What Is Moonbounce (EME) in Ham Radio?
Definition of Earth-Moon-Earth Communication
Earth-Moon-Earth (EME), also known as moonbounce, is a form of radio communication where radio waves are transmitted from Earth, bounce off the surface of the Moon, and are received back on Earth. This mode requires high power, sensitive receivers, and large antenna arrays to overcome path losses, making it a true test of a station's capabilities and an operator's skill in weak signal work. Unlike satellite communication, there is no active repeater or transponder involved — the Moon itself reflects the signal, functioning purely as an inert celestial mirror with all the inefficiency that entails.
Brief History of Moonbounce from Military Origins to Amateur Radio
The origins of EME trace back to post-World War II military experiments, with the first successful detection of lunar echoes achieved on January 10, 1946, during Project Diana by the US Army Signal Corps at 111.5 MHz using 3 kW of power and a high-gain antenna. This astonishing result proved that a radio signal could survive a round trip of nearly half a million miles and return detectable on Earth.
Well before the first artificial earth-orbiting satellite was placed around our planet, the moon was used to bounce radio signals off its surface for establishing communication between radio stations on earth. Since the first two-way amateur QSO via the moon took place in 1960, it has since been followed by many others on every amateur band from 28 MHz to 47 GHz. The earlier contacts were made using slow-speed CW and large antenna arrays were driven by transmitter output of 1 kW or more.
Why Hams Are Fascinated by EME Communication
Moonbounce is the ultimate long path DX. It is exciting and allows you to literally work the world on VHF and UHF. Earth-Moon-Earth, EME or Moonbounce propagation is a really challenging, but interesting form of radio propagation for radio amateurs to use. Moonbounce propagation presents a number of significant technical and operating challenges, but in this it provides a real sense of achievement and enjoyment when a contact has been successfully achieved. For many operators, the appeal is deeply personal — the idea of sending your voice or data signal 477,000 miles round trip via a natural celestial body, then hearing it return, touches something fundamental about the amateur radio spirit of experimentation and pushing limits.
How Far Is the Moon and What Does That Mean for Your Signal
One-way distance to the moon is in the range of 384,000 km, much higher than any other mode of communication in amateur radio. The path length is approximately 800,000 km round trip, the signal arrives back on Earth roughly 2.5 seconds after transmission, and the free-space path loss is enormous — around 250–260 dB on 144 MHz. This delay is not just a curiosity — it has real implications for how digital EME modes are structured, how echo monitoring works, and why proper timing synchronization between stations is absolutely essential for any EME contact.
How Moonbounce Works: The Science Behind EME
Signal Path and Round-Trip Distance of 477,000 Miles
Earth-Moon-Earth (EME) communication bounces signals off the Moon's surface to enable long-distance contacts between stations on Earth, primarily using VHF and UHF frequencies above 50 MHz. The Moon serves as a passive reflector, with signals traveling approximately 770,000 km round-trip, resulting in significant path loss of around 271 dB at 1296 MHz due to free-space propagation and lunar surface scattering. This method overcomes line-of-sight limitations of direct radio propagation, allowing global communication without artificial satellites.
Path Loss: Understanding the Massive Free-Space Loss
The surface of the Moon also reflects only about 6% of the radio signal power that reaches it. Added to the path loss for the signal travelling to and from the Moon, the overall path loss is at best approximately 252 dB on 144 MHz and 271 dB on 1296 MHz.
At first, the idea that any amateur signal could survive a round trip to the Moon seems impossible — 250 dB of path loss means the received signal power is a factor of 10^25 weaker than the transmitted power. But modern weak-signal digital modes like JT65 can decode signals 28 dB below the noise floor — signals that are completely inaudible and invisible on any meter. Combining a high-gain antenna (a large dish or Yagi array), reasonable transmit power (100–1500W), and JT65's remarkable sensitivity, the link budget becomes achievable.
The Moon as a Passive Reflector: Efficiency and Signal Return
The Moon itself is an inefficient and irregular reflector — only about 6.5% of incident radio energy is reflected back towards Earth — but enough returns for modern equipment to detect. 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.
This fading is termed libration fading, caused by the libration movement. 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.
Doppler Shift and Its Effect on EME Signals
Because the moon moves in relation to Earth, there is a slight Doppler shift on EME signals. The amount of Doppler shift is proportional to frequency. It is about 350 Hz maximum on 144 MHz, more on higher frequencies. At moonrise, the Doppler shift is upward in frequency, reaching zero as the moon passes overhead, and then going negative as the moon heads toward set. For digital modes this Doppler compensation is handled automatically by WSJT-X when set up correctly, but CW operators must retune manually as the Moon transits the sky.
Polarization Rotation and Faraday Effect
Faraday rotation can cause loss in the EME link because the transmitted wave can suffer polarisation distortion. This page describes a model of Faraday rotation: a linearly polarised wave is launched. It is distorted by some angle as it transits the ionosphere under influence of the Earth's magnetic field. At frequencies of 1296 MHz and above, Faraday rotation is not a problem, but on 432 MHz rotations up to 360 degrees are common, and below this the signal may rotate through several complete revolutions. This may result in stations only being able to communicate in one direction at times.
To overcome the orientation uncertainties of the linearly polarized signals arriving at the receiver during EME Moonbounce communication, the antennas are often set up as Crossed-Yagi with a polarization switch or a polarization diversity arrangement. Such schemes would ensure that the polarization mismatch loss could never be more than -3 dB which is equivalent to the maximum possible 45° mismatch.
Frequency Bands Used for Moonbounce
50 MHz (6 Meters) EME Operations
Comparatively few EME, Moonbounce contacts are made on 50 or 70 MHz in view of the local noise as well as building the sort of antennas that would be needed to provide the required gain. Six-meter EME is possible but demanding, requiring very large Yagi arrays to achieve adequate gain. The band is subject to higher galactic and terrestrial noise levels compared to 2 meters. That said, dedicated operators using Q65-30A and large stacked arrays have completed impressive 6-meter EME DX, and the challenge of the band makes each contact especially satisfying.
144 MHz (2 Meters): The Most Popular EME Band
For 144 MHz the sensitive receivers and transmitter exciters are relatively commonplace. A good preamplifier is needed at the antenna, and a high power linear amplifier is needed to develop the maximum legal power. Antennas are manageable even at 144 MHz, but high gains are required. Feeder losses must be kept to an absolute minimum. The 2-meter band is the sweet spot for beginning EME operators. Equipment is widely available, community activity is highest, and the digital EME frequencies around 144.120 MHz are continuously populated during moon windows.
432 MHz and 1296 MHz EME Activity
Though right from 50 MHz to 47 GHz have been used for EME, most commonly used bands are 2 m, 70 cm and 23 cm. At 432 MHz, higher antenna gain is achievable from smaller physical antenna structures, but path loss is greater and Faraday rotation is still a significant concern. At 1296 MHz (23 cm), dish antennas become the preferred option, path loss is around 271 dB, and the EME community on this band is active and growing thanks to modern low-noise amplifier technology and Q65 digital mode capabilities.
Microwave EME Bands: 2.3 GHz, 3.4 GHz, 5.7 GHz, and 10 GHz
As the selection of the relevant amateur band moves into the UHF portion of the spectrum, there is a steady move from Yagi antennas to parabolic reflector or dish antennas, and it becomes more difficult to generate the levels of power needed to drive the antenna. The 10 GHz (3 cm) band has seen remarkable growth in recent years. After many years of collecting bits and pieces for 3 cm, it all came together on Sunday afternoon 22nd March 2026 when at 1500 UTC a first 10 GHz EME contact was successfully completed. Dishes from 1.2 meters to 3 meters are standard at these frequencies, and signals can be decoded with very modest power when the LNA is excellent.
Band Characteristics and Tradeoffs for EME Work
Lower bands (2 m and 70 cm) offer easier equipment availability and more forgiving polarization effects, but require physically larger antennas for comparable gain. Higher microwave bands provide more antenna gain per square meter of aperture, but suffer greater path loss and require more sophisticated transverter and feed system engineering. Most active EME stations eventually operate on multiple bands, using the 2-meter band as their primary workhorse and adding microwave capability as their skills and budget develop.
Antennas for Moonbounce: What You Really Need
Why High-Gain Antennas Are Essential for EME
To overcome the losses and enable amateur radio communications to be established using Moonbounce, very high radio transmitter powers, directive antennas and very sensitive receivers are required. With the distance of the Moon from the Earth being between 360 and 405 thousand kilometers and its diameter being 3475 kilometres it subtends an angle of only 0.52 degree to observers on the Earth. In order to illuminate the Moon with little wasted power either side, enormously directive antennas are required. Also these antennas must be completely steerable to be able to track the steadily changing position of the Moon.
Yagi Arrays: Single vs. Multiple Stacked Arrays
Calculations indicate that antenna gains of around 20 dBd are needed on 144 MHz and 23 dBd on 432 MHz are needed to achieve Morse contacts. For digital EME using JT65 or Q65, the requirements are relaxed considerably. A single high-gain Yagi of 20+ elements (providing approximately 14–16 dBd gain) is the practical minimum for JT65 EME on 2 m. Many operators start with a commercial 2 m EME Yagi from Innovantennas, M2 Antennas, or similar. Four-Yagi stacked arrays are the most common next step, offering approximately 6 dB improvement over a single antenna and enabling contacts with a much wider range of stations including modest DX stations worldwide.
Dish Antennas for Microwave EME Bands
For 23 cm and above, parabolic dish antennas become the dominant antenna choice. A 2.4 meter dish on 1296 MHz provides roughly 28–30 dBi of gain, more than adequate for digital EME contacts with other dish-equipped stations. Surplus satellite dishes — particularly offset-fed dishes — are frequently repurposed by EME operators, with custom feed systems designed for the EME frequency of interest. Approximately 50 stations decoded a 1296 MHz beacon, using antennas ranging from 1.5 m 'cooker' dishes to 10 m.
