B-Sandwich Radome
Understanding B-Sandwich Radome Construction
A radome must be transparent to radar signals while protecting the antenna from weather, aerodynamic forces, and physical damage. The radome wall acts as a dielectric transmission line: the RF signal passes through multiple layers of different dielectric constant and thickness. If the layers are properly designed, the transmitted waves interfere constructively and the wall appears nearly transparent. The design parallels multi-section impedance transformers in microwave engineering, where each section (layer) provides a step in impedance between free space (377 Ω) and the skin material (typically 170 to 220 Ω).
The B-sandwich adds a third skin and second core compared to the A-sandwich. This is analogous to moving from a two-section to a three-section quarter-wave transformer: the additional section widens the bandwidth over which the impedance match is maintained. The three skins can be made thinner than in an A-sandwich while maintaining equivalent structural strength (through the truss-like action of the five-layer stack), reducing the total dielectric loading and improving RF performance.
Radome Wall Transmission Analysis
Zlayer = Z0 / √εr
Skin (εr = 4): Z = 377/2 = 188.5 Ω
Core (εr = 1.1): Z = 377/1.05 = 359 Ω
Optimal Layer Thickness (quarter-wave):
t = λ0 / (4 × √εr)
At 10 GHz, skin (εr=4): t = 30mm/(4×2) = 3.75 mm
Transmission Loss (normal incidence, single layer):
IL = 10 log(1 − |Γ|²) dB per interface
where Γ = (Z2 − Z1)/(Z2 + Z1)
Radome Wall Construction Comparison
| Type | Layers | Bandwidth | Insertion Loss | Structural Strength | Weight |
|---|---|---|---|---|---|
| Monolithic | 1 (solid skin) | 5-10% | 0.3-1.0 dB | Moderate | Heavy |
| A-Sandwich | 3 (skin-core-skin) | 10-15% | 0.3-0.8 dB | High | Moderate |
| B-Sandwich | 5 (S-C-S-C-S) | 20-30% | 0.3-0.5 dB | Very high | Moderate-heavy |
| C-Sandwich | 7+ layers | 30%+ | 0.2-0.5 dB | Very high | Heavy |
| Space Frame | Structural lattice | Very wide | Variable | Extreme | Heavy |
Frequently Asked Questions
What is the difference between A-sandwich and B-sandwich radome construction?
An A-sandwich has three layers (skin-core-skin), achieving 10 to 15% bandwidth. A B-sandwich has five layers (skin-core-skin-core-skin), achieving 20 to 30% bandwidth. The extra layers act as additional quarter-wave impedance transformer sections, similar to multi-section matching networks. The B-sandwich provides wider bandwidth at the cost of increased weight and manufacturing complexity.
How is a radome wall designed for RF transparency?
Each layer is treated as a transmission line section with characteristic impedance Z = Z0/√εr. Skins (εr = 3 to 5) have Z ≈ 170 to 220 Ω. Cores (εr ≈ 1.1 to 1.3) have Z ≈ 330 to 360 Ω. Layer thicknesses are optimized for constructive interference of transmitted waves across the operating bandwidth, including incidence angle effects that vary across the radome surface curvature.
Why are B-sandwich radomes used on military aircraft?
Military radars operate across wide bandwidths (10%+ for fire control, multi-octave for EW). An A-sandwich would attenuate band-edge signals, reducing detection range. The B-sandwich maintains < 0.5 dB insertion loss across the full bandwidth while providing structural strength for aerodynamic loads, bird strikes, and environmental extremes. The weight and complexity trade-off is accepted for performance-critical platforms.