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Low-mass transmission lines for a lunar low frequency array

The cosmic Dark Ages, the period between the formation of the cosmic microwave background radiation and the formation of the first stars, is a critical stage in the evolution of our universe. During this period, the intergalactic medium consisted almost entirely of neutral Hydrogen, and consequently one of the only ways to study this epoch is through the highly redshifted 21-cm spectral line of Hydrogen. This line occurs at frequencies of tens of MHz during the Dark Ages, where sensitive observations from Earth are limited by the ionosphere, interference, and solar radio noise. The far side of the Moon offers a uniquely advantageous location for cosmological studies of the Dark Ages, but only if a low frequency radio array with sufficient collecting area can be deployed. The large area required implies a very large number of simple antennas, which in turn requires that the mass of each antenna must be very low (grams). Several designs for low-mass antennas that sit directly on the lunar regolith have been studied, and at least one design (passive dipoles deposited on thin polyimide film for easy deployment) also requires transmission lines to bring signals from each antenna to a central electronics box containing the receivers. The transmission lines may be many tens of meters in length. This paper will consider transmission line designs that are low mass and low loss, and the frequency range over which this approach is likely to be viable. For frequencies below about 10 MHz, balanced parallel-conductor transmission lines are usable over distances up to 100 meters within a factor of two of the dipole resonant frequency. At frequencies well below resonance extremely high VSWR (voltage standing wave ratio) values occur at the antenna feed point, and at higher frequencies conductor losses become prohibitive. At frequencies of several tens of MHz, superconducting material would allow low loss operation during the lunar night. Otherwise the higher frequencies will require active electronics at the individual dipole antennas.