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

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 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 >90dB

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

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

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

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)

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

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

Optical-zonu fiber link RF-over-fiber link currently under test $980/pair (?) Adds <5K noise

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

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

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

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