How does a short backfire antenna work?

The short backfire antenna consists of a circular ground plane (diameter approximately 2 to 2.5 wavelengths) with a raised rim (height approximately 0.5 wavelength), a driven element (typically a dipole or crossed dipole) at the center, and a small sub-reflector (diameter approximately 0.4 wavelength) located about 0.5 wavelength above the ground plane. The driven element radiates downward toward the ground plane, which reflects the energy upward. The sub-reflector redirects some of this energy back down, creating a resonant cavity effect between the ground plane and sub-reflector. This multiple-reflection process produces a high-aperture-efficiency illumination of the ground plane, achieving gains of 15 to 20 dBi with a physically compact structure. The rim around the ground plane reduces edge diffraction and improves the front-to-back ratio to 25 dB or more.

What is the difference between backfire and Cassegrain?

Both use a sub-reflector, but the geometry and scale are fundamentally different. A Cassegrain antenna uses a large parabolic main reflector (many wavelengths in diameter) with a convex hyperbolic sub-reflector at the focal point. The feed is located at the vertex of the main reflector, looking toward the sub-reflector. It is a large, high-gain antenna (30 to 60 dBi) used for satellite earth stations and radio telescopes. A backfire antenna is much more compact: the ground plane is only 2 to 3 wavelengths in diameter, and the sub-reflector is a simple flat or slightly curved disk. The resonant cavity effect between the closely-spaced reflectors (0.5 wavelength) provides the gain enhancement. The backfire principle works best at lower gains (15 to 25 dBi) where the compact size advantage is most valuable.

Where are backfire antennas used?

Short backfire antennas are used in applications requiring moderate gain (15 to 20 dBi) in a low-profile package. Spacecraft and satellite platforms: the compact axial length is critical where volume is limited. NASA used SBAs on the Space Shuttle for S-band communication. LEO satellite telemetry and tracking: provides adequate gain for data downlinks while fitting within spacecraft envelope constraints. Point-to-point microwave links: where a dish would be too large or have too much wind loading. Military communications: low-profile, rugged construction suitable for vehicle mounting. The SBA has largely been superseded by patch arrays in many applications, but retains advantages in circular polarization (using crossed dipoles) and in applications requiring very low side lobes.

Antenna Design

Backfire Antenna

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Compact high-gain antenna using sub-reflector above ground plane. Short backfire (SBA): 2-2.5λ diameter ground plane, 0.5λ rim, 0.4λ sub-reflector, dipole feed. Gain: 15-20 dBi. Axial length: ~λ/2. F/B: 25+ dB. Resonant cavity effect between reflectors. Long backfire: parabolic, 20-25 dBi. Used: spacecraft (NASA Shuttle), LEO telemetry, P2P links, military comms.

Gain: 15-20 dBi

Length: ~λ/2

F/B: 25+ dB

Understanding Backfire Antennas

The backfire antenna achieves remarkably high gain for its physical size by exploiting a resonant cavity effect. Rather than using a large reflector like a parabolic dish, it creates multiple reflections between a ground plane and a small sub-reflector spaced about half a wavelength apart. Each reflection improves the aperture illumination, producing gain comparable to a dish antenna several times larger.

The short backfire antenna was invented by Ehrenspeck in the 1960s and found its most famous application on the NASA Space Shuttle for S-band communication. Its combination of moderate gain, circular polarization capability (with crossed dipoles), and extremely compact axial length made it ideal for spacecraft where volume is at a premium.

Backfire Antenna Equations

SBA gain (empirical):
G ≈ 10log[(πD/λ)² × η_a] dBi
D = 2.5λ, η_a = 0.65:
G ≈ 10log[(π×2.5)² × 0.65] = 18 dBi

Dimensions:
Ground plane: D = 2-2.5λ
Rim height: h = 0.4-0.5λ
Sub-reflector: d = 0.35-0.45λ
Spacing (GP to sub): s ≈ 0.5λ

Beamwidth:
BW_3dB ≈ 70λ/D degrees
= 70/2.5 = 28° for D=2.5λ

Compact High-Gain Antenna Comparison

Type	Gain	Profile	BW (%)	Application
Short backfire	15-20 dBi	0.5λ	5-10%	Spacecraft, LEO
Patch array 4x4	18-22 dBi	0.05λ	5-15%	Radar, 5G
Helical	12-18 dBi	4-8λ	40-70%	Satellite, GPS
Horn	15-25 dBi	3-10λ	Broadband	Feed, test
Parabolic (small)	20-30 dBi	0.3-0.5 f/D	Broadband	P2P, SATCOM

Common Questions

Frequently Asked Questions

How does SBA work?

Ground plane (2-2.5λ) with rim (0.5λ), dipole feed at center, small sub-reflector (0.4λ) at 0.5λ height. Dipole radiates downward, GP reflects up, sub-reflector redirects down: resonant cavity. Multiple reflections improve aperture efficiency. Rim reduces edge diffraction, F/B > 25 dB. 15-20 dBi in λ/2 height.

vs Cassegrain?

Cassegrain: large parabola (many λ), hyperbolic sub-reflector, 30-60 dBi, earth stations. Backfire: compact (2-3λ dia), flat/curved sub-reflector, 15-25 dBi, resonant cavity effect. Different scale: backfire for moderate gain + compact size. Cassegrain for maximum gain at any size.

Applications?

Spacecraft (NASA Shuttle S-band). LEO satellite telemetry/TTC. P2P links where dish too large/heavy. Military vehicle-mount. Circular polarization with crossed dipoles. Largely superseded by patch arrays in many apps, but retains advantages in CP purity and low sidelobes.

Related Terms

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