helical antennas using shape memory...
TRANSCRIPT
Helical Antennas Using Shape Memory Metal
Joel Villasenor
16 April 2010
Why Nitinol?
Need to compress a huge volume into a tiny space Require structures that will self deploy and maintain shape Structures must be light weight Shape memory alloy used for precisely these reasons! Nitinol a good starting material, most commercially available
Nitinol has excellent spring properties, can be compressed with little strain deformation
Shape memory regained once heated above temperature AF (lunar daylight)
Loses tensile strength below temperature MS (lunar night) Can maintain shape with a combination of nitinol + support
structure Lower gravity (1/6 gE) and absence of disturbances means the
support structure can be minimal
Goals
Investigate different antenna designs (emphasis on support)
Construct protoype unit @137MHz to demonstrate deployment
Characterize mechanical repeatability/reliability of antenna
Characterize electrical properties of antenna Optimize electrical properties Iterate build/design cycle
Helix Design
Prototype design @137 MHz readily comparable with commercial quadrifilar antenna
End- fire antenna, gain ~ 18 or 12.5 dB for N=5 turns Pitch angle θ = 13.5 degrees, S = .75 cm Diameter = 69.7 cm
a
S
D
(Ground Screen)
Antenna Fabrication Steps
Fabricate Mandrels
Wind/fix Wire around
Mandrel Anneal Test Mechanical
Properties
stands, with fixed spacing holders
Options: - Oven (initial trial) - resistive heating (larger antennas ~10m long)
Heat Treatment Must “shape-set” the coil to its final form Accomplished through annealing at @500C Create a mandrel (machinable ceramic) to hold antenna form in place
during shape set Antenna fabrication underway
Nitinol sample: alloy-BB from SAES Memry used in initial run
Vacuum chamber with controlled heating elements which will raise the temperature to 500C.
Shape Set with Resistive Heating
Eventually, will need larger ovens to accommodate full size antennas (e.g., company such as Solar Atmospheres can do 2m dia, ~12m length. But $$$!)
Will investigate simpler method of resistive heating Estimate:
Nitinol resistivity ~ 80 ohm-cm, heat capacity ~ 0.077 cal/gm-K, 3mm dia wire, 11m long, with a power supply providing 30A requires only 30s to get T~ 590C (assume perfect insulation)
Open air heat treatment done routinely for other metals Currently investigating methods such as those used at
the MIT Plasma Science and Fusion Center to heat treat large coils