Numerical Simulations and Field Tests of Lunar Polyimide Film AntennasK.P. Stewart1, E. Polisensky1, B.C. Hicks1, R. MacDowall2, D. Jones3, J. Lazio3, K.W. Weiler1

1Naval Research Laboratory2Goddard Space Flight Center

3Jet Propulsion LaboratoryThe Moon is a unique platform for both cosmological and solar observations at low radiofrequencies due to the very tenuous lunar ionosphere and diminished RFI, particularly onthe far side. Science antennas constructed of metal dipoles deposited on thin dielectricfilm substrates would have the advantages of convenient packaging and easy deployment.An array of dozens of adjacent dipole antennas, deposited on dielectric sheets, e.g., 25-μm thick polyimide, 1 to 2 m wide, up to several hundred meters long, could be wound ona spool for transport and unrolled robotically on the lunar surface. We report numericalsimulations and measurements of the electromagnetic properties (gain, response pattern,and feedpoint impedance) of a 16-m long thin-film dipole that would be useful for solarradio studies at frequencies around 10 MHz. Broadband drift scans from 1 to 30 MHzlasting for two days indicate that the antenna was not sensitive enough, due to losses inthe moist, clay soil on which the antenna was deployed, to conclusively observe thediurnal variation of the Galactic background emission, although variations in ionosphericpropagation of MF and HF broadcast stations were observed. Simulations withappropriate lunar regolith properties predict that antennas similar to this design wouldwork well for radio heliophysics or lunar ionospheric studies.

The LUNAR consortium is funded by the NASA Lunar Science Institute(via Cooperative Agreement NNA09DB30A) to investigate concepts forastrophysical observatories on the Moon.

Regolithε = 3, tan δ = 0.01

8 m

Cu

Kapton

Figure 1. The antenna was tested on moist, clay, grass-covered soil.We tested a prototype thin-film dipole antenna constructed from a 5-µm thick Cu layerdeposited on a 25-µm thick Kapton film. Each dipole arm was 8 m long and 30.5 cm wide.The inner 1 m of each arm tapered to a point at which a 1:1 wideband balun was attached(Fig. 1). Coaxial cable connected the balun to an RFSpace SDR-14 software-definedreceiver. The feed point impedance was measured with an AIM4170 vector impedanceanalyzer.

Simulations with soil parameters appropriate to the lunar regolith indicatethat, despite the low gain, an array of these dipoles would be useful forlow-frequency solar radio observations. However, our testing with thisantenna in close proximity to the lossy ground reduces the antennasensitivity and the observable amplitude of the diurnal variation in theGalactic background radiation. Diurnal variations are seen from 10-15 MHzbut the data is noisy and could be correlated with the rising and setting ofthe Sun rather than the Galactic Plane (Fig. 6). Further field testing at alocation with soil properties closer to the lunar surface where the diurnalamplitude will be greater and observable over a larger frequency band,e.g., very dry desert, are planned.

Figure 4. Received power vs. LST (horizontal axis) and frequency (MHz, vertical axis).

CST Microwave Studio 3D electromagnetic simulation software was used to model theantenna on different types of ground (Fig. 2). The dielectric constant and conductivity ofthe ground were adjusted to give the best fit to the measured feedpoint impedance. Thebest-fit values for the soil on which the antenna was deployed are ε = 7 and σ = 2 mS/m.The response pattern is nearly isotropic over the sky in this frequency range (Fig. 3).

The receiver recorded a spectrum from 1 – 30 MHz every 10 minutes for a period of justover two days. The spectral resolution was 1.017 kHz. Fig. 4 shows the received power asa function of frequency and Local Sidereal Time (LST). Local noon occurred at LST ~ 15 hrs.The decrease in power below 7 MHz is evident at these times, due to absorption ofterrestrial signals by the D layer of the ionosphere (Fig. 5).

Figure 2. Antenna model. Regolith parameters are from the Apollo Lunar Sourcebook.

Figure 5. Daytime absorption by the D layer of the ionosphere was observed below ~7 MHz around ~15 hrs LST.

Figure 3. Antenna far-field response pattern at 10 MHz: E-plane (top), H-plane (bottom). The x-y plane is the ground surface and the zenith is in the +z direction. Calculated results are similar from 1 – 30 MHz.

Figure 6. Average antenna power (blue) compared to a simulation of the Galactic back-ground diurnal variation (pink). The antenna power appears to show a diurnal variation but not conclusively due to the Galactic background.