High-Gain Parabolic Antenna
3 m CFRP Parabolic Reflector with Cassegrain Feed
3-meter-diameter rigid carbon-fiber-reinforced-polymer (CFRP) parabolic reflector providing ~46 dBi gain at X-band (and ~57 dBi at Ka-band) for the Direct-To-Earth (DTE) link.
Purpose
Provide the high directivity (narrow beam) needed to close a multi-Mbps downlink from the lunar surface to a 34 m DSN ground station with modest spacecraft transmit power, while withstanding lunar dust, vacuum, and thermal cycling.
Context
Mechanical interface to L3-COM-ANT-POINT (gimbal/positioner) and RF interface to L3-COM-ANT-FEED (corrugated feed horn). Designed for fixed installation at the highest illuminated point near Shackleton crater rim. Sized for ~46 dBi X-band gain to match standard DSN tracking margin.
Principles
- ▸Parabolic reflector gain G = η · (πD/λ)², with typical aperture efficiency η ≈ 0.55–0.65 for well-illuminated Cassegrain feeds
- ▸For D=3 m at 8.4 GHz (λ=35.7 mm), G ≈ 60% × (π × 3 / 0.0357)² ≈ 46 dBi; 3 dB beamwidth ≈ 0.7°
- ▸Cassegrain configuration places feed at vertex (reduces feed-line loss), with hyperbolic subreflector providing 'magnification' of feed pattern
- ▸Surface accuracy requirement is λ/50 RMS for <0.2 dB gain loss (~0.7 mm at X-band, 0.2 mm at Ka-band)
- ▸CFRP achieves near-zero CTE in the fiber direction, critical for maintaining figure across –173°C night to +127°C day
- ▸Reflector mass scales as ρ·t·πD²/4; for CFRP sandwich (15 mm honeycomb, ~3.5 kg/m²), 3 m dish is ~25 kg
Typical implementations
- ▸Mars Reconnaissance Orbiter (MRO) HGA: 3 m CFRP, X- and Ka-band, ~46 dBi/57 dBi gain, ~30 kg total — direct heritage reference
- ▸Lunar Pathfinder X-band HGA: parabolic dish ~0.8 m on top deck, demonstrated for DTE link
- ▸MAVEN HGA: 2 m CFRP, ~43 dBi at X-band
- ▸Cassini HGA: 4 m aluminum honeycomb, X- and Ka-band with dual-frequency feed
- ▸JUICE HGA: 2.5 m CFRP, X- and Ka-band Cassegrain
Lunar considerations
- ▸Lunar dust adhesion (electrostatic + Van der Waals) degrades surface accuracy; passive surface coating or electrodynamic dust shield (L2-DST-EDS) required for long-term performance
- ▸Thermal gradients across reflector during sunrise/sunset create figure distortion — CFRP layup balanced to minimize this; multi-layer insulation on rear of reflector
- ▸Micrometeoroid impacts over 100 years statistically expected; sandwich construction provides some self-healing margin, but surface degrades slowly
- ▸Mounted on stable foundation (sintered regolith or pile) to avoid pointing errors from settlement
- ▸Earth visibility from south pole site means HGA elevation typically <10° — long path through lunar terrain horizon, sensitive to topographic occultation
Specifications
Functional
| primary function | Provide high-directivity bidirectional RF aperture for X-band (and Ka-band) Earth link |
| inputs | RF transmit signal from feed (5 W X-band), Pointing position commands from L3-COM-ANT-POINT, Mechanical mounting reaction from L3-COM-ANT-POINT |
| outputs | Radiated beam toward Earth (46 dBi X, 57 dBi Ka), Received signal coupled into feed, Mechanical structural mass to be supported |
| diameter m | 3.0 |
| focal length m | 1.05 |
| f d ratio | 0.35 |
| frequency range ghz | 7.0, 32.0 |
| gain dbi x band | 46 |
| gain dbi ka band | 57 |
| beamwidth 3db x deg | 0.7 |
| beamwidth 3db ka deg | 0.18 |
| surface accuracy rms mm | 0.15 |
| sidelobe first db | -22 |
| cross polarization db | -25 |
| configuration | Cassegrain with hyperbolic subreflector |
Physical
| mass kg | 28 |
| dimensions | 3.0 m diameter × 1.2 m depth (including subreflector struts) |
| materials | M55J carbon fiber prepreg with cyanate ester resin (low outgassing), Aluminum honeycomb core (perforated for venting), Vapor-deposited aluminum reflective surface (RF-conductive), Tedlar / black Kapton MLI on rear, Invar mounting inserts (low CTE for stability) |
| operating temperature c | -180, 130 |
| thermal gradient max c per m | 50 |
| first resonant frequency hz min | 10 |
| ultimate load g | 6 |
Operational
| power consumption w | 0 |
| thermal range c | -180, 130 |
| lifetime years | 100 |
| mtbf hours | passive — limited by micrometeoroid and dust degradation |
Interfaces
Provides
- Reflected/focused RF beam for transmit and receive at feed phase center
- Aperture aerodynamic profile (here: dynamics inertia) for pointing control
Requires
- Two-axis articulation, structural support, vibration damping
- Feed pattern illuminating dish to maximize efficiency
- Electrodynamic dust shield or passive coating to keep surface clean
- Stable foundation pad on sintered regolith or anchored mast
Cite this entry
Lunar Ark Codex. "High-Gain Parabolic Antenna" (L3-COM-ANT-HGA). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-COM-ANT-HGA
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.