Monolithically Integrated RF-MEMS Actuated Patch-Slot Element for X-Band Reconfigurable Reflectarrays
Dragos Dancila
The Ångström Laboratory
Solid State Electronics - Microwave group Uppsala University
email: Dragos.Dancila@angstrom.uu.se
Outline
• Introduction: NANOTEC – Dem.1 and Uppsala University’s expertise
– Reflect Array Antenna for Wake Vortex Detection Radar – Unequally Spaced and Unequal phase Planar Reflect-Array
• Different approaches
– Single element design reference at 2 GHz
• MEMS Advanced integration
– X-band reflectarray element – RF MEMS design
– Surface losses for open and shorted open slot
• Phase quantization and impact on beam-steering
• Phases of the reflected wave
– Selected MEMS commutations – Investigation vehicle
• Measurements automation
• Conclusions
NANOTEC - Demonstrator 1:
Reflect Array Antenna for Wake Vortex Detection Radar
• Uppsala University focuses on the design and specification of the reflect array antenna elements and phase shifters for achieving necessary beam- steering and requirements for power handling. The antenna elements are selected in order to allow for the maximum necessary beam-steering and for small side-lobe levels of the antenna array.
• Establishment of technical and functional designs and specs. for:
- Elementary miniaturised switched capacitors and array of switched capacitors (component level)
- Phase shifter (function level)
- Partial Reflect Array Antenna (system level)
• In terms of:
- RF performances, power, reliability - Packaging, environmental conditions - Functionalities and architectures - Definition of final test procedure
Introduction - Unequally Spaced and Unequal phase Planar Reflect-
Array for Radar Systems
• Reflect-arrays scatter EM field to form a radiation maximum in a desired direction
• To do so the reflection phase of the individual element is modified
• These are substitutes for parabolic reflector antennas
• Previous work has focused on the patch antenna as the main array element
• Vary patch size, stub length and tilt angle to modify the reflection phase.
• The resonant nature results in poor phase agility and bandwidth
• Small side-lobe levels of the antenna array due to the unequally spaced and unequal phase planar reflect-array
Reference:
D. G. Kurup, M. Himdi and A. Rydberg:
“Design of an unequally spaced reflectarray”, IEEE Antennas and Wireless Propagation Letters, pp. 33-35, Vol.-2, No.
1, June. 2003.
Reflect-array illuminated by an offset horn
100 element reflect-array for excitation with offset antenna
Approaches
Single element design
• Investigation of the reflection phase of a radiating element: linear reflection phase as function of frequency
• The element is designed for minimal loss and significant phase swing at a central frequency of 9.55 GHz (X-band)
• Reference:
RF MEMS Actuated Reconfigurable Reflectarray Patch-Slot Element
HarishRajagopalan, Yahya Rahmat-Samii, William A.
Imbriale, IEEE Transactions on Antennas and Propagation, vol. 56, issue 12, pp. 3689-3699
characteristic dimensions @ 2 GHz:
patch: 47 x 47 mm, slot: 12 x 2.8 mm RT duroid 5880: εr = 2.2, tanδ = 0.0009
HFSS simulation of E field
Phase shifting mechanism:
implementation using commercially available RADANT MEMS
• When a slot is cut in the ground plane, the resonant frequency of the patch is changed following the length of this slot
• The adaptation of the scattered waveform will be realized using an
ensemble of MEMS to short the slot in the ground plane
Reference:
RF MEMS Actuated Reconfigurable Reflectarray Patch-Slot Element Harish Rajagopalan, Yahya Rahmat- Samii, William A. Imbriale, IEEE Transactions on Antennas and Propagation, vol. 56, issue 12, pp.
3689-3699.
MEMS Advanced integration – X-band reflectarray element
• Patch antenna is patterned on the back side of the wafer
• Capacitive serial MEMS switches ensure phase shift
HRSi 5000 Ωm, 1500µm
layer 1 SiO2, 2µm layer 2 SiO2, 700nm
Bias lines, MIM gold 500nm Back side metalization,
gold 5µm
MEMS
DC contact, for wirebonds patch antenna,
gold 5µm
MEMS RF design
MEMS Advanced integration – X-band reflectarray element
patch antenna L = W = 3300µm
2 slots in back side metallization
Biasing lines
MIM decoupling capacitors
MEMS Stack:
Gold 5µm
HRSi 5000 Ωm, 1500µm layer 1 SiO2,1µm
bias Gold, 500nm
layer 2 dielectric, 700nm Gold 5µm
DC pads Array element
L = W = 18mm
MEMS Advanced integration – X-band reflectarray element
Bias lines DC pads
PCB
DC bias
MIM decoupling capacitor
E field and Surface losses for open and shorted slot
Slot open
Slot shorted Slot open Slot
shorted
Patch antenna
Phase quantization and impact on beam-steering
• The number of phases depends on the slot quantization
• Because of symmetries, the number of states is lower than the number of bits: total 2 x 28 : 512 states
• Directivity loss is evaluated as follows:
– 1-bit (2 states) Dloss = 4 dB – 2-bit (4 states) Dloss = 0.8 dB – 3-bit (8 states) Dloss = 0.2 dB – 4-bit (16 states) Dloss = 0.05 dB
Reference: “On the Selection of the Number of Bits to Control a Dynamic Digital MEMS Reflectarray”., Billy Wu, Adrian Sutinjo, Mike E. Potter, and Michal Okoniewski, Antennas and Wireless Propagation Letters, IEEE , vol.7, no., pp.183,186, 2008
Selected MEMS commutations and phase of the refled wave
9.00 9.20 9.40 9.60 9.80 10.00
-200.00 -150.00 -100.00 -50.00 0.00 50.00 100.00 150.00 200.00
cang_deg(S(1,1)) [deg]
10l_biasHRSi_5000Ohmm3
XY Plot 3
-37.8710
-147.3234 -2.2599 156.0831 152.0704
-25.0127
155.5986
-79.4302
-116.9787 156.0661 132.7572
38.7593 154.2657 156.2316
20.5836 146.2735
Other commutations
Return Loss : S11< 2.5 dB in the band
No MEMS
with MEMS and biasing
Investigation vehicle
l1= 0.20 L l2= 0.25 L l3= 0.30 L l4= 0.35 L l5= 0.40 L
l1 l5
MEMSx
lx
The results of the investigation vehicle is a matrix of combinations covering virtualy almost all
implementations with MEMS.
9 variations possible on 3’’ wafer
Network Analyzer
Port 1 Port 2
Procedure : The measurement is a 2 port measurement.
• short circuit at the output of the taper (mechanical support screwed without phase-shifter circuit) and record the amplitude and phase
• insert the phase shifter between the taper and the mechanical support and measure data. Then substract this result to the reference values.
10 or 20 dB APC7 or SMA attenuator
APC7 or SMA coax to wave guide transition
High directivity X wave guide coupler (typically, coupling = 10 or 20 dB, directivity = 30 dB)
Measurement set-up
Standard X coax to wave guide transition
Taper
Multilayer circuit
containing Phase-shifter and biais lines
Small waveguide length (flange)
Mechanical support
Connector for biais signals
X-band taper
Measurements automation
reflectarray element 16 MEMS
Control board 16 relays
PC Labview VNA X-band taper
16 DC lines
USB
RJ45
Excell sheet RF
Conclusions
• Within NANOTEC, UU designs demonstrator 1, a highly integrated MEMS based Reflect Array Antenna for Wake Vortex Detection Radar
• UU has the expertise in Reflect Array Antenna design, focusing on the side-lobe level reduction by using unequally spaced and unequal phase planar reflect-array
• The X-band reflectarray element design is directly inspired by an implementation with commercially available MEMS, at 2 GHz
• The size of one X-band element is 18 x 18 mm , comprising the patch antenna (3.3 x 3.3 mm) and biasing network
• The number of possible MEMS commutations and phases available is so high that phase quantization has no impact on the beam-steering
• The current design targets an investigation vehicle allowing to comute a large number of phases: a number of combinations will be investigated (2x28=512),
targeting variation of slot position (lx = 0.20L to 0.40L) and different MEMS per slots.
• Future plans consist in launching the fabrication and planning the measurements of the element in a waveguide, while presently, the control system is investigated.
