EME Moonbounce: Bouncing Signals Off the Moon

You transmit a signal from your backyard. It travels 238,000 miles to the lunar surface, bounces off the regolith, and returns to Earth where another station hears it. The round trip takes about 2.5 seconds. That's moonbounce, and yes, it actually works.

EME (Earth-Moon-Earth) sits at the intersection of amateur radio's technical challenge and pure wonder. It's not easy. It's not cheap. But it might be the most satisfying QSO you'll ever make.

How EME Actually Works

The physics are straightforward. The moon reflects radio signals. It's a terrible reflector — only about 7% of the signal energy bouncing back toward Earth — but it's always there and it's free. No infrastructure needed, no satellites to schedule, no repeaters to maintain.

Path loss is the challenge. A signal traveling ~480,000 miles round-trip through space gets absolutely crushed. We're talking roughly 250+ dB of path loss at 144 MHz. To put that in perspective: your signal arrives back on Earth something like 10^-25 of its original power. The numbers are almost absurdly small.

So how does anyone manage it? Three things:

• High transmit power (hundreds to a thousand watts)
• High-gain antennas (big yagis, arrays, dishes)
• Extremely sensitive receiving systems (low-noise preamps, narrow digital modes)

The moon itself isn't a smooth mirror. It's rough, uneven, covered in craters and rocks. This creates a phenomenon called libration fading — as the moon's apparent rotation changes the reflecting surface, your signal fades in and out, sometimes rapidly. This is why EME contacts often sound broken up or ghostly even when everything's working perfectly.

The Bands People Use for EME

EME is possible from 50 MHz through microwaves, but each band has different trade-offs:

144 MHz (2 meters): The most popular EME band. Path loss is manageable. Antennas (yagis) are a practical size. Equipment is affordable compared to microwave. Most beginners start here.

432 MHz (70 cm): Second most active. Better antenna gain for a given physical size (because of shorter wavelength), but higher path loss. The sweet spot for operators who want more challenge after 144.

1296 MHz (23 cm): This is where dishes start making sense. A 3-meter dish at 1296 gives serious gain. Fewer operators, but dedicated community. Activity weekends are organized monthly.

Microwave bands (2.3 GHz and up): Extreme niche. Typically requires large dishes (3m+) and very high power. Contacts at 10 GHz EME are genuinely rare achievements. A handful of operators worldwide.

For your first EME project, 144 MHz is the practical answer. The largest community, the most activity, and the most forgiving in terms of equipment requirements.

Equipment: What You Actually Need

Let me break this into tiers.

Minimum Viable Station (144 MHz, digital JT65/Q65)

The digital mode revolution — particularly JT65 and now Q65 — has lowered the EME barrier dramatically. These modes can decode signals well below the noise floor, meaning you don't need the monster antenna arrays that CW EME requires.

• Transceiver: Any all-mode 2m rig (IC-9700, IC-7100, FT-991A, TS-2000)
• Amplifier: 300-500W output minimum (look used: TE Systems, Mirage, BEKO)
• Antenna: Single yagi with 12-14 dBd gain (15+ elements, DK7ZB or YU7EF designs popular)
• Preamp: Mast-mounted GaAsFET LNA, noise figure under 0.5 dB (SSB Electronics, G4DDK kits)
• Feedline: Low-loss coax (LMR-600 or hardline) — every dB matters on receive
• Tracking: Azimuth-elevation rotator capable of pointing at the moon
• Software: WSJT-X (free, handles the digital modes and Doppler tracking)

Total investment if buying everything new: $3,000-6,000. With patience on the used market, you can cut that significantly.

Competitive CW/SSB Station

This is where things get serious (and expensive):

• Power: Legal limit (1500W in the US)
• Antennas: 4x or more yagis in a phased array (think 4x 17-element yagis on an H-frame)
• Elevation rotator: Heavy-duty, capable of handling the wind load of an array
• Preamp: Best available, sequenced with the amplifier
• Polarity switching: Ability to rotate antenna polarization (or use circular) to combat Faraday rotation

We're talking $10,000+ in antenna infrastructure alone. The big-gun stations running 8-yagi arrays have invested considerably more. Not trying to discourage you. Just setting expectations.

Setting Up Your First EME Station

Let me walk through the practical steps for a digital-mode-focused 144 MHz setup.

Antenna and Rotator

You need an antenna that can point at the moon. This means elevation control — the moon moves from horizon to horizon, and you need to track it.

A single long-boom yagi (4-5 meters) on an azimuth-elevation rotator is your starting point. The Yaesu G-5500 combo (az + el) is the standard budget option. It handles a single yagi fine but might struggle with an array's wind load.

Mount the yagi for horizontal polarization to start. Faraday rotation at 144 MHz means the signal's polarization rotates as it passes through the ionosphere — sometimes your horizontal antenna will be perfect, sometimes you'll lose 10+ dB to cross-polarization. This is normal. You work around it by being patient and trying at different times.

I'll get hate for this, but: I think single-yagi EME with digital modes is undersold. The r/amateurradio crowd sometimes makes it sound like you need a field of antennas and a kilowatt. With Q65 and a single long-boom yagi at 500W, I've worked 35 DXCC on 2m EME over 18 months. Not fast. Not easy. But doable without a second mortgage.

The Preamp Problem

Your receive system needs to be absurdly sensitive. Signals arriving from the moon are below the noise floor — sometimes 20+ dB below. The preamp (Low Noise Amplifier / LNA) goes at the antenna, before the coax loss eats your weak signals.

Critical details:

• Mount it as close to the antenna feedpoint as physically possible
• Use a sequencer to protect it during transmit (the preamp sees your full TX power otherwise — expensive smoke)
• Target 0.3-0.5 dB noise figure. Every tenth of a dB matters when your target signal is -30 dB SNR

The G4DDK Anglian series LNA kits are popular in the EME community. About $80-100 for the kit, you solder it yourself. Works beautifully. Commercial options from SSB Electronics or ARR run $200-400.

Feedline

At 144 MHz, coax loss isn't as brutal as at UHF/microwave. But you're dealing with signals so weak that every dB counts double — it hurts you on both transmit AND receive.

For a typical 75-foot run:

• RG-213: ~2.5 dB loss. Usable but painful.
• LMR-400: ~1.5 dB loss. Decent.
• LMR-600: ~1.0 dB loss. Good.
• 7/8" hardline: ~0.5 dB loss. Best practical option.

I used LMR-400 for my first year because I already had it. It worked. I upgraded to surplus CATV hardline (free from a local cable company) and noticed immediate improvement in my decodes. Not night-and-day, but a couple more stations per session.

Sequencing

When you transmit with 500-1500W through a system that has a delicate preamp at the antenna, timing matters. A sequencer ensures:

1. Preamp is bypassed/protected BEFORE the amplifier keys
2. Amplifier keys BEFORE PTT activates
3. On receive: everything reverses in order

Skipping the sequencer is how people destroy $200 preamps. Don't learn this lesson the hard way.

Software: WSJT-X and Moon Tracking

WSJT-X handles everything digital for EME:

• Moon position calculation (azimuth/elevation for your rotator)
• Doppler shift compensation
• JT65B encoding/decoding (the classic EME digital mode)
• Q65 (newer, even more sensitive, rapidly becoming the standard)
• Automatic sequencing of EME QSO exchanges

A standard EME digital QSO exchange:

CQ K1ABC FN42
         K1ABC W9XYZ EM48
W9XYZ K1ABC +00
         K1ABC W9XYZ R-12
W9XYZ K1ABC RRR
         K1ABC W9XYZ 73

Each transmission is 60 seconds. A complete QSO takes 6+ minutes minimum. Patience is mandatory.

For moon tracking, many operators use PstRotatorPlus or WSJT-X's built-in tracking connected to their rotator controller. You point at the moon, start calling, and wait.

Realistic Expectations for Beginners

Look, I want to be straight with you about this. Your first EME session will probably be frustrating. Here's what's normal:

• Calling CQ for an hour with no response
• Seeing decodes appear and disappear as libration fading hits
• Getting one-way decodes (you hear them, they don't hear you) because your station is borderline
• Needing 3-4 sessions before your first completed contact

The activity windows matter enormously. The moon is only above the horizon for both you AND the station you're trying to work for limited periods. Weekend activity contests (like the ARRL EME contest in September/October) concentrate dozens of stations on the moon simultaneously — your best chance for contacts.

After the initial learning curve, a modest digital EME station (single yagi, 500W, good preamp) can realistically work:

• 15-30 stations on a good activity weekend
• Stations on 3-4 continents
• Rare DXCC entities that you'd struggle to work on HF

Not thousands of contacts. Not rapid-fire. But each one feels like an achievement because it genuinely is.

Degradation Factors and Timing

Not all moonpasses are equal. Factors that affect your success:

Moon declination: When the moon is high in the sky (high declination for your latitude), you get longer passes and less ground noise pickup. Low-elevation moonpasses are harder.

Ground noise: When the moon is near the horizon, your antenna is also looking through more atmosphere and picking up terrestrial noise. Ideally work the moon above 15-20° elevation.

Lunar distance (apogee vs perigee): The moon's orbit is elliptical. At perigee (~356,000 km) vs apogee (~406,000 km), you get about 1.2 dB difference. Matters at the margins.

Faraday rotation: Polarization mismatch caused by the ionosphere. Worse at lower frequencies and during high solar activity. Sometimes you wait 20 minutes and the rotation shifts in your favor.

Sky temperature: The area of sky behind the moon affects your system noise temperature. Moon near the galactic plane = more background noise.

Where to Go From Here

Start with WSJT-X installed and just listening. Decode what you can on 144.120-144.160 MHz during an activity weekend. You'll see EME signals appearing even with a modest antenna — it's motivating to know the signals are really there before you invest in the full station.

The Ping Jockey chat site and Moon-Net reflector are where the EME community coordinates. Lurk for a month, absorb the culture, then start building.

73 and good moonbounce.