RF Shielding — When Filters Are Not Enough, You Need a Faraday Cage
Every electronics engineer eventually discovers that there is a frequency above which filters stop working — not because the filter is bad, but because the noise is bypassing it through the air. At this point you are no longer fighting an electrical problem. You are fighting a radio problem. And the solution is the same one Faraday demonstrated in 1836: surround the offender (or the victim) with a conductor connected to ground. Every high-frequency current wants to flow somewhere. Give it a low-impedance path to ground that is shorter than the path through the air, and it takes that path. That is the entire theory of RF shielding.
The Faraday Cage Principle
A conductive enclosure connected to ground terminates electric field lines on its surface. External fields induce surface currents, which redistribute to cancel the field inside. The result: zero (ideally) electric field inside the cage.
Two types of fields behave differently:
- Electric field (E-field): blocked by any conductive surface, even a thin foil. A 0.1 mm aluminium sheet is sufficient.
- Magnetic field (H-field): harder to block. Requires a thick, high-permeability material (steel, mu-metal). Thin aluminium provides almost no magnetic shielding at DC and low AC frequencies. At RF frequencies (above ~10 MHz), the skin effect means even thin conductors attenuate H-fields adequately.
For most EMC purposes (30 MHz and above), aluminium or steel enclosures work well for both E and H fields. Below 1 MHz, magnetic shielding requires steel enclosures or specialised shielding materials.
Three Types of Shielding in Electronics
1. PCB Shield Cans
A metal cover, typically stainless steel or tin-plated steel, soldered to a continuous ground fence on the PCB. Surrounds a specific sub-circuit — most commonly an RF module (GSM, WiFi, Bluetooth, LTE chipset).
Requirements for an effective shield can:
- Continuous soldered perimeter: the ground fence must be an unbroken ring of copper poured on the PCB surface, and the can must solder to it continuously. Any gap in the fence is a slot antenna.
- Via fence spacing: vias connecting the surface ground fence to the main ground plane must be spaced < λ/20 at the highest frequency. For 2.4 GHz (λ = 125 mm): via spacing < 6.25 mm. Standard practice: 1–2 mm spacing.
- Apertures: U.FL or I-PEX antenna connectors pass through small apertures in the can wall. Keep aperture diameter < λ/20. At 2.4 GHz, < 6.25 mm diameter is safe.
- Removable lids: some designs use a two-piece shield (fence + snap-on lid) for debug access. Lid must make contact all around its perimeter when closed.
Example: the ESP32 module (ESP-WROOM-32) ships with a metal shield can already soldered over the RF section. This is why it passes FCC certification as a pre-certified module — your end product may be able to inherit this certification if you do not modify the RF design.
2. Chassis and Enclosure Shielding
A fully metal enclosure (steel or aluminium die-cast or sheet metal) for the entire product. Provides the highest level of shielding but requires careful engineering of every aperture and seam.
Shielding effectiveness (SE) of common materials:
| Material | Thickness | SE @ 100 MHz | SE @ 1 GHz |
|---|---|---|---|
| Cold-rolled steel | 1 mm | 80–100 dB | 70–90 dB |
| Aluminium | 1 mm | 60–80 dB | 70–90 dB |
| Aluminised ABS plastic | — | 20–40 dB | 15–30 dB |
| Uncoated ABS plastic | — | 0 dB | 0 dB |
A 1 mm aluminium enclosure provides ~70 dB SE at 1 GHz — which means an electric field that would be 10 V/m outside is reduced to 0.3 mV/m inside. Excellent. But this is for a perfect, seamless enclosure. Real products have seams, gaps, buttons, displays, ventilation holes, and cable entry points.
3. Cable Shielding
Cables are among the most effective antennas in your product — long conductors driven by common-mode currents generated by the PCB. A shielded cable wraps a braided or foil conductor around the signal conductors.
Single-point ground (shield grounded at one end only): prevents ground loops at low frequencies. Used below ~1 MHz, or where the cable connects equipment at different AC ground potentials.
Double-ended ground (shield grounded at both ends): maximises shielding at high frequencies. Used above 1 MHz. Requires that both grounds are at low-impedance RF ground — not always possible with long cables between separate equipment.
Practical note: an unshielded cable exiting a shielded enclosure through a filtered connector immediately undoes the enclosure shielding if it carries common-mode current. Always filter the cable at the chassis wall penetration point, not somewhere inside the box.
The Seam Problem
This is where most chassis shielding designs fail. A perfect metal box with a 1 mm gap along one seam will leak RF as if the gap were a slot antenna. The radiated field from a slot aperture depends on the longest dimension of the aperture, not the area.
A 100 mm long seam gap, even if only 0.1 mm wide, is a 100 mm slot antenna. At 1.5 GHz (λ = 200 mm), a 100 mm slot is a half-wave dipole — extraordinarily efficient.
Solutions:
- Conductive gaskets: beryllium copper finger strips (e.g., Leader Tech CF series), conductive foam (with metal coating), wire mesh. Must maintain electrical contact along the entire seam at all positions and temperatures.
- Overlapping seams: design the enclosure so the lid overlaps the base by 5–10 mm, reducing effective slot length.
- EMC-rated fasteners: regular painted screws are non-conductive at the contact surface. Use captive nuts with toothed star washers to bite through anodising or paint and make metal-to-metal contact.
Rule: shielding effectiveness is determined by the largest aperture, not by the overall enclosure. A 70 dB aluminium box with a 20 mm gap is a 30 dB box.
PCB Ground Plane as a Shield
A solid, continuous ground plane on an inner PCB layer:
- Provides a reference plane that reduces loop area of all signals routed above it (reducing radiated emissions)
- Offers a low-impedance return path for high-frequency currents (reducing common-mode current on the PCB itself)
- Partially shields components below the plane from components above
Never split the ground plane under a high-frequency trace to route a signal through the gap — this forces the return current to take a long loop around the split, dramatically increasing loop area and radiation.
Via Fence and Stitching Vias
A via fence is a row of vias connecting top-layer ground copper to the ground plane, placed:
- Around the perimeter of a shield can footprint
- Along the edges of the PCB (prevents edge-launched radiation)
- Along coplanar waveguide transmission lines (prevents field fringing)
- Around sensitive analog sections to isolate them from digital sections
Spacing rule: < λ/20 at the highest frequency of concern.
| Highest Frequency | λ | Max Via Spacing |
|---|---|---|
| 433 MHz | 692 mm | 34.6 mm |
| 2.4 GHz | 125 mm | 6.25 mm |
| 5 GHz | 60 mm | 3 mm |
| 5.8 GHz | 51.7 mm | 2.6 mm |
For WiFi and BT designs, 2 mm via spacing around the shield can footprint is standard practice.
Shielded Product Cross-Section
Aperture Management
Every hole in your chassis is a potential leak.
Display apertures: an LCD display behind a plastic window in a metal enclosure is fine. An LCD mounted in an aperture cut in the metal enclosure is a large radiating slot. Solutions: conductive bezel that contacts the enclosure perimeter, transparent conductive coating on the display glass (ITO or wire-mesh coated glass).
Ventilation holes: individual round holes < λ/20 diameter cause minimal leakage. If you need large ventilation areas, use honeycomb waveguide panels — hexagonal cells whose depth-to-diameter ratio makes them act as waveguide-below-cutoff for RF:
$$f_c = \frac{1.75c}{d}$$
where d is the inscribed diameter of the hexagonal cell and c is speed of light. A 5 mm diameter honeycomb cell has f_c = 105 GHz — it blocks all RF below 105 GHz while passing airflow.
Button holes: use plastic buttons that do not create a continuous metal path to the outside.
When to Shield vs When to Filter
| Situation | Approach |
|---|---|
| Conducted noise on power/signal lines | Filter (ferrite, LC, Pi) |
| Radiated noise from a specific IC or clock | PCB shield can over the source |
| Radiated noise from the whole PCB | PCB layout (reduce loop area), then chassis shielding |
| Noise entering through cables | Ferrite clamp on cable + filtered chassis connector |
| Noise entering through enclosure apertures | Close apertures or add waveguide-below-cutoff |
| Crosstalk between board sections | Ground plane, via fence, separate the sections |
The general order of attack: reduce emissions at source first (layout, slew rate control, decoupling), then filter conducted paths, then shield what remains. Shielding is the last resort, not the first. It is also the most expensive.
Practical Tips
Ferrite clamp-on cores on cable exits: Snap-on ferrite cores (e.g., Fair-Rite 0431164281, 31 material, for 1–300 MHz) attenuate common-mode current on cables acting as antennas. A single clamp placed at the chassis wall exit can reduce cable radiation by 10–20 dB. No soldering required — tool-free fit to existing cables.
Chassis bonding strap: the PCB ground plane must connect to the metal chassis with a short, wide, flat strap — not a thin wire. A 100 mm wire has ~100 nH of inductance, which at 100 MHz has an impedance of 63 Ω. A 25 mm wide flat strap of the same length has ~5 nH and 3 Ω at 100 MHz. Use copper braid or flat copper strap, as short as possible.
Conductive coating on plastic enclosures: zinc-arc spray, electroless nickel plating, or conductive paint can convert a plastic enclosure to a partial shield (~30–40 dB). Not as good as metal, but possible when the industrial design demands plastic.
Measuring Shielding Effectiveness
For product development, a practical measurement of SE uses two antennas: one inside the enclosure (transmit) and one outside (receive), or vice versa. Connect a signal generator to the transmit antenna, a spectrum analyser to the receive antenna. Measure received power with the lid off (open), then with the lid on (shielded). The difference in dB is the SE at that frequency.
Key frequencies to test: 100 MHz (typical worst case for many digital products), 433 MHz (ISM band), 915 MHz, 2.4 GHz (WiFi/BT), and any frequency your product generates intentionally.
Expected results for typical enclosures:
- Aluminium extrusion, no gaskets, sheet screws every 50 mm: ~30–45 dB SE
- Same enclosure with EMC gasket on the lid seam: ~60–70 dB SE
- Die-cast aluminium with machined mating surfaces: ~70–80 dB SE
- Sheet steel with RF gaskets and bonded seams: ~80–100 dB SE
Shield Can Rework and Debug Access
A fully soldered shield can prevents accessing the circuit beneath for debugging. Professional designs address this in two ways:
Two-piece can design: a perimeter frame (fence) is soldered to the PCB permanently. A removable lid snaps onto the fence with spring contacts. During development, the lid is removed for probing. For production, the lid is snapped shut (or soldered). Supplier: Laird Connectivity offers snap-on EMI shield solutions in standard sizes.
Access holes: some designs add a small (< λ/20 diameter) test point access hole in the shield can for probing without removing the can. The hole's diameter must satisfy the aperture size limit for the shielding requirement.
Post-rework re-test: any time you open a shield can and resolder it, re-test emissions. Solder joints that are partially bridged or have voids create effective slot antennas that can degrade SE by 10–30 dB.
Key Takeaway
RF shielding is architecture, not an afterthought. Plan for shield cans when you place RF ICs, plan for a chassis ground connection when you design the PCB, and plan for filtered connectors at every chassis penetration when you design the enclosure. A 70 dB enclosure with an unmanaged seam is a 30 dB enclosure. Shielding effectiveness is set by the worst aperture, not by the best panel. Fix the layout and filters first — shielding what remains is far cheaper than shielding everything from the start.