Using the WLAN Pi M4+ in the Field

After a couple of weeks with the WLAN Pi M4+ on my desk, it has earned a permanent spot in my bag. If you live in Wi-Fi like I do, or you’re an IT admin who has ever stared at a flaky wireless problem wishing you could just see the frames, this device is the answer I wish I’d had years ago. It’s a Raspberry Pi Compute Module 4 in a small, PoE-powered (or external-battery) carrier board, fitted with an Intel BE200 Wi-Fi 7 radio that does proper monitor-mode capture across 2.4, 5 and 6 GHz.

The makers ship a genuinely thorough manual that goes through all the options, so I’m happy to skip those. What I find more useful is to give you a feel for what the thing actually does in practice, with a few concrete examples, and just as importantly, where it stops

What it does well

Capturing 802.11 frames from all three bands you otherwise can’t see

The M4+ is, in my opinion, primarily a wireless sensor, and it delivers. The onboard radio supports monitor mode, so the M4+ pulls down management, control and data frames on whichever channel I point it at, including the full 6 GHz band.

A typical session for me: SSH into the Pi from my laptop, start a capture on the target channel, and the frames stream straight into Wireshark in real time. On macOS I drive it with AirTool2. I’m not using Windows here, as I only use Windows for testing.

One example: I used it to check whether all the APs had the same country code.

There was a strange problem where clients weren’t roaming properly, so I went to capture the Beacon traffic. After looking at the results it turned out one of the APs was set to US instead of NL, and in the US you have more channels available that EU clients normally can’t use. After changing it, problem solved!

In my own lab at home, using sensor with iPhone tools (AirTool Pi together with WiFi Explorer Pi), I could see some neighbours running 160 MHz on their mesh network (luckily far enough away), and drilling down showed their AP wasn’t using the correct country code either. (Oh dear.) Within five minutes, easy!

Profiling what a client is really capable of

The built-in Profiler is the feature I show people first. The M4+ creates a hotspot, Point a client at it, let the device send an association request, and the Pi spits out a clean report of what that client claims to support: spatial streams, channel widths, 802.11k/v/r, 6 GHz capability, and more. It solves the discussion when someone says “the laptop supports Wi-Fi 7, the vendor said so” argument in about thirty seconds.

Below is an example of my Phone:

Being a pocket-sized test rig

Beyond capture, it quietly replaces a bag of other tools. A few I reach for regularly:

• iperf server: stand it up on the wired side and run throughput tests from any client without dragging a laptop along.

• Wi-Fi scanner: a quick read on the RF neighbourhood across all three bands

• Remote sensor: PoE in, drop it at a remote site, and capture or scan over SSH from wherever I am. It runs happily off a single network cable.

• Switch-port/helpers: reachability checks and a port blinker for when you need to find which jack you’re actually plugged into

Where it stops

None of this makes it magic, and it’s worth being honest about the edges so you don’t buy it expecting the wrong thing.

• It’s a Wi-Fi sniffer, not a spectrum analyzer. It decodes 802.11 frames beautifully, but it can’t show you the raw RF energy. So non-Wi-Fi interference like a microwave, a video bridge or a noisy cordless device won’t show up. For that you still want a proper spectrum tool.

• One radio means one channel at a time. The M4+ carrier board has a single M.2 Wi-Fi slot, so you capture on one channel and width per session. If you need to watch a client roam across several channels simultaneously, you need more than one sensor (or the dual-radio Pro). Yes, there’s a ‘dwell’ option, but it’s not the same.

• Encrypted data stays encrypted. You’ll see all the management and control frames, but without the keys the data payloads are just ciphertext, the same as with any other sniffer.

• It works best as a host on a network. The capture experience really shines when it’s streaming to Wireshark on your machine. And the CM4 is capable but modest, so don’t expect line-rate everything.

Why I couldn’t just use my laptop

(or anything else that I had)

Before the M4+, I stubbornly tried to do this with the hardware I already owned and burned a lot of hours on it, because I'm not a fan of spending money unnecessarily (read: cheap). After all that effort, though, it's worth sparing you the same dead ends.

On Windows, capturing raw 802.11 frames in monitor mode is effectively a non-starter: the built-in adapters and drivers simply don’t expose it, and the usual workarounds range from fragile to impossible.

On macOS, monitor mode itself is more cooperative, but I hit a harder wall: my Mac’s built-in card is Wi-Fi 6, not 6E or 7. It has no 6 GHz radio at all. And even if you add a 6 GHz NIC, the Mac won’t let you put it into monitor mode, so that’s money wasted.

If you do happen to have a newer Mac with a 6 GHz band, this device is still worth your while: the ‘sensor’ won’t drop your Mac’s own Wi-Fi connection while it monitors, whereas doing it natively does, which is annoying at the least.

That’s the gap the M4+ fills. It puts a real, monitor-mode-capable radio for all three bands in the path and lets my laptop go back to what it’s good at: running Wireshark and showing me the result. For 6 GHz work specifically, a dedicated capture radio isn’t a nice-to-have; it’s the only way in.

The verdict

The WLAN Pi M4+ isn’t trying to be everything. It’s a focused, affordable (though not so cheap you’d buy one just to see what it does), PoE or battery powered capture and test box that does the one thing my laptops, and most laptops, can’t: see 802.11 frames across all three bands, plus a pile of useful side jobs with analytics tools around it. Know that it’s a frame sniffer and not a spectrum analyzer, accept the single-radio trade-off, and it’s hard to beat for the money.

For anyone doing serious wireless troubleshooting, or who just wants to see what’s really out there, it’s become the first thing I pack. It gives you, and the customer you’re trying to help, just the edge you need to be useful.

Reference:

https://userguide.wlanpi.com/hardware/wlan-pi-m4+

Tools I used on my Mac: Airtool2, Wireshark
Tools I used on Iphone (not all free), WiFi Explorer PI, Airtool PI and WLAN PI