Moving metal
Cranes, forklifts, doors, and vehicles carry large metal surfaces through the RF path, so a link that measured fine can fade whenever one of them passes.
On this page5 sections
Is this your problem? It presents like this
- Short drops or retry bursts that line up with crane travel, forklift traffic, door cycles, or passing vehicles.
- Fixed devices drop too, not only mobile ones.
- The same spot fails when a particular machine is in a particular position.
- Signal strength on the affected link swings with no change in configuration.
- The survey looked fine because it ran with the metal parked.
Why it happens
Large metal in motion changes the channel even when your devices stand still. NIST lists moving metal objects such as forklifts and cranes among the hazards of industrial RF, and its factory measurements found machinery, conveyors, forklifts, and vehicles scattering radio signals. A field study of fixed industrial links found that moving operators, logistics vehicles, and gantry cranes create paths as strong as the direct one. With a vehicle moving nearby, received signal on a fixed link swung between -46 and -33 dBm1.
When the object blocks the direct path, the result is a shadow. NIST describes how a large object rolling into an area can drop a link, and recommends path redundancy because handing off to another access point can be too slow. For control traffic, the second path has to exist before the shadow arrives.
Surveys miss this because the metal was parked. One industrial Wi-Fi guide warns that coverage on a parked forklift does not prove that a route works, since antenna orientation and shielding by the load change while it moves. Test with the metal moving, or you are testing a different plant.
How to confirm it
- Log signal strength and retries on the affected link at a short interval for a full shift.
- Log crane, door, and vehicle positions or events with timestamps from the controller or MES, and line them up with the dips.
- Move the suspect object through its range on purpose and watch the link.
- Check whether the affected device can still hear a second access point when the first is blocked.
- Rule out interference. A shadow lowers the signal; interference raises the noise floor.
The fix
- Add a second path, an access point or antenna that sees the device from a different angle, so one object cannot block both.
- Raise or move antennas so the moving object does not cross the direct path.
- On vehicles, place antennas where the load and the vehicle body do not shadow them.
- Use smaller cells and more access points where large metal moves through.
Prevent it at design time
- Survey with cranes, doors, and vehicles in their working positions and moving at production speed.
- Design for path redundancy wherever large metal moves.
- List moving machinery in the survey brief so the surveyor tests around it.
About this page
Built from 5 sources: 3 standards bodies and labs, 1 research paper or thesis and 1 other source. Researched and drafted with AI assistance, then reviewed and approved by Ben Rutter on . How pages are made
- First published
- Last updated
Change history (1)
- First published
Cite this page
Plain
Ben Rutter. "Moving metal." OT Wireless, published October 5, 2026. https://otwireless.com/causes/moving-metal/
APA 7
Rutter, B. (2026, October 5). Moving metal. OT Wireless. https://otwireless.com/causes/moving-metal/
BibTeX
@misc{rutter2026movingmetal,
author = {Rutter, Ben},
title = {{Moving metal}},
year = {2026},
howpublished = {\url{https://otwireless.com/causes/moving-metal/}},
organization = {OT Wireless},
}