Electrical coupling of a horizontal dipole antenna to a dielectric half-space: applications to radio astronomy from the lunar surface

Abstract

The far side of the Moon, shielded from terrestrial radio frequency interference and beyond the influence of Earth's ionosphere, should offer a uniquely quiet environment for radio astronomy and cosmological experiments. The radio sky below 30 MHz is largely unexplored and is thought to contain spectral signatures of new physics in the early, high-redshift Universe. Achieving precision measurements in this band requires accurate understanding of antenna performance and systematics. For upcoming lunar surface radio astronomy missions, this modeling will be challenging because antennas will deploy at heights that are only a small fraction of a wavelength above the lunar regolith, where strong coupling between the antenna and the surface can significantly alter impedance, radiation patterns, and efficiency. The challenge is compounded by the layered dielectric structure of the regolith and the tendency for permittivity to increase with depth, both of which are difficult to represent faithfully in numerical simulations. In this work, we review theoretical predictions for the behavior of a simple horizontal dipole above a dielectric half-space, representing the lunar regolith, and compare them with simulation results obtained using the Ansys HFSS integral equation solver. We quantify how the antenna impedance and beam pattern couple to the sky for a representative lunar surface radio astronomy experiment. The results show that surface induced effects decrease rapidly, even for modest increases in antenna height above the regolith. Conversely, a dipole antenna placed on or very near the lunar surface will exhibit complex spectral response that renders systematics control very difficult without detailed information on regolith properties.

Figures (9)

• Figure 1: Isometric view of the HFSS simulation. The dipole antenna consists of two 3-meter long perfect electrical conductor strips that are 1 mm wide. The two monopoles are separated by 1 mm and are excited by a 50 $\Omega$ lumped-port, shown in red.
• Figure 2: Real (left) and imaginary (right) components of the complex antenna impedance $Z_A$ as simulated for a simple 6-meter tip-to-tip dipole some height above a dielectric half-space.
• Figure 3: Real (left) and imaginary (right) components of the complex antenna impedance as simulated for a simple 6-meter tip-to-tip dipole 0 meter (green), 0.1 meter (blue), and 1 meter (brown) high with varying dielectric constants.
• Figure 4: Representative 3D antenna gain plots simulated by HFSS at 1 MHz, 25 MHz, and 50 MHz for a 6-meter tip-to-tip dipole 0.5 meters above a dielectric half-space with dielectric constant $\epsilon_r=4$.
• Figure 5: Representative radiation plots at 1 MHz, 25 MHz, and 50 MHz for a 6-meter tip-to-tip dipole 0 meters, 1 meter, and 5 meters above a dielectric half-space with dielectric constant $\epsilon_r=4$. The blue lines are the extracted H-plane simulated by HFSS. The red dashed lines are theoretical predictions (TE) from the formalism defined in text.