Chassis
Understanding Chassis
In RF engineering, the chassis is far more than a mechanical housing. It serves as the system ground plane, the primary thermal conduction path from active devices to the heatsink or cold plate, and the first line of defense against radiated emissions and susceptibility. A power amplifier module dissipating 50 W, for example, relies on the chassis baseplate (machined from 6061-T6 aluminum with a thermal conductivity of 167 W/m·K) to conduct heat to the system cold plate with a thermal resistance below 0.5 °C/W. Simultaneously, the chassis walls must prevent the amplifier's harmonics and spurious products from radiating into adjacent receiver channels.
At millimeter-wave frequencies, chassis design becomes especially critical because the internal cavity dimensions approach half-wavelength resonance. A 25 mm × 25 mm cavity resonates at approximately 8.5 GHz (TE10 mode), which falls within many radar and satcom IF bands. Engineers suppress these modes by partitioning the cavity with internal walls, applying lossy absorber to the lid, and using dense arrays of grounding screws or conductive gaskets to maintain continuous electrical contact along every seam. For MMIC modules above 40 GHz, cavity pockets are typically kept below 3 mm to push the first resonance above 50 GHz.
Cavity Resonance and Shielding
fmnp = (c/2) × ((m/a)² + (n/b)² + (p/d)²)½
Dominant Mode (TE10) Cutoff:
fc = c / (2a)
Shielding Effectiveness (plane wave):
SE = 20·log10(1 + (Z0 / (2Zs))) + Aabsorption + Rreflection [dB]
Where a, b, d = cavity dimensions (m), c = speed of light, Z0 = 377 Ω (free-space impedance), Zs = shield surface impedance. For 2 mm thick 6061-T6 Al: SE > 100 dB from 1 MHz to 18 GHz.
RF Chassis Material Comparison
| Material | Density (g/cm³) | Thermal Cond. (W/m·K) | CTE (ppm/°C) | Typical Use |
|---|---|---|---|---|
| 6061-T6 Al | 2.70 | 167 | 23.6 | General-purpose RF modules |
| 7075-T6 Al | 2.81 | 130 | 23.4 | High-strength airborne units |
| C101 Copper | 8.94 | 401 | 17.0 | High-power PA baseplates |
| Kovar (ASTM F15) | 8.36 | 17 | 5.5 | Hermetic GaAs/InP packages |
| CuMo (85/15) | 9.95 | 170 | 7.0 | CTE-matched MMIC carriers |
Frequently Asked Questions
What materials are used for RF chassis and why?
Aluminum alloys (6061-T6 and 7075-T6) dominate because they combine low density (2.7 g/cm³), good thermal conductivity (167 W/m·K for 6061), excellent machinability, and adequate electrical conductivity. For higher shielding, chassis are plated with nickel, silver, or gold. Copper (C101) offers the best conductivity (401 W/m·K) but is heavier and more expensive, reserved for high-power amplifier baseplates. Kovar and Invar are used for hermetic packages requiring CTE matching to alumina or GaAs substrates.
How is cavity resonance suppressed inside an RF chassis?
An enclosed chassis acts as a rectangular cavity, and any internal dimension exceeding half a wavelength can support resonant modes. The dominant TE10 cutoff is fc = c/(2a). Suppression techniques include internal partition walls, lossy RF absorber on the lid, conductive gaskets along seams, and ground via arrays around critical circuits. For MMICs above 40 GHz, cavity dimensions are kept below 3 mm to push the first resonance well above the operating band.
What shielding effectiveness does a typical RF chassis provide?
A machined aluminum chassis with continuous conductive gaskets provides 80 to 120 dB of shielding from 100 MHz to 18 GHz. Above 18 GHz, shielding degrades if gaskets are insufficiently compressed or if apertures exceed λ/20. Sheet-metal chassis with riveted seams typically achieve 40 to 60 dB. MIL-STD-461G RE102 and RS103 tests verify that the chassis meets radiated emissions and susceptibility limits for military applications.