EME — Earth-Moon-Earth (Moonbounce)
EME — Earth-Moon-Earth, commonly called moonbounce — is the ultimate weak signal propagation mode: bouncing VHF and UHF radio signals off the surface of the Moon for contacts between stations on opposite sides of the Earth. 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. Yet amateur radio operators routinely complete EME contacts using dish antennas and modern digital modes, and some dedicated operators use surprisingly modest stations. EME represents the pinnacle of weak signal operating achievement.
Why EME works at all
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 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.
Window requirements
For an EME contact, both stations must have the Moon above their horizon simultaneously. The Moon is above the horizon for roughly half of each 24-hour period at any location — but both stations must have it up at the same time. The EME window between two stations depends on their geographic relationship. Stations at similar latitudes on opposite sides of the Atlantic or Pacific have overlapping EME windows for several hours per day. The Moon's distance also affects EME — at perigee (closest approach) the path loss is about 1.5 dB less than at apogee, which meaningfully affects the link budget for marginal stations. EME scheduling tools calculate windows and Moon position for any station pair.
Minimum station for JT65 EME
The "big gun" EME stations of the past needed enormous dish antennas to make CW contacts. JT65 changed EME forever — stations that would have been laughed at in the CW era now make routine EME contacts. A practical minimum 2m EME station today is approximately: 100W output, a single long Yagi (4m EME Yagi, typically 20+ elements, 15+ dBd gain), and a low-noise preamplifier at the antenna. Many operators start EME this way and progressively upgrade. A 10m dish, or stacked arrays of 4 Yagis, opens significantly more contact opportunities and enables CW EME.
JT65 for EME
JT65 (included in WSJT-X) was designed specifically for EME. It uses 65-tone FSK modulation with strong forward error correction, 60-second transmission periods, and can decode signals at signal-to-noise ratios as low as -28 dB. The 2.5-second round-trip time to the Moon means that your received signal arrives from the Moon while you are still transmitting — you need to account for the echo delay in your monitoring setup. Standard JT65A is used for 2m EME (the A designation indicates 1 Hz tone spacing). The WSJT-X digital modes guide provides setup instructions specific to EME operation.
Scheduling and coordination
EME contacts are almost always pre-arranged because pointing a large antenna at the Moon and waiting for a random caller is inefficient. The ON4KST EME chat room (on4kst.com) is the global standard for EME scheduling — operators log in, post their availability, and arrange sked times with stations they want to contact. The WSJT-X user community forums and EME-specific mailing lists (like the Moon-Net reflector) are also active coordination venues. Most EME operators publish their activity schedules in advance so potential contact stations can plan accordingly.
The EME community
The EME community is small but exceptionally dedicated and welcoming to new entrants. Experienced operators are generally willing to schedule skeds with new stations to help them make their first EME contact, even if the new station's setup is modest. The EME Contest (October, organised by ARRL) is the largest annual EME event and brings out many stations that are not normally active on EME. The DXCC EME award and the ARRL EME award provide achievement recognition for operators who have worked EME stations in many DXCC entities or grid squares.
What is the minimum antenna for 2m EME?
A single high-gain Yagi of 20+ elements (providing approximately 14–16 dBd gain) is the practical minimum for JT65 EME on 2m. Many operators start with a commercial 2m EME Yagi from Innovantennas, M2 Antennas, or similar. A low-noise preamplifier (NF below 0.5 dB) mounted at the antenna feedpoint is as important as the antenna itself for receive performance. Without a good preamp, receive sensitivity severely limits your contact rate even with a large antenna.
What bands are used for EME?
The primary EME bands are 144 MHz (2m), 432 MHz (70cm), 1296 MHz (23cm), and various microwave bands up to 10 GHz. 144 MHz has the most activity because the path loss is lowest and antenna sizes are manageable. At higher frequencies, path loss increases but beamwidth narrows and dish antennas become practical — 23cm is popular with dish antenna operators. The higher microwave bands (3.4 GHz, 5.7 GHz, 10 GHz) are the domain of highly specialised EME stations with large dish arrays.
Can I hear my own echo off the Moon?
Yes — with enough transmit power and a good receiving system, you can detect your own signal returning from the Moon 2.5 seconds after you transmit. This is called hearing your own echo and is a significant milestone for any EME operator. You need a system that can switch between transmit and receive very quickly (or a separate receive antenna) because the echo arrives while you are still transmitting on most equipment. The echo is very weak — detecting it requires the same sensitivity needed to copy other EME stations.