How to Design Radio Coverage for Hazardous Areas in Process Plants

Evertac Engineering Team · Updated November 2026

In refineries and chemical plants, radio problems usually show up as one-way communication: the control room can be heard on site, but replies from inside the process units break up or never arrive. The cause is the architecture, not the handsets. Signal pushed in from base stations at the plant edge cannot get back out through dense steel. The reliable fix is to carry the signal over fiber to certified explosion-proof remote units and antennas inside the classified areas, so every handset is close to a signal source — and to monitor that equipment so degradation is seen before an incident.

Why process areas are hard to cover

A process area is a dense maze of columns, vessels, pipe racks and multi-level steel structures. Every one of them reflects or blocks radio signal, and the layout changes as you walk a few metres.

Most plants started with one or more outdoor base stations at the edge of the site. That works in open areas and on the roads, but inside the units the result is uneven: good signal at one spot, nothing two levels down. Workers learn to walk to a particular place before they call — which is exactly what nobody can do in an emergency.

The less visible problem is the imbalance between the two directions:

The result is a call that sounds as if it works: the operator hears the instruction, but the dispatcher cannot confirm it was received. In normal operation this is a nuisance. During an abnormal event it is more dangerous than an obvious outage, because people assume the message got through.

Bring the signal into the units

Raising handset power or adding another perimeter base station does not change the geometry. The approach that does is "centralised processing, distributed coverage":

1. Keep the base station in the safe area. One master base station holds all channel resources, typically in the control building or an equipment room.
2. Carry the signal over fiber. Optical fiber runs into the process areas. Fiber carries no RF power, is immune to interference and loses very little over long distances — and there are no long coaxial feeder runs through the classified area.
3. Generate RF locally with explosion-proof remote units. Certified remote units placed in or next to the classified areas feed explosion-proof antennas close to where people work. The handset is now metres from a signal source instead of hundreds of metres, so uplink and downlink become balanced.
4. Cover each area to its own needs. Process units, tank farms, loading areas and control buildings are covered as separate zones on the same system, and users move between them without changing channels.

Expansion follows the same logic: a new unit or tank farm adds remote units and antennas on the existing fiber, not another base station or another frequency licence.

Certification across the whole chain

In a classified area, the system is only as compliant as its least certified component. Remote units, antennas, junction boxes and handsets used inside the area each need explosion-protection certification appropriate to the zone (typically Zone 1 or Zone 2 for gas) and to the gas group and temperature class on the area classification drawing. Equipment in the safe area — master base station, combiners, splitters — does not, which is another reason to keep as much as possible outside.

Practical points that often decide a project:

• Start from the area classification drawing. It sets which equipment has to be certified, to what level, and where cables cross zone boundaries.
• Check certification per product, not as a general claim for the system, and make sure the certificate documents are delivered for the authority that will inspect the site.
• Process areas are outdoors and corrosive. Enclosures with a high ingress-protection rating (IP66 or above) and stainless steel housings reduce maintenance in salt, dust and humidity.
• Handsets used inside the area must be certified too — a fully certified coverage system with uncertified radios is still a gap.

Monitor the system before it fails

Radio coverage in process plants rarely fails all at once. Antenna connections corrode, a feeder takes on water, a remote unit starts to overheat — and call quality in one area slips over days. Without monitoring, the first sign is often a call that does not get through when it matters.

A monitored system reports the status of the base station, every remote unit and the antenna feed (including abnormal VSWR), shows it on a site map with clear normal / warning / fault states, and sends alarms to the maintenance team's phones. In one of our process-plant projects, moving from manual patrol checks to continuous monitoring cut the time to detect a coverage fault from more than 24 hours to under 4 hours.

Common mistakes

• Testing only whether field staff can hear the control room, not whether the control room can hear them.
• Adding perimeter base stations or higher-power handsets to fix what is really an uplink geometry problem.
• Running long coaxial feeders through classified areas instead of fiber.
• Certifying the coverage equipment but not the handsets — or the other way round.
• Treating radio as finished at handover, with no monitoring of antennas and remote units.

Design checklist

• Area classification drawing, with zones, gas groups and temperature classes.
• Coverage areas: process units by level, tank farms, loading bays, pipe racks, control and substation buildings.
• Acceptance targets measured in both directions, per area and per level.
• Fiber routes from the safe area into each process area, and positions for remote units and antennas.
• Certification documents for every device that sits inside a classified area, including handsets.
• Enclosure ratings for outdoor and corrosive conditions.
• How emergency functions — man-down, lone-worker, SOS with location — will work on the same system.
• Who monitors the system after handover and how alarms reach them.

How Evertac approaches process plants

Our DistriNET hazardous-area coverage keeps the master base station in the safe area and carries the signal over fiber to explosion-proof remote units and MAEX explosion-proof antennas inside the process areas. In one plant this raised measured radio coverage inside the process units from 78% to 99%. The NetFlex platform adds equipment monitoring and personnel safety functions, and multi-mode terminals work on DMR inside the fence and switch to 4G outside it. See our energy and chemical plant solution for more.

FAQ

What certification does radio equipment need in a hazardous area?

Every device installed or carried inside a classified area needs explosion-protection certification appropriate to its zone, gas group and temperature class. Requirements are taken from the site's area classification drawing and confirmed for each product during design.

Why use fiber instead of coaxial cable in a process area?

Fiber carries no RF power, is not affected by electrical interference and loses very little over long distances, so it can bring the signal deep into the process area without long coaxial feeder runs through the classified zones.

Why can field workers hear the control room but not reply?

The control room transmits from a high-power base station, while a handheld radio transmits at low power from body height. Inside dense steel structures the weaker uplink is blocked first. Putting remote units and antennas close to the users balances the two directions.

Can safe areas and hazardous areas share one radio system?

Yes. Standard remote units serve offices, control rooms and warehouses, while explosion-proof remote units serve the process areas, all connected to the same master base station.

Do we need more frequency licences to extend coverage to a new unit?

Usually not. A fiber-distributed system extends the existing base station's channels to new remote units, so new areas are added without another base station or frequency.