Faraday Effect & Enclosure Design

Faraday Effect in IoT Enclosures

Context: Cloudlock loses battery level transmission when container is closed (metal case suspected)

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What is Faraday Cage

A closed conductive enclosure redistributes external EM fields along its surface, preventing them from penetrating inside (and vice versa). Result: signals inside can’t escape, signals outside can’t enter.

Metal case attenuation:

• Steel 2mm: 40–60 dB @ 900 MHz
• Aluminum: 50–80 dB
• Small gaps/seams: still substantial attenuation

Problem for Cloudlock: Cellular GPS signals (~-130 dBm weak) get attenuated by 20–40 dB → undetectable.

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Diagnosis: Is It Faraday Effect?

Quick Test (No Equipment)

1. Read card.wireless via Notecard API with case open → record RSSI, RSRP
2. Close case, wait 60 seconds, read again
3. If RSSI drops >10 dB → definitely shielding

Example Results

Open case:   RSSI: -75 dBm, RSRP: -95 dBm
Closed case: RSSI: -95 dBm, RSRP: -115 dBm
Difference:  -20 dB (significant!)

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Material Choice for Enclosure

RF-Transparent Materials

Material	Dk	Df	Attenuation @ 1 GHz	Best for
Polycarbonate (PC)	2.9–3.2	0.001–0.010	<0.5 dB	✅ Best choice
ABS	2.4–3.5	0.005–0.019	<1 dB	✅ Good
ASA	2.9–3.5	0.008–0.020	<1.5 dB	✅ Good
PMMA (Acrylic)	2.6–3.7	0.005–0.060	1–3 dB	⚠️ Absorbs water
Glass	5–8	high	2–5 dB	❌ Too much loss

Dangerous Materials (For RF)

• Carbon black (pigment) — conductive, used in “UV stabilizers” for black plastic → 50–100 dB attenuation even in small amounts
• Graphite/carbon fiber — essentially metal
• Aluminum/steel — complete Faraday cage

Recommendation

Choose white or transparent PC/ABS without carbon black UV package. When ordering molding, specify: “RF-transparent UV stabilizer (HALS, not carbon black)“

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Three Solutions for Metal Enclosure

If locked into metal case (required for mechanical strength):

Option A: External Antenna (Screw-Mount)

• One hole, one antenna, one IP67 gasket
• Antenna: 2J Antennas 2J6941BGF (combo LTE+GPS, screw mount)
• Cost: ~€45 antenna + gasket
• Advantage: No redesign needed, proven industrial approach

Option B: Hybrid Enclosure

• Bottom/sides: metal for strength
• Top: RF-transparent plastic (PC) section
• Cost: More complex molding

Option C: RF Window (Dielectric Insert)

• Small PC window in metal case
• Antenna inside, radiating through window
• Requirement: Window size ≥ 120×120 mm for LTE 700 MHz

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Antenna Placement Best Practices

Keep-Out Zone (No Metal Nearby)

Minimum λ/4 distance from any conductive object:

• GPS 1575 MHz: 48 mm (use 100 mm to be safe)
• LTE 700 MHz: 107 mm

Ground Plane

• GPS patch antenna: needs 50 mm radius clean ground plane
• LTE monopole: 107 mm radius @ 700 MHz, 42 mm @ 1800 MHz

Connection Cables

• Use shielded u.FL to SMA cables
• Keep cables away from metal parts
• Add ferrite beads if over 10 cm length

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Color Myths vs. Reality

Black plastic with carbon black = RF killer (verified, attenuation >30 dB). Don’t use.

White PC with TiO₂ pigment = RF-transparent (correct, <0.5 dB @ 1 GHz).

Blue/red organic pigments = RF-neutral (correct, <0.5 dB).

Conclusion: Color matters only if black is made with carbon black. White, transparent, or light colors with proper UV package are equivalent.

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Testing (If You Have Instruments)

With TinySA or spectrum analyzer:

1. Place antenna outside enclosure, set up TX signal (signal generator or NanoVNA)
2. Measure power received with RX antenna
3. Close enclosure around TX antenna
4. Re-measure power
5. Attenuation (dB) = Power_open - Power_closed

Example: 20 dB attenuation = signals 100× weaker inside vs. outside.

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For Cloudlock (Recommendations)

✅ If redesigning enclosure:

• Use PC or PC+ABS (white/clear)
• Ensure UV stabilizer is HALS, not carbon black
• External SMA connectors for cellular + GPS antennas
• Gaskets for IP67 seal

✅ If stuck with metal case:

• Add external combo antenna (Option A, cheapest)
• Confirm antenna placement 100+ mm from metal frame
• Use high-gain active GPS antenna (28 dB LNA built-in)

⚠️ Current symptoms (battery level missing):

• Likely EMI from servo (not pure Faraday) — see rf/antenna-interference
• But combined metal case + servo EMI = worst case
• Quick fix: move antenna location away from servo
• Proper fix: external antenna + EMI filtering