Wireless Thread Signal Strength & Path Loss — Practical Design Note

Wireless Thread Signal Strength & Path Loss — Practical Design Note (AU, 2025)

Note:  Freeair distance values for 2.4 GHz often cite performance indicative of an external antenna.  Placing products with internal antennas in enclosures will diminish performance.  

1) What “signal strength” means here

• Technology: Thread (IEEE 802.15.4 @ 2.4 GHz, 250 kbps PHY) as used by DALI+ over Thread.
• Power: We assume device EIRP (Tx power + antenna gain − cable loss) up to 8 dBm. Where your hardware differs, substitute its EIRP.
• Receiver: Practical design targets are stronger than absolute sensitivity; we reference −70 dBm (robust/lowlatency lighting control) and −90 dBm (edgeofcoverage, not a design target indoors).
• Key idea: Range is set by the link budget = EIRP − required RSSI (dBm). Every wall/door/duct eats into that budget.

2) How materials eat your range (attenuation at 2.4 GHz)

Indicative onepass losses; buildings vary — always verify on site.

*Range multiplier ≈ 10^(−Loss/20). Example: an extra 20 dB loss cuts range to ×0.1 (onetenth).

3) Worked examples (8 dBm EIRP)

1. Open corridor, clear line of sight → target −70 dBm: freeair ≈ 40m. Add one solidcore fire door (≈12 dB): 79 m × 0.25 ≈ 15 m.
2. Through one 200 mm reinforced concrete wall (≈25 dB) → 79 m × 0.056 ≈ 4.4 m beyond the wall.
3. Plant room (equipment/ducts causing ~30 dB mixed loss) → 79 m × 0.032 ≈ 2.5 m reliable; place a router inside the room and one just outside.
4. Amenity block (tiles + plumbing, ~10–15 dB) → 79 m × 0.18–0.32 ≈ 14–25 m; put routers near entries and midblock if long.

These are firstorder estimates. Multiple obstructions add in dB (e.g., 12 dB door + 8 dB wall ≈ 20 dB total → ×0.1 of freeair).

4) Environmentspecific guidance

Offices & openplan

• Typical extra loss per partition: 2–6 dB.
• Router spacing: 10–15 m LOS; 8–12 m where partitions or furniture are dense.
• Keep luminaires/routers ≥300–500 mm from metal (ceiling grid, trays) and well clear of WiFi APs.

Corridors & fire doors

• Long LOS looks great until a door closes.
• Treat each closed fire door as 10–15 dB; space routers each side of doors or every 10–15 m.

Toilets / amenities

• Tiled walls, plumbing chases, mirrors → 10–15 dB typical.
• Place routers near entries and midamenity where runs exceed 15–20 m.

Plant / facility rooms

• Motors, VFDs, switchboards, ducting → 20–30 dB mixed attenuation + reflections.
• Provide one router inside and one outside each room; avoid mounting radios on ducts or inside electrical switchboards.

Car parks (basement)

• Thick reinforced slabs, pillars, vehicles → 20–30 dB; humidity often high.
• Run routers down aisles at 10–15 m spacing; add at corners/ramps and near entry shutters (metal).

Roof spaces & cavities

• Foilbacked insulation can be nearblocking (25–40 dB+).
• Keep antennas below foil, with ≥300 mm clearance to metal and ducts; use approved remote antennas if needed.

Stairs & lifts

• Lift shafts are effectively RF dead zones; do not rely on crossshaft links.
• Place routers at landings every level or every second level depending on construction; avoid metal enclosures.

5) Picking a target RSSI and margin

• For responsive lighting control and multicast scenes, design for ≥−70 dBm at two neighbour routers.
• Keep fade margin ≥10–20 dB over your minimumacceptable RSSI to ride out doors closing, people moving, and humidity changes.
• Emergency lighting meshes (sparser devices) must add routers at exits/stairs/plant rooms to restore two dissimilar paths.

6) Quick cheatsheet

• 8 dBm EIRP, free air: ~40 m at −70 dBm; every +6 dB of extra loss halvestothirds your range (×0.5–0.35).
• Concrete wall (200 mm RC): treat as ~25 dB → ×0.056 of freeair distance.
• Fire door closed: ~12 dB → ×0.25 of freeair distance.
• Foil sarking: ≥25 dB; drop antennas below foil.
• Near big ducts/trays: assume 20–30 dB unless you can maintain Fresnel clearance.

7) Notes for DALI+ / application controllers

• The above guidance applies regardless of vendor. For zencontrol or other WACs, keep controllers out of electrical switchboards, distribute them per area, and maintain wired Ethernet.
• Stagger hightraffic tasks (commissioning bursts; emergency test windows) so you’re not pushing weaker links at the same time.

8) Disclaimer

These figures are indicative engineering values to support preliminary design and review. Buildings vary — always validate on site with surveys and acceptance testing. Where lifesafety outcomes are involved (e.g., emergency egress illumination), RF is for control and reporting only; illumination performance is not dependent on comms.