Earth-Moon-Earth (EME) communication, also known as moon bounce, is a technique that uses the Moon as a passive reflector to establish radio contact. Radio waves travel approximately 384,400 km to the Moon and back, taking about 2.5 seconds for the round trip. Since the Moon is visible from roughly half of Earth's surface at any given time, EME theoretically enables communication with stations anywhere in the world that can see the Moon simultaneously.

However, EME has historically been one of the most challenging forms of amateur radio. The Moon's surface reflects only 7-12% of incident radio waves, and the round-trip path loss through approximately 770,000 km of space results in signal attenuation of 250-310 dB. Traditional EME setups have required massive parabolic dishes or stacked Yagi arrays, high-power transmitters (500W to 1kW or more), extremely sensitive receivers with low-noise preamplifiers, and sophisticated tracking systems to follow the Moon's movement across the sky.

In Japan, the first amateur EME contact was achieved in 1975 by JA6DR. Since then, Japanese radio amateurs have conducted EME experiments using borrowed facilities such as KDDI's 32-meter dish at the Ibaraki Satellite Communication Center and JAXA's 18-meter parabola at the Katsuura Space Communication Station. While digital modes like WSJT have made EME possible with lower power levels (around 50W with good antennas), it remains the "ultimate challenge" in amateur radio.

Introducing the open.space Project

"open.space" is an ambitious open-source hardware and software initiative designed to dramatically lower the barrier to entry for EME communication. The project's stated mission is to provide "all the tools needed to experience the thrill of space communication" through an open-source software-defined phased array antenna.

A phased array antenna consists of multiple small antenna elements arranged in a planar configuration, with electronic control over the phase and amplitude of each element. This enables rapid electronic beam steering without mechanical movement, offering a compact yet high-performance alternative to traditional large dish antennas.

Product Lineup and Specifications

open.space offers three product tiers designed for different use cases, with shipping expected to begin in March 2026.

Quad: Software-Defined Radio Tile

The smallest building block is a 4-antenna SDR (Software-Defined Radio) tile that can function as a standalone SDR or serve as a component for larger phased arrays.

  • Frequency: 4.9-6.0 GHz (C-band), full duplex
  • Per-antenna bandwidth: 40 MHz; 8+8-bit I/Q
  • Tx power: 1 W per antenna
  • Rx noise figure: ~1.2 dB
  • Polarization: RHCP (Tx), LHCP (Rx)
  • MEMS TCXO with ~1.4 ps jitter
  • FPGA: Lattice ECP5
  • Latency: < 1 ms
  • Size: 13 cm
  • Price: $49-99 (TBD)

The Quad is drop-in compatible with Raspberry Pi pipelines including GNU Radio, Python/C++, and SoapySDR. Standalone applications include general-purpose 4×4 MIMO SDR, fox hunting, direction of arrival (DOA) detection, Open WiFi routers, and drone HD links.

Mini: Starter Phased Array

A compact entry-level phased array composed of 18 Quad tiles (72 antenna elements).

  • Array size: 52 cm
  • Array gain: ~34.0 dBi
  • EIRP: ~52.6 dBW
  • Beam steering range: ~60°
  • Power: 12 V DC (~450 W peak)
  • Price: $899-$1,499

Suitable for high-gain backhaul links, low-Earth orbit satellite downlink reception, long-range drone telemetry, and hands-on phased array experimentation. Extension boards can be added later for array expansion.

Moon: High-Aperture EME Array

The flagship model designed specifically for Earth-Moon-Earth communication, featuring 60 Quad tiles (240 antenna elements).

  • Array size: 100 cm (1 meter)
  • Array gain: ~39.3 dBi
  • EIRP: ~63.1 dBW (equivalent to ~2 kW effective radiated power)
  • Beam steering range: ~60°
  • Power: 12 V DC (~1.5 kW peak)
  • Price: $2,499-$4,999

Beyond EME, the Moon array supports Milky Way C-band imaging, RF sky surveys, terrestrial RF imaging, and atmospheric/ionospheric sensing experiments. It features a coherent distribution network with GPSDO-ready timing for precise beamforming.

Technical Innovation and Discussion

According to discussions on Hacker News, the creator (mrtnmcc) is an RF/DSP engineer from the University of Illinois at Urbana-Champaign (UIUC). A working demonstration was shown at Pacificon, showcasing an end-to-end transmission chain from GNU Radio to a receiver.

Notable technical innovations include custom MASH ΣΔ (sigma-delta) ADCs built from discrete differential pair transistors costing approximately $0.08 each, achieving clean 50 MHz baseband bandwidth. The DACs also use ΣΔ modulation through FPGA LVDS pins with modulating DSP.

The technical community has debated whether the specifications are sufficient for reliable EME. At 5.76 GHz, free space path loss to the Moon reaches approximately 283 dB. The project team's calculations suggest received power of approximately -150.8 dBm with the full array, while the noise floor at 1.2 dB noise figure and 500 Hz bandwidth is -151.9 dBm, yielding an SNR of about +1.1 dB, which the team describes as "easily detectable by ear with CW."

The product page notably includes a disclaimer: "Not intended for radar applications. Core functionality needed for radar not included due to export control restrictions."

Regulatory Considerations

Operating open.space requires appropriate amateur radio licensing. In the United States, a Technician class license or higher is required. The product listing indicates "1 per person" for the Moon array, with "country restrictions apply" noted for all models.

In Japan, the 5.6 GHz amateur band (5650-5850 MHz) overlaps partially with the device's frequency range. Japanese operators would need to verify compliance with Ministry of Internal Affairs and Communications (MIC) regulations regarding effective radiated power. For EME operations exceeding 500W, additional approvals involving extensive documentation and inspections are typically required.

A New Era for Amateur Radio

The emergence of open.space represents a potential paradigm shift in EME accessibility. While the $2,499-$4,999 price range for the Moon array is not inexpensive, it is dramatically more affordable than traditional parabolic dish systems with their associated infrastructure costs. The 1-meter form factor also makes installation feasible in locations where massive antenna structures would be impractical.

Modern features like digital beamforming, electronic Moon tracking, Raspberry Pi integration, and software-defined radio capabilities bring EME into the 21st century. The developer has expressed enthusiasm for achieving "mobile moon bounce," the possibility of EME communication during portable operations.

Japan has a proud tradition of EME experimentation, viewing moon bounce as the "ultimate expression" of amateur radio. The romance of using the Moon as a relay station to communicate with fellow enthusiasts around the world embodies the original spirit of ham radio: making connections with strangers across vast distances.

Projects like open.space have the potential to bring this dream to more people worldwide. How is amateur radio and EME enjoyed in your country? Have you participated in any challenging communication experiments or worked with specialized equipment? We'd love to hear about your experiences and perspectives on the future of space communication.

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