Last Updated on September 7, 2025 by John Berry
I gave this presentation online to the Mid Sussex Amateur Radio Society on the Friday 11th October 2024. It gives an overview of moonbounce/EME. The texts and pages below elaborate for anyone wanting more information on EME.
I also recommend the following information sources:
- Moon ephemeris charts for 2025.
- EME Moonbounce Radio Communications Facebook page;
- Moon Net groups.io group sharing technical information: and
- HB9Q logger showing stations currently active on each band.
EME Basics
Radio hams have been bouncing signals off the moon, and hence having QSOs across the world via the moon, for the past seventy years. It’s termed moonbounce or EME (Earth-Moon-Earth communications). The attraction is obvious: just to be able to say, “I bounce signals off the Moon”, has kudos. Here’s some background.
EME path loss
The EME path loss relies on the Moon being an effective reflector. The transmitted power incident on the Moon is initially captured and then re-radiated. In such a scenario, the path loss between transmitter and receiver comprises the loss in the first leg, the loss or gain of the reflection and the loss in the second or return leg. Here’s the detail.
EME path budget
Before setting out on an expensive project, I wanted to know why EME works and where the sensitivities lie. I couldn’t find any site that described this properly. So, I’ve set out my calculations and rationale for this exciting element of the hobby of ham radio here. It certainly shows the knife edge 144MHz moonbounce sits on – it works, but only just.
The Moon as reflector
The Moon as reflector is complicated. I’ll try to build an explanation here and on other pages. When viewed from a station on Earth, the received signal will be a vector sum of all arrivals from the reflection scenario. The receiver will experience fading, with a median value and discrete values in time distributed about that median. This fading is termed libration fading, caused by the libration movement. Here’s more.
Libration fading
One EME station points its antennas at the moon and transmits. Another EME station points its antennas at the Moon and receives the weak signal returned. The signal received by the second station is the aggregate of multiple reflections and scatters from the varied terrain on the Moon. The received signal suffers from libration fading. Here’s a description of the phenomenon.
Faraday rotation
Faraday rotation can cause loss in the EME link because the transmitted wave can suffer polarisation distortion. It’s geometry and frequency dependent. This page describes a model of Faraday rotation: a wave launched vertically polarised is distorted towards horizontal as it transits the ionosphere under influence of the Earth’s magnetic field.
Geometric polarisation rotation
If something happens along the EME path and a wave has its polarisation distorted by some angle relative to that when launched, there will be a loss of signal received. EME polarisation distortion has two parts: from geometry of the path, and from the Earth’s magnetic field. This describes geometric rotation – polarisation occurring simply as a result of station location.
Ground gain
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. Here’s an explanation.
Sky noise
The sky is noisy – some bits of it more than others. And any noise from celestial radio sources degrades the radio amateur’s ability to communicate. It’s not a good idea to attempt EME communications when your antennas point coincidentally at Moon and Sun. The effects from the Milky Way are lesser. Here’s why sky noise is an issue.
Does the Moon reflect or scatter?
Scattering and reflection of radio waves is complex. Many radio hams mis-categorise. Categorisation matters when trying to understand signal returns from solid objects like the Moon, particularly in allowing for reflection or scattering loss in the link budget. So, does the Moon reflect or scatter? And how should we account for the Moon as a sphere, rather than a flat plate?
EME bibliography
A page listing various useful references associated with EME. Critical examples include the determination of the sensitivity of JT65 with reference to the Shannon limit, and typical noise figures of 144MHz receivers. More…