5 GHz DFS Channels: Radar Detection, Channel Switching, and When to Avoid Them

The 5 GHz band offers plenty of channels, but not all are equal. Some share spectrum with radar systems, and Wi-Fi must step aside when radar appears. Those are the DFS channels, and they explain a good share of mysterious 5 GHz drops.

What DFS is

Dynamic Frequency Selection (DFS) is a rule that requires Wi-Fi devices using certain 5 GHz channels to detect radar signals and vacate the channel if they find any. It protects weather radar, military systems, and in some regions airport radar.

Which channels are affected

In most regions, DFS applies to the UNII-2A (channels 52 to 64) and UNII-2C (channels 100 to 144) ranges. Channels 36 to 48 and 149 to 165 are generally non-DFS, though exact rules vary by country.

How it works in practice

1. Channel Availability Check (CAC). Before using a DFS channel, the access point listens for radar for a set period, typically 60 seconds. For some weather-radar channels the period is much longer, around 10 minutes. During this time, no clients can connect on it.
2. Operation. The access point keeps monitoring while it transmits.
3. Radar detected. The access point must stop transmitting on that channel within a short time, notify clients, and move to another channel. It then cannot return to the affected channel for a period (the non-occupancy period, typically 30 minutes).

Symptoms of a DFS event

• Brief disconnections and reconnections with no apparent cause
• A channel that keeps changing in the router log
• Clients that cannot see the network after a switch, because some devices do not scan DFS channels well
• Worse effects if the access point has no fallback and has to run the check again

When DFS channels are a good choice

• You are in a crowded area where non-DFS channels are saturated.
• You are far from radar sources, and your logs show no radar events.
• You need wide channels (80 or 160 MHz) and cannot get them elsewhere.
• Your devices handle DFS well.

When to avoid them

• You are near an airport, weather radar, or coastal or military installation, where radar hits are frequent.
• You run mission-critical links such as voice or point-of-sale that cannot tolerate interruptions.
• Your clients are old, and some do not connect on DFS channels.
• You see repeated radar events in your logs.

Best practices

• Use a non-DFS channel as the primary choice for critical areas.
• In multi-access-point deployments, spread DFS and non-DFS channels sensibly rather than putting everything on DFS.
• Review access point logs for radar events.
• Check your country's rules, because the list of DFS channels and timing requirements varies.
• Remember that some vendors let you exclude DFS channels from automatic selection.

In Uganda, confirm the DFS channel list and timing with the Uganda Communications Commission (UCC) or your equipment vendor's regional profile — they can differ from the US list above.

See also Channel Widths Explained and Designing High-Density Wi-Fi.

Next Steps

Mysterious 5 GHz drops near Entebbe's airport corridor or other radar sources are a problem our Networks & Infrastructure team sees regularly. We plan channels that stay stable.

Request a Free Site Survey if your 5 GHz network keeps switching channels on its own.