)
Array G/T Performance
LNA
RF Loss before LNA
LNA Noise Figure
Peak G/T at 0.0 deg. Scan
Peak G/T at max. Scan
EOC beam box G/T at max. scan
Array Power And Weight
DC Power
Dissipation
Weight
Value
Units
35.5
34.9
60
2.6
2200
inch
inch
inch
λ
43.5
45.5
52.2
0.4
1463
64
8.5
85
-1.5
GHz
GHz
dBi
Degree
Degree
%
dBi
0.5 dB
2 dB
21 dB/K
19.55 dB/K
18 dB/K
850 Watt
850 Watt
630 lbs
2.0 Rotman lens/Array/RFD Column Computed
Realized Gain
The 8 element active and 44 element passive uplink
lens/RFD/array/realized gain (GR) was predicted using
Figure 3 - 44 Element Passive Array & lens/RFD at
NSI NFR with Mounting Fixture & Near Field Probe
the previously measured lens array factor gain (AFG) and
computed embedded element realized gain (GE). The
AFG was measured using a HP8510C automatic network
analyzer. According to IEEE Std 145-1983, define AFG
as “array factor” as follows:
“array factor. The radiation pattern of an array antenna
when each array element is considered to radiate
isotropically.
15
10
5
R ea lized G ain (d B iL )
-25
n 2πd
G
R
(θ ) = G E (θ )∑ S nB e
G
R
(θ ) = G E (θ ) AFG (θ )
λ
5 0.00
6 0.00
7 0.00
70 .0 0
4 0.00
60 .0 0
3 0.00
50 .0 0
2 0.00
0.00
1 0.00
-1 0.00
-2 0.00
5
sin(θ )
(2)
n =1
(3)
0
-5
43.5 GHz H-Plane Realized Gain (dBiL)
-10
45.5 GHz H-Plane Realized Gain (dBiL)
-15
-20
-25
40 .0 0
30 .0 0
20 .0 0
10 .0 0
0 .0 0
-10 .0 0
-20 .0 0
-70 .0 0
-30
-30 .0 0
(1)
Since the element spacing is d/λ = 2.6, and the maximum
coupling between any two of the widely spaced array
elements is -36 dB, the gain of all elements is identical
and the embedded element gain is essentially equal to the
isolated element gain:
j
-3 0.00
10
sin(θ )
Where SnB is the measured lens/RFD transmission
scattering parameter from beam port B to array port n.
A
-4 0.00
-7 0.00
15
-40 .0 0
e
Angle From Boresight (deg)
CST H-Plane Radiation Pattern
20
-50 .0 0
nB
λ
-5 0.00
-30
-60 .0 0
n =1
n 2πd
45.5 GHz E-Plane Realized Gain (dBiL)
-20
R ea lize d G ain (d B iL )
G R (θ ) = ∑ G E (θ )S
j
43.5 GHz E-Plane Realized Gain (dBiL)
-10
-15
Assuming that the linear array is disposed along the Yaxis with its normal pointed to θ=0 in the principal Y-Z
plane (φ = π/2):
A
0
-5
-6 0.00
NOTE: When the radiation patterns of individual array
elements are identical, and the array elements are
congruent under translation, then the product of the array
factor and the element radiation pattern gives the
radiation pattern of the entire array.”
Pyramidal Horn Element CST MWS Computed
Realized Gain E-Plane Radiation Pattern
20
Angle From Boresight (deg)
Figure 4 - Pyramidal Horn Element Computed vs.
Measured Realized Gain Radiation Patterns
The realized gain of the pyramidal horn using CST MWS
TDS compared with that measured using Nearfield
Systems Inc.'s (NSI) Near Field Range is plotted in
Figure 5. The maximum difference between the computed
realized gain and that measured is less than 0.2 dB.
18.7
(4)
Embedded Element Gain term GE(θ) in (3) is that
computed using CST Microwave Studio Time Domain
Solver (CST MWS TDS).
3.0 Pyramidal Horn Element Computed and NFR
Measured Realized Gain
Since the primary goal is development of the Rotman
lens/RFD beamformer, and not the development of an
array element, it was decided to use linearly polarized
pyramidal horn elements for the radiation pattern related
testing of the beamformer. For a Phase III spacecraft
antenna, a circularly polarized element with high aperture
illumination efficiency would be designed. The element
design selected would probably be a multimode conical
or pyramidal horn with integrated polarizer. The
pyramidal horn selected for this Phase II contract effort
has well understood radiation patterns, is relatively
efficient, and is extremely low risk.
Plots of CST MWS TDS computed realized gain
radiation patterns GE(θ) compared with those measured
for horn number 8 are presented in Figure 4.
18.65
18.6
18.55
18.5
18.45
Gain (dBiL)
GR(θ) dBi = GE(θ) dBi + AFG(θ) dB
18.4
18.35
18.3
18.25
18.2
CST MWS Computed Realized Gain (dBiL)
18.15
NFR Measured horn 7 (dBiL)
18.1
NFR Measured horn 8 (dBiL)
18.05
18
43.5
43.7
43.9
44.1
44.3
44.5
44.7
44.9
45.1
45.3
45.5
Frequency (GHz)
Figure 5 - Pyramidal Horn Element CST MWS TDS
Computed vs. Measured Realized Gain
The CST MWS computed E-Plane realized gain GE(θ)
dBi in (4) and the measured lens array factor AFG(θ) dB
is used to compute the realized gain of the 8 element
active array and 44 element passive array integrated with
the Deliverable lens/RFD. The data required is at the 44
beam peak angles from -8º to +8º with respect to
pyramidal horn aperture normal. The difference statistics
between the CST MWS computed realized gain and the
NFR measured realized gain (mean measured gain of
horn S/N007 and S/N008) at each beam peak angle and at
all 5 frequencies is documented in Table 2. The mean
gain difference between model and measured data over
the 220 points is 0.015 dB. The maximum P-P difference
between measured and computed gain is 0.480 dB at the
worst-case frequency. The standard deviation averaged
over 5 frequencies measured 0.072 dB.
Table 2 - Pyramidal Horn Element NFR Measured CST Computed Realized Gain Statistics
Pyramidal Horn Element NFR Measured - CST Computed Realized Gain
44 Beam Port F (GHz)
Angles
43.50
Mean
MAX
MIN
P-P
1 sigma
-0.150
-0.097
-0.188
0.091
0.028
F (GHz) F (GHz) F (GHz) F (GHz)
44.00
44.50
45.00
45.50
-0.027
0.172
-0.116
0.287
0.080
-0.130
0.074
-0.405
0.480
0.128
0.170
0.299
0.101
0.198
0.060
0.059
0.151
-0.082
0.233
0.062
5 Frq
-0.015
0.299
-0.405
0.480
0.072
measured data over the 65 points is 0.048 dB. The
maximum P-P difference between measured and
computed gain is 1.269 dB at the worst-case frequency.
The standard deviation averaged over 5 frequencies
measured 0.271 dB.
Table 3 - Eight Element Active Array/lens/RFD NFR
Measured - CST Computed Realized Gain Statistics
8 Elt Active Array ([NFR Measured Realized Gain]
-[AF + CST MWS Computed Horn Gain])
13 Beam Port F (GHz) F (GHz F (GHz) F (GHz) F (GHz)
Angles
43.50 44.00 44.50
45.00
45.50
5 Frq
Mean
MAX
MIN
P-P
1 sigma
0.048
1.640
-0.860
1.269
0.271
-0.548
-0.113
-0.860
0.747
0.259
-0.429
0.128
-0.853
0.980
0.234
-0.141
0.379
-0.492
0.871
0.241
0.952
1.640
0.371
1.269
0.343
0.404
1.097
0.072
1.025
0.279
4. Eight Element Lens/RFD Active Array NFR Test
Results
The 8 element active array integrated with the
Deliverable lens/RFD was tested for realized gain using
a Planar 12’ x 12’ NFR. Gain for all 44 beam ports was
measured.
To provide a means of predicting the measured NFR
gain of the eight element active array integrated with the
lens/RFD, the assembly shown in Figure 2, but without
the 8 pyramidal horns, was measured using an HP8510C
ANA. Array Factor was computed for Beam Ports B02,
B06, B10, B14, B18, B22, B23, B27, B31, B32, B35,
B39, and B43. An 8 element active computed Array
Factor rosette using the HP8510C measured S-parameter
data at 44.5 GHz is presented in Figure 6. Note that the
HPBW for the 8 element active beams is about 2º,
whereas the HPBW for the 44 element passive beams is
0.4º (Figure 10) due to the 5.5 times larger passive array
aperture.
Figure 6 - Array Factor Calculated Lens/RFD 8
Element Active Rosette at 44.5 GHz 13 Beam Ports
Figure 7 is an overly plot of 8 Element Active Array
lens/RFD Calculated and NFR Measured Realized Gain
for the same 13 beam ports as those computed for
AFG(θ) dB in Figure 6. The method for computing the
realized gain (dashed lines) in Figure 7 is to add CST
MWS TDS computed realized gain GE(θ) dBi in Figure
4 to AFG(θ) dB from Figure 6 according to equation (4).
The data required is at the 13 beam peak angles from -8º
to +8º with respect to pyramidal horn aperture normal.
The difference statistics between the CST MWS/Array
Factor computed realized gain and the NFR measured
realized gain for the 8 element active assembly at each
beam peak angle and at all 5 frequencies is documented
in Table 3. The mean gain difference between model and
Figure 7 - Eight Element Active Array Deliverable lens/RFD
Calculated and Measured Realized Gain Overlay Bench(dashed
lines), NFR(solid lines) 44.5 GHz 13 Beam Ports
Figure 8 shows measured NFR realized gain of the 8
element active array of Figure 2 for all 44 beam ports at
44.5 GHz.
Figure 8 - Eight Element Active Array lens/RFD Measured
Realized Gain NFR 44.5 GHz for all 44 Beam Ports
The standard deviation averaged over 5 frequencies
measured 0.174 dB.
Figure 9 - 44 Element Passive Array/lens/RFD at NSI
NFR with Mounting Fixture
5. Forty-Four Element Lens/RFD Passive Array NFR
Test Results
The 44 element passive array integrated with the
Deliverable lens/RFD was tested for realized gain using a
Planar 12’x18’ NFR. Gain for all 44 beam ports was
measured. The passive array mounting in the NFR test
setup is illustrated in Figure 9.
Figure 11 is an overly plot of 44 Element Passive Array
lens/RFD calculated and NFR Measured Realized Gain
for all 44 beam ports. The method for computing the
realized gain (dashed lines) in Figure 11 is to add CST
MWS TDS computed realized gain GE(θ) dBi in Figure 4
to AFG(θ) dB from Figure 10 according to equation (4).
The difference statistics between the CST MWS / Array
Factor computed realized gain and the NFR measured
realized gain at each beam peak angle and at all 5
frequencies is documented for the 44 element passive
assembly in Table 4. The mean gain difference between
model and measured data over the 220 points is 0.329 dB.
The maximum P-P difference between measured and
computed gain is 0.879 dB at the worst case frequency.
Figure 10 - Array Factor Calculated Lens/RFD 44
Element Passive Rosette at 44.5 GHz 44 Beam Ports
Realized Gain (dBiL)
To provide a means of predicting the measured NFR gain
of the 44 element passive array integrated with the
lens/RFD, the lens/RFD assembly shown in Figure 2 but
without the 44 pyramidal horns was measured using an
HP8510C ANA. Array Factor was computed for all 44
Beam Ports. A 44 element passive computed Array Factor
rosette using the HP8510C measured S-parameter data at
44.5 GHz is presented in Figure 10. Note that the HPBW
for the 44 element passive beams is 0.4º as expected.
Figure 11 - 44 Element Passive Array lens/RFD
Calculated(dashed) and Measured(solid) Realized
Gain
Table 4 - 44 Element Passive Array/lens/RFD NFR
Measured - CST Computed Realized Gain Statistics
44 Elt Passive Array ([NFR Measured Realized Gain]
-[AF + CST MWS Computed Horn Gain])
44 Beam Port F (GHz) F (GHz) F (GHz) F (GHz) F (GHz)
Angles
43.50
44.00
44.50
45.00
45.50 5 Frq
Mean
MAX
MIN
P-P
1 sigma
0.214
0.598
-0.144
0.742
0.164
0.103
0.441
-0.278
0.719
0.170
0.299
0.572
-0.274
0.847
0.173
0.454
1.022
0.142
0.879
0.177
0.573
0.902
0.229
0.673
0.188
0.329
1.022
-0.278
0.879
0.174
5. NFR Measurement Accuracy
The NFR testing was performed at Nearfield Systems
Inc., Torrance, Calif., on 5/8/07 - 5/11/07, using their
Planar 12’ x 12’ NFR. The RF test block diagram is
shown in Figure 12.
Gain Standard Uncertainty. For both direct and
comparison gain measurements, a gain standard is
required, and the uncertainty in the gain of the standard
is the largest contributor to the uncertainty in the gain of
the AUT. The calibration was performed at the 2.4mm
coaxial terminal using the 3 Antenna Method. For these
measurements, the gain standard estimated uncertainty is
0.2 dB.
Impedance Mismatch Factor. One or more cable-toantenna connections must be changed to accomplish the
gain measurement, and since the AUT, gain standard,
and cables are not perfectly matched, a mismatch
correction should be applied. The mismatch was not
calculated for the current measurements, and from
similar measurements this causes an uncertainty of 0.05
dB.
RF Source
Coupler
Multiplier
Probe
AUT
Pad
Test
Mixer
Test IF
x4
11.25 GHz
45.0 GHz
-10 dB
LO
LO
Ref
Mixer
Ref IF
Truncation of the near-field data for the standard gain
horn. The near-field data for the gain standard was
truncated in steps with an analysis script to show that the
truncation uncertainty was approximately 0.02 dB.
Bias error leakage within the receiver. An analysis
script was used to estimate the bias leakage signal level
for the gain horn data. It was found that the error signal
was 105 dB below the peak of the near field peak and
would cause less than a 0.02 dB uncertainty in the farfield peak.
Room scattering. Room scattering measurements were
not performed, but the absence of distortion on the peak
of the horn main beam indicates that the uncertainty due
to this source is less than 0.1 dB. Table 5 summarizes
the estimated uncertainties for a comparison gain
measurement
Table 5 - Probable uncertainties in peak far-field gain
Term
dB
Gain Standard 0.20
Mismatch
0.05
SGH FF Peak 0.15
Total (RSS)
0.25
7. Summary
The primary emphasis of this paper was to compare the
accuracy of predicting the realized gain using
fundamental array theory with NFR measurements. An 8
element active array and a 44 element passive array were
both tested. The mean gain difference between model and
measured data is 0.048 dB for the active array and 0.329
dB for the passive array. Overall NFR peak gain
measurement accuracy is estimated as 0.25 dB.
8. REFERENCES
LO to Ref
LO Source
LO/IF Unit
LO to Test
15.00667 GHz
Ref
Mixers operate in
3rd harmonic mode
Multiple Reflections between the horn and probe. This
is usually evaluated with multiple-Z measurements, but
these were not obtained. The reflections cause a
distortion on the peak of the beam, which was not seen
at any of the measurement frequencies. It is estimated
that this uncertainty is about 0.1 dB or less.
Sig
Panther Receiver
[1] Maybell, M.J., Chan, K.K. Simon, P.S., “Rotman
Lens Recent Developments 1994-2005”, IEEE AP S
Proceedings, July 2005, Washington, D.C.
Receiver displays Sig/Ref
9. ACKNOWLEDGMENTS
Figure 12 NFR RF Test Block Diagram
Peak Far-Field Peak Amplitude for Gain Standard. For
the comparison gain method, a near-field measurement
must be performed on the gain standard and the data
processed to determine its Far-Field Peak (FFP). There
is some uncertainty in this FFP, and the primary error
sources in planar measurements are:
The authors wish to thank Mr. Joseph Chavez, program
manager for contract FA9453-05-C-0033, Air Force
Research Laboratory, Space Vehicles Directorate,
Kirtland Air Force Base, N.M. In addition, thanks to Dr.
Alan Cherrette of NGST for invaluable technical
guidance and NGST test facilities’ support during the
contract.