EME Moonbounce for Beginners: Bouncing Signals Off the Moon

EME moonbounce is the most extreme long-distance path available to radio amateurs. Instead of relying on the ionosphere or the weather, you point your antenna at the Moon, transmit, and listen for your signal and other stations’ signals reflected back from the lunar surface. The Moon acts as a huge, passive reflector that can link any two stations on Earth that can see it at the same time. It is the longest communications path any two stations on Earth can use.

For decades EME was reserved for operators with huge antenna arrays and high-power amplifiers. That has changed. Modern digital modes and signal processing allow contacts with far more modest stations, and a single good Yagi on 2 meters with a reasonable amount of power can now hear and work the larger EME stations. This guide explains how moonbounce works, which bands and modes to use, what kind of station you need, and how to make your first contact.

How Moonbounce Works

As the overview of Earth–Moon–Earth communication describes, EME relies on radio waves transmitted from Earth, reflected from the surface of the Moon and received back on Earth. The average distance to the Moon is about 384,400 kilometres, so the signal travels roughly twice that distance, and the round trip takes about two and a half seconds. You can literally hear your own echo return after you stop transmitting.

The challenge is the path loss. The signal spreads out enormously on the way to the Moon, only a small fraction is reflected back, and that reflection spreads out again on the way home. The returning signal is extremely weak, far below what most operators are used to on terrestrial paths. Everything in an EME station, from antenna gain to receiver noise and mode choice, is about recovering that weak echo.

Effects you will notice

• Echo delay: about two and a half seconds round trip, varying slightly with the Moon’s distance.
• Doppler shift: the relative motion of Earth and Moon shifts the frequency of the echo. The amount depends on the band and the geometry, and EME software can calculate and compensate for it.
• Libration fading: the Moon’s slight rocking motion makes the reflected signal fluctuate rapidly, giving EME signals their characteristic flutter.
• Polarisation effects: on the lower EME bands, the ionosphere can rotate the polarisation of the signal, and the geometry between distant stations adds further polarisation mismatch. At times this makes signals fade out completely for a while.

Bands and Modes for EME

Amateurs have made EME contacts on bands from 50 MHz up into the microwave region, but most activity is on 2 meters, 70 centimetres and 23 centimetres. Each band has its own character.

Modes fall into two groups. Experienced stations with large arrays still make many EME contacts on CW, and occasionally SSB when the link budget allows. For smaller stations, digital modes are the key. Weak-signal digital modes in the WSJT-X software, such as JT65 and Q65, were designed with paths like EME in mind and can decode signals far below the threshold of hearing. Q65 in particular is widely used on EME today. If you have not set up WSJT-X before, start with our guide to a WSJT-X setup that works the first time; the same audio and rig control setup applies.

What Kind of Station You Need

EME stations range from a single Yagi to arrays of many antennas and large dishes. The more capable your station, the more stations you can work. But you can start smaller than many people expect.

Antenna

Antenna gain is the most important single factor, because it helps on both transmit and receive. On 2 meters, a single long Yagi is the realistic entry point; stations with arrays of two or four Yagis can work far more stations. The antenna must point at the Moon in both azimuth and elevation, which means a rotator that can tilt as well as turn, or a simpler fixed-elevation arrangement that catches the Moon near moonrise and moonset. Our antenna rotator guide covers sizing and computer control, which is useful for tracking the Moon.

Receive system

Because the echo is so weak, receiver noise matters enormously. A low-noise preamplifier mounted at the antenna, before the feed line loss, is standard on EME stations. Feed line loss before the preamp adds directly to the system noise, so keep that part of the system short and low-loss. Protect the preamplifier from your own transmitter with a proper sequencer and relay arrangement.

Transmit power

EME stations often use significant power, but digital modes have reduced the requirement. The overview linked above notes that modern signal processing allows EME contacts with powers in the order of 100 watts and a single Yagi, at low data rates, typically with larger stations at the other end. Whatever power you run, your licence limits apply, and EME transmissions with long, continuous digital periods put a heavy duty cycle on amplifiers.

Frequency accuracy and timing

Digital EME modes depend on accurate frequency and accurate time. A radio with a stable reference oscillator, or one locked to a GPS-disciplined reference, makes a big difference, especially on the higher bands. The computer clock must be accurate, as with any WSJT-X mode.

Sequencing, Safety and Reliability

An EME station switches repeatedly between a sensitive receiver and a high-power transmitter on the same antenna, often every minute for an hour or more. That makes sequencing essential. A sequencer switches the coaxial relay, the preamplifier and the amplifier in the correct order, so the preamp is never connected while RF is present and the relay never switches under power. A single mistake can destroy an expensive masthead preamplifier in a fraction of a second.

Reliability matters just as much. Masthead equipment must be weatherproof, relays must be rated for the power and frequency, and connectors on the higher bands should be of a constant impedance type. High-gain antennas also concentrate RF energy, so check RF exposure distances around the antenna, especially when it points low towards the horizon near moonrise or moonset.

Software for EME

Software does much of the heavy lifting in a modern EME station. The WSJT-X package provides the digital modes and also includes tools for EME work, such as calculating the Moon’s position and the Doppler shift between two stations. Many operators combine it with tracking software that steers the rotator automatically, logging software, and the chat pages where EME operators announce activity and arrange schedules. Spend time learning the software settings before your first serious attempt: correct Doppler handling, transmit period and frequency settings make the difference between a decode and silence.

Planning Around the Moon

The Moon is your repeater, so its position decides when you can operate:

• Mutual window: both stations must see the Moon at the same time. EME software and online tools show the common window between your location and another station’s.
• Elevation: a Moon low on the horizon can help stations with fixed-elevation antennas, thanks to ground reflection gain, but local obstructions and noise near the horizon matter.
• Distance: the Moon’s distance changes during its orbit. When it is closer, path loss is slightly lower and conditions are easier.
• Sky noise: when the Moon passes in front of noisy parts of the sky, or close to the Sun, the background noise rises and weak signals become harder to decode.

A simple habit helps beginners a lot: before each session, write down the Moon window, the expected elevation range, the Doppler shift on your band and the stations you hope to hear. Comparing those notes with what you actually decoded quickly shows whether a problem lies in pointing, timing or the receive system.

Many EME operators plan their activity around weekends with favourable conditions, when more stations are on the air. Activity weekends and EME contests are excellent times to try for your first contacts.

Making Your First EME Contact

1. Listen first. Before transmitting, set up your station and try to decode the larger EME stations during a good Moon window. Hearing echoes from big stations proves that your receive system and antenna pointing work.
2. Check your own echoes. If your station is capable enough, transmitting a short test and listening for your own echo is the ultimate system check. Many smaller stations cannot hear their own echoes, which is normal.
3. Find activity. EME operators coordinate schedules and announce activity through dedicated chat pages and reflectors. Arranging a schedule with a large station is the most reliable way to get a first contact.
4. Use a digital mode. Q65 or JT65 give a small station the best chance. Follow the conventions for transmit periods, frequency offset and Doppler correction used by the EME community on your band.
5. Complete the contact properly. EME contacts follow defined minimum exchanges, including call signs and reports or acknowledgements. Make sure both sides have received what is needed before logging.
6. Log the grid. EME contacts often link very distant grid squares, and many operators chase grids and countries via the Moon. Our guide to grid squares explains the locator system.

EME Compared With Other Weak-Signal Paths

EME belongs to a family of weak-signal techniques on VHF and above. Meteor scatter uses brief ionised trails and covers medium distances; our guide to meteor scatter with MSK144 describes it. Amateur satellites use active transponders in orbit and need far less station capability. Tropospheric ducting depends on the weather. EME is unique because it does not depend on propagation conditions in the atmosphere at all: whenever the Moon is visible to both stations and the station is capable enough, the path exists.

Common Beginner Mistakes

• Too much feed line before the preamp. Every fraction of a decibel of loss before the first amplifier stage costs receive performance.
• Poor antenna pointing. A small error in azimuth or elevation with a high-gain antenna can lose the Moon entirely. Calibrate your rotator readings carefully.
• Ignoring frequency drift. A radio that drifts during a long transmission period makes decoding harder for the other station.
• Expecting instant success. Polarisation fades, sky noise and libration can all spoil a session. Persistence matters as much as power.
• Underrated amplifier cooling. Long digital transmit periods demand a continuous-duty rating.

The Bottom Line

EME moonbounce sends your signal on a round trip of roughly three quarters of a million kilometres and back, with an echo delay of about two and a half seconds. The path loss is enormous, but modern digital modes such as Q65 and JT65 have opened moonbounce to stations far smaller than the arrays of the past. Start on 2 meters with the longest Yagi you can manage, a low-noise preamp at the antenna, accurate frequency and time, and careful Moon tracking. Listen to the big stations first, arrange schedules, and be patient with fading and noise. Your first decode of a station via the Moon is one of the most memorable moments in amateur radio.

Frequently Asked Questions

What is EME moonbounce?

EME, or Earth–Moon–Earth communication, is a technique in which radio signals are reflected off the surface of the Moon to reach another station on Earth. Any two stations that can see the Moon at the same time can potentially make contact.

How long does a moonbounce echo take?

The round trip to the Moon and back takes about two and a half seconds on average, varying slightly with the Moon’s distance.

Which band is best for starting EME?

2 meters is the most popular EME band, with the largest number of active stations. 70 centimetres and 23 centimetres are also very active, especially for stations using dishes at the higher bands.

Can I do EME with a single Yagi?

Yes. With digital modes such as Q65 or JT65, a single long Yagi, a low-noise preamplifier and moderate to high power can work larger EME stations, especially during favourable Moon conditions.

Which modes are used for EME?

Large stations often use CW and sometimes SSB. Smaller stations mainly use weak-signal digital modes from WSJT-X, such as Q65 and JT65, which decode signals far below audibility.

Why do EME signals fade in and out?

Libration of the Moon causes rapid fluctuations, and polarisation effects, including rotation in the ionosphere on lower bands, can cause deeper fades. Sky noise also varies as the Moon moves across the sky.