Downhole system for ARA station1

1
Downhole system for ARA station1

• Peter Gorham for UH ARA group

2
Working specifications I

• Radio frequency
• Number of holes and antennas
• There will be 4 boreholes of nominal depth 200m,
  with lt20m mean separation
• There will be 4 antennas per hole, 2 Hpol and 2
  Vpol
• The polarization H and V will be grouped
  vertically adjacent to each other in pairs, the
  pairs will be separated by lt20m vertical
• The antennas will have dipole or bicone-like
  response and be as efficient and broadband as
  possible covering 150-850 MHz
• Low-noise pre-filter and pre-amplifier
• Each antenna will have a passband-defining filter
  and low-noise preamplifier as close as possible
  to it, with noise figure as low as possible,
  preferably below 1 dB for both filter and LNA
• The pre-amplifier module will be protected from
  external and internal electromagnetic
  interference to gt90dB

3
Working specifications II

• Radio frequency (cont.)
• Transmission to surface
• The RF signals from the pre-amp module will be
  transmitted to the surface receivers with lt 1dB
  of differential loss across the passband
• Transmission to surface will be done on a single
  signal line per antenna, and will not be up- or
  down-converted for transmission
• Mechanical Environmental
• Downhole systems
• The downhole cable systems will include strength
  members to ensure that they are self-supporting,
  and a surface deployment system capable of
  lowering the cables and antennas without damage

4
Working specifications III

• Mechanical and Environmental (cont)
• Downhole systems (cont)
• All downhole systems must fit easily into
  boreholes of 15 cm diameter and move smoothly
  down potentially rough holes
• All downhole electrical systems must be sealed
  against snow dust ingress
• All downhole systems must be able to operate at
  -55C or colder
• All downhole systems must be designed to survive
  scraping along the sides of the hole during
  deployment
• Deployment system
• Deployment systems must operate smoothly in
  blowing snow and temperatures of -40 C
• Deployment systems must be manageable by
  operators wearing thick gloves

5
Antennas

• Current baseline antennas
• UH wire-frame bicones for Vpol
• Tested and operating in current ARA testbed
• Should be redesigned for smaller central
  pass-through to achieve better impedance coupling
• Also, probably lengthened 5-10 for better
  low-pass
• UW quad-slot-cylinders for Hpol (A. Laundrie)
• Still requires some final testing to verify
  efficiency and gain, but results so far look
  good, though can't match bicone bandwidth

6
Bicone response

• Bicones turn on about 175 Mhz, so we need to grow
  them slightly, about 15 in length
• Should try also to get SWR from 31 down to lt21
  (75 ? 90 eff)

7
Preamps

• Current baseline passband filters
• Lark Engineering
• Passband from 120-840MHz, notch at 450MHz
• Found CW noise at 127MHz, from aircraft comms at
  SP
• Intermittent during summer, not on during winter
• Not clear if we should keep current passband or
  move up
• 9 usable filters on-hand, need at least 32 per
  station spares
• Baseline LNAs
• Miteq AFS series amplifier seems to be excellent
• Includes protection diode, still gives 0.9 dB NF

8
Transmission to surface

• Currently 3 options under consideration
• 1. Digitizer down the hole
• The good transmit digital to surface, not RF
• The bad need a more complex housing, requires
  local triggering, more complex synching
• 2. Coaxial cable
• The good simple, low-risk equalizer seems
  straightforward triggering at top
• The bad very bulky and heavy cables and
  deployment
• 3. RF-over-Fiber
• The good lossless transmission, trigger at top
• The bad so far seems to be a bit power-hungry

9
Optical-zonu fiber link

• RF-over-fiber link currently under test
• 980/pair (?)
• Adds lt5K noise

10
Costs for options

• RF-over-fiber appears best for cost, IF we build
  our own cable assemblies using existing
  I-temp-rated cabled fibers
• This option comes with cost risk as well!
• Mil-spec cable assembly fr RF-over-fiber doubles
  the cost
• Coaxial cable next in line, probably low-risk
• Downhole digitizer costs driven by environmental
  rating for cables

11
Recommendations

• Antennas
• UH Taiwan should work together for production
  and testing of the antennas, with Christian Miki
  as lead engineer
• Pre-amplifiers
• Recommend NTU subcontract these to UH, with final
  assembly and testing at NTU
• Avoids problems Lark Eng. won't accept foreign
  purchase
• Miteq prices are much better for US customers
• Transmission option
• RF-over-fiber with in-house cable assembly
  appears best
• If so, optical Tx would be part of pre-amplifier
  module subcontract
• Need to develop EMI-proof optical fiber
  feedthrough!
• This is a non-trivial RD task, need to start now

12
Glenair integrated E/O cable

• Integrated electro-optical cable
• Dual SM fiber
• Dual 18 AWG conductors
• Mil-D-38999 EMI-rated connectors and bulkheads at
  either end
• Costly!
• But low risk

13
Tactical fiber-optic cable

• From Cables Plus, Mil-rated assembly with
  EMI-rated F/O connector, no conductors
• Price for 10x 4-fiber cables bulkhead
  connector 20K