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