1. Downhole system for ARA station1
Peter Gorham for UH ARA group
Downhole system for ARA station1

2. Working specifications I
Radio frequency
Number of holes and antennas
There will be 4 boreholes of nominal depth 200m, with <20m 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 <20m vertical
The antennas will have dipole or bicone-like response and be as efficient and broadband as possible covering 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 >90dB

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 < 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 3:1 down to <2:1 (75% → 90% eff)

7. Preamps Current baseline passband filters: Lark Engineering
Passband from MHz, 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 <5K 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: Pre-amplifiers Transmission option:
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 R&D task, need to start now

12. Glenair integrated E/O cable
Integrated electro- optical cable:
Dual SM fiber
Dual 18 AWG conductors
Mil-D 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