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.

Level 1Column

Lens(50)

Level 2Row lens (44)

ActiveReceivePlanarArray

(44x50)

RF SW Matrix/Combiner(1463x64)

35”

60”

36”

Level 1ColumnActive1x44Array

Level 1Column

Lens(50)

Level 2Row lens (44)

ActiveReceivePlanarArray

(44x50)

RF SW Matrix/Combiner(1463x64)

35”

60”

36”

Level 1ColumnActive1x44Array

Figure 1 – EHF Active Uplink Array; 64 Beams 2200 Elements

Table 1 - EHF Uplink 2200 Element Active Planar Array Design Goals

Receive Active Array Design Goals

Parameter Value UnitsReceive Array Size

Aperture Length 35.5 inchAperture Width 34.9 inchAperture Payload Depth 60 inchColumn Spacing 2.6 λNumber Array Elements 2200

Array Beam PerformanceOperating Frequency (min.) 43.5 GHzOperating Frequency (max.) 45.5 GHzPeak Gain 52.2 dBiHalf Power Beamwidth 0.4 DegreeNumber Pixel Earth Beams 1463Number Simultaneous Comm. Beams 64FOV Radius (Geo) 8.5 DegreeElement Aperture Efficiency 85 %Element FOV Relative Gain (min.) -1.5 dBi

Array G/T PerformanceLNARF Loss before LNA 0.5 dBLNA Noise Figure 2 dB

Peak G/T at 0.0 deg. Scan 21 dB/KPeak G/T at max. Scan 19.55 dB/KEOC beam box G/T at max. scan 18 dB/K

Array Power And WeightDC Power 850 WattDissipation 850 WattWeight 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

WR-19/2.4mmEnd LaunchTransition

Rotman lens/RFD

0.086” Semi-Rigid CoaxLNA

HPFL/ExtensionPyramidal

Horn

WR-19 Shim

WR-19/2.4mmEnd LaunchTransition

Rotman lens/RFD

0.086” Semi-Rigid CoaxLNA

HPFL/ExtensionPyramidal

Horn

WR-19 Shim

Figure 2 – EHF Active Uplink Array 8 Element RF Chain and Lens/RFD

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.

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.”

Assuming that the linear array is disposed along the Y-axis with its normal pointed to θ=0 in the principal Y-Z plane (φ = π/2):

∑=

=A

n

dnj

nBR eSGG E1

)sin(2)()( θλπ

θθ (1)

Where SnB is the measured lens/RFD transmission scattering parameter from beam port B to array port n.

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:

∑=

=A

n

dnj

nBER eSGG1

)sin(2

)()( θλπ

θθ (2)

)()()( θθθ AFGGG ER = (3) GR(θ) dBi = GE(θ) dBi + AFG(θ) dB (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.

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-70.

00

-60.

00

-50.

00

-40.

00

-30.

00

-20.

00

-10.

00

0.00

10.0

0

20.0

0

30.0

0

40.0

0

50.0

0

60.0

0

70.0

0

Angle From Boresight (deg)

Rea

lized

Gai

n (d

BiL

)

43.5 GHz H-Plane Realized Gain (dBiL)

45.5 GHz H-Plane Realized Gain (dBiL)

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-70.

00

-60.

00

-50.

00

-40.

00

-30.

00

-20.

00

-10.

00

0.00

10.0

0

20.0

0

30.0

0

40.0

0

50.0

0

60.0

0

70.0

0

Angle From Boresight (deg)

Rea

lized

Gai

n (d

BiL

)

43.5 GHz E-Plane Realized Gain (dBiL)

45.5 GHz E-Plane Realized Gain (dBiL)

Pyramidal Horn Element CST MWS Computed Realized Gain E-Plane Radiation Pattern

CST H-Plane Radiation Pattern

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-70.

00

-60.

00

-50.

00

-40.

00

-30.

00

-20.

00

-10.

00

0.00

10.0

0

20.0

0

30.0

0

40.0

0

50.0

0

60.0

0

70.0

0

Angle From Boresight (deg)

Rea

lized

Gai

n (d

BiL

)

43.5 GHz H-Plane Realized Gain (dBiL)

45.5 GHz H-Plane Realized Gain (dBiL)

-30

-25

-20

-15

-10

-5

0

5

10

15

20

-70.

00

-60.

00

-50.

00

-40.

00

-30.

00

-20.

00

-10.

00

0.00

10.0

0

20.0

0

30.0

0

40.0

0

50.0

0

60.0

0

70.0

0

Angle From Boresight (deg)

Rea

lized

Gai

n (d

BiL

)

43.5 GHz E-Plane Realized Gain (dBiL)

45.5 GHz E-Plane Realized Gain (dBiL)

Pyramidal Horn Element CST MWS Computed Realized Gain E-Plane Radiation Pattern

CST H-Plane Radiation Pattern

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

18.05

18.1

18.15

18.2

18.25

18.3

18.35

18.4

18.45

18.5

18.55

18.6

18.65

18.7

43.5 43.7 43.9 44.1 44.3 44.5 44.7 44.9 45.1 45.3 45.5

Frequency (GHz)

Gai

n (d

BiL

)

CST MWS Computed Realized Gain (dBiL)

NFR Measured horn 7 (dBiL)

NFR Measured horn 8 (dBiL)

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 Gain44 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 -0.150 -0.027 -0.130 0.170 0.059 -0.015MAX -0.097 0.172 0.074 0.299 0.151 0.299MIN -0.188 -0.116 -0.405 0.101 -0.082 -0.405P-P 0.091 0.287 0.480 0.198 0.233 0.4801 sigma 0.028 0.080 0.128 0.060 0.062 0.072

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 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

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 -0.548 -0.429 -0.141 0.952 0.404 0.048MAX -0.113 0.128 0.379 1.640 1.097 1.640MIN -0.860 -0.853 -0.492 0.371 0.072 -0.860P-P 0.747 0.980 0.871 1.269 1.025 1.2691 sigma 0.259 0.234 0.241 0.343 0.279 0.271

Figure 6 - Array Factor Calculated Lens/RFD 8 Element Active Rosette at 44.5 GHz 13 Beam Ports

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

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.

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 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.

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

Figure 10 - Array Factor Calculated Lens/RFD 44 Element Passive Rosette at 44.5 GHz 44 Beam Ports

Rea

lized

Gai

n (d

BiL

)

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 0.214 0.103 0.299 0.454 0.573 0.329MAX 0.598 0.441 0.572 1.022 0.902 1.022MIN -0.144 -0.278 -0.274 0.142 0.229 -0.278P-P 0.742 0.719 0.847 0.879 0.673 0.8791 sigma 0.164 0.170 0.173 0.177 0.188 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-to-antenna 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

x4

Multiplier Coupler

LO Source

-10 dB

RefMixerLO

Ref IF

Probe PadAUT

TestMixer

Test IF

LO

Panther Receiver

Ref Sig

LO to Ref

LO to Test

15.00667 GHz

11.25 GHz 45.0 GHz

LO/IF Unit

Mixers operate in3rd harmonic mode Receiver displays Sig/Ref

RF SourceRF Source

x4

Multiplier Coupler

LO SourceLO Source

-10 dB

RefMixerLO

Ref IF

Probe PadAUT

TestMixer

Test IF

LO

Panther Receiver

Ref Sig

LO to Ref

LO to Test

15.00667 GHz

11.25 GHz 45.0 GHz

LO/IF Unit

Mixers operate in3rd harmonic mode Receiver displays Sig/Ref

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:

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.

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 far-field 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 dBGain Standard 0.20 Mismatch 0.05SGH FF Peak 0.15Total (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

[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.

9. ACKNOWLEDGMENTS

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.