Last Updated on December 4, 2024 by John Berry
Ground gain must be the simplest practical application of a piece of theory ever!
Ground gain available
Simply, in EME, if the Moon is rising or setting there is a point where a signal rise is experienced. It occurs because the reflections from the ground add to the signal from the direct path. As I show in the first figure (graph) on the Fresnel Zone page on this site, a maximum of 6dB is available (read off the Y-axis ‘dB from Free Space’ value for full First Fresnel Zone clearance for R=1). This gain is available to both the transmit and receive paths.
Here’s the geometry.
The geometry is critical. I give the formula for the Fresnel Zones in the Fresnel Zone page. The First Fresnel Zone is about 55m in radius 5km from the ham station at 432MHz. The First Fresnel Zone reduces in radius with frequency by √(1/f).
Timing and application
Ground gain is available for the time for which the geometry applies – for a matter of minutes while the Moon rises and sets. It’s longest for the few days when the Moon’s declination and rotation causes it to appear in a long arc just above the local horizon. It’s available at 144MHz, and perhaps 432MHz, but its effect is reduced above that. The time for which it applies becomes too short to be useful at higher frequencies.
Some EME beginners and occasional operators with modest kit and fixed elevation antennas aim to exploit ground gain to improve their chances. But of course, the time for which the transmission is within the ionosphere is increased with such a glancing path. Faraday rotation may therefore reduce any benefit.
And as the Moon rises, the geometry changes to reveal reflection from the second and higher order Fresnel Zones. Reflections from even number Fresnel Zones cause big nulls. Since the second Fresnel zone is just 83m in radius at 432MHz there’s not long to exploit the gain as the Moon rises or sets.