Earth-Moon-Earth (EME) — Moonbounce Ham Radio

Of all the exotic propagation modes available to amateur radio operators, Earth-Moon-Earth (EME) — universally known as moonbounce — is the most extraordinary. You transmit a signal from Earth, it travels 384,000 km to the Moon, reflects off the lunar surface, and returns 384,000 km back to Earth — a round trip of 768,000 km in 2.56 seconds. If both you and the receiving station have the Moon above your horizon at the same time, you can communicate with anyone in the other station's footprint, regardless of distance. EME contacts between stations on opposite sides of Earth — the maximum possible distance — are routine for well-equipped operators.

The challenge, of course, is that the path loss for an EME signal is staggering. The Moon is a poor reflector — its cratered, dusty surface scatters rather than mirrors, returning only a small fraction of the energy that hits it. Combined with the free space path loss over a 768,000 km round trip, the total path loss exceeds 250 dB. This is why EME traditionally required very large antenna arrays, high transmitter power, and extremely sensitive receivers. However, modern digital weak-signal modes have transformed EME — a station with a modest setup can now complete EME contacts that were simply impossible 20 years ago.

The EME round-trip covers 768,000 km with a 2.56 second delay. The total path loss exceeds 250 dB including free space spreading loss on both legs and the 10 dB lunar reflection loss from the Moon's poor reflective surface.

View Larger

What EME Is

Earth-Moon-Earth propagation uses the Moon as a passive reflector for radio signals. The Moon orbits Earth at a mean distance of approximately 384,400 km, varying from about 356,500 km at perigee (closest approach) to 406,700 km at apogee (furthest point). This variation matters for EME because path loss decreases as distance decreases — the Moon at perigee offers about 2.5 dB less path loss than at apogee, which is a meaningful improvement for marginal station setups.

The concept of using the Moon as a reflector was proposed as early as the 1940s, and the first successful EME contact was made in 1953 — just a few years after radar had demonstrated the Moon's surface was capable of reflecting radio waves. The US military maintained operational EME communication links during the Cold War before satellites became available. These military systems used massive antenna arrays and high-power transmitters specifically designed to overcome the enormous path losses involved. Amateur radio operators began attempting EME in the early 1960s, and the first amateur EME contact was completed in 1964.

Today, EME is a well-established amateur propagation mode with thousands of active operators worldwide. The WSJT-X software suite — the same software used for meteor scatter — transformed EME from an elite activity requiring hundreds of thousands of dollars of equipment into something achievable by an operator with a modest antenna and a 100-watt transceiver.

Path Loss Calculation

The free space path loss for an EME signal can be calculated using the standard free space path loss formula applied to the one-way transmitter-to-Moon distance:

Wait — that seems impossibly large. How can any signal survive 374 dB of free space loss plus additional reflection losses? The key is the concept of effective signal level at the receiver, which depends not just on path loss but on the transmitter power, antenna gain on both ends, and receiver sensitivity. We will work through the complete link budget shortly, but first we need to understand the lunar reflection process.

Lunar Reflection Characteristics

The Moon's surface is not a smooth mirror. It is a cratered, rocky, dust-covered sphere with very low electrical conductivity. When a radio wave hits the lunar surface, only a small fraction of the energy is reflected back toward Earth — the rest is absorbed by the surface material or scattered in random directions away from Earth. The fraction reflected back toward the transmitting station is described by the radar cross-section of the Moon.

For radio wavelengths, the Moon's radar cross-section is approximately 7% of its physical cross-section — meaning only about 7% of the signal that hits the Moon is reflected back in the direction of Earth. This translates to a reflection loss of roughly 10 dB (0.07 = -11.5 dB, approximated to -10 dB for this discussion). Combined with the fact that the reflection is scattered over a hemisphere rather than focused back at the transmitter, this is why the total path loss is so enormous.

The reflection is also not perfectly specular — the lunar surface is rough at radio wavelengths, and the scattered signal comes from a large area of the lunar disk rather than a single point. This means the received signal has a slight frequency spread due to Doppler effects from different parts of the lunar surface moving at different velocities relative to the transmitter. This spreading is called lunar libration fading and produces a characteristic fluttering of the received signal.

EME Link Budget — Worked Example

Let us work through a complete 2m EME link budget for a typical amateur station to understand what is actually achievable:

This example shows why the complete link budget calculation is essential for understanding EME. A naive look at "250 dB of path loss" might make the mode seem impossible, but when you account for 20 dBi receive gain and the extraordinary sensitivity of JT65B, the math works out. The same calculation with a single 9-element Yagi (about 12 dBi) and 100 watts would give an SNR of about -28 dB — right at or just below the JT65B threshold for reliable decoding, making it marginal but occasionally possible under the best conditions.

Operating Modes — CW, SSB, and JT65

Traditional CW and SSB EME

Before digital modes, EME was the exclusive domain of operators with very large antenna arrays — typically four to sixteen high-gain Yagis, 1 kW or more of transmitter power, and extremely low-noise receive preamplifiers. A 2m EME contact via CW required the signal to be audible above the background noise — a demanding requirement that needed extraordinary stations at both ends. SSB EME was even more demanding because the wider bandwidth of SSB requires a much higher received signal level to produce intelligible voice.

The classic 2m EME station of the 1970s–1990s used four or more 16-element Yagi antennas on an elevation-azimuth tracking mount, a pair of vacuum tube amplifiers running 1,500 watts, and a low-noise GaAs FET preamplifier directly at the antenna feedpoint. Building and maintaining such a station was a major engineering project, and the number of operators capable of it was small.

JT65 — The Mode That Democratized EME

The JT65 digital mode, developed by Joe Taylor K1JT around 2003, is specifically optimized for EME and other extremely weak signal paths. JT65 uses 65 tone multi-FSK modulation with strong error-correction coding (Reed-Solomon code), allowing decoding of signals more than 25 dB below the noise floor in a 2,500 Hz bandwidth. In practical terms, JT65B (the variant used for 2m EME) needs a received signal about 20 dB weaker than a CW signal to achieve the same contact success rate.

This 20 dB sensitivity advantage completely changed what was required for EME. Instead of needing four high-gain Yagis, an operator could potentially work EME with a single large Yagi and 100–200 watts into JT65B if the other station had a large array. "Single Yagi EME" or "SBSG" (Single Biggun, Single Biggun) became a category of EME operation where both stations use modest single-antenna setups — previously unthinkable.

The JT65 transmission period is 60 seconds per frame — both stations transmit on alternating 60-second periods, ensuring that each can decode the other's signal. The software handles synchronization, decoding, and display of decoded messages automatically. The operator simply monitors the screen and confirms contacts in the software's log.

Station Requirements

The receive preamplifier is as important as antenna gain and transmitter power. A GaAs FET or PHEMT low-noise amplifier (LNA) mounted directly at the antenna feedpoint can achieve a noise figure of 0.3–0.6 dB at 144 MHz. This reduces the system noise temperature dramatically compared to having the preamplifier in the shack at the end of a feedline. Every 3 dB of additional noise figure at the receive end requires 3 dB more transmitter power at the far station to compensate — which is why a high-quality LNA is considered essential equipment for serious EME work.

Frequencies Used for EME

As frequency increases, path loss increases (more dB), but antenna gain also increases for the same physical aperture. A 3-meter dish at 1,296 MHz has a gain of about 30 dBi. The same 3-meter dish at 144 MHz would have a gain of only about 17 dBi. So higher frequencies benefit from the ability to focus more power into a narrow beam with a physically manageable antenna. The net effect is that EME performance does not degrade simply because path loss increases with frequency — the antenna compensates.

Window Periods and Scheduling

The fundamental constraint on EME operations is that both stations must have the Moon above their horizon simultaneously. At any given moment, the Moon is visible from roughly half the Earth's surface. As the Moon moves across the sky during its 29.5-day lunar cycle, the geographic area that can see it changes. The area of mutual Moon visibility between two stations is the EME window for that pair.

For two stations diametrically opposite on Earth — say, one in the USA and one in Japan — the window may only be a few hours per day when both can see the Moon. For stations on the same continent, the window may be 6–10 hours per day. The Moon's elevation at each station affects signal strength too — higher Moon elevation means the signal passes through less of the absorbing horizon atmosphere, and the antenna can be aimed more efficiently.

Most EME contacts are pre-arranged. Operators use internet-based scheduling tools (the EME Logs website, the ON4KST DX chatroom, and dedicated software) to coordinate contact attempts. Before attempting a contact, both operators confirm the Moon is above their horizon, agree on a frequency, and confirm the transmission sequence (who transmits first). During major EME contest weekends — the ARRL International EME Contest held in October and November — dozens of operators are active simultaneously and random contacts without pre-arrangement are common.

Doppler Shift on EME Paths

Because the Moon is in constant motion relative to both the transmitting and receiving stations, EME signals experience a Doppler shift. When the Moon is rising (moving toward the station), the signal frequency appears slightly higher than transmitted. When the Moon is setting (moving away), the frequency appears slightly lower.

At 144 MHz, the maximum Doppler shift from the Moon's orbital motion is about ±300 Hz. For CW and SSB, this shift is compensated by tuning to where the signal actually appears. For JT65, the decoding software automatically searches ±1,000 Hz around the expected frequency to find and decode shifted signals. This automatic Doppler compensation is one of the advantages of digital modes for EME — no manual tuning is required as the Moon's velocity changes throughout the contact window.