## The Problem
Everything was fine yesterday — today nothing works:
– Weather station barely reachable — only works at very close range
– Garage remote has severely reduced range
– RaspberryMatic / ioBroker reports `No ACK for Bidcos TX` for multiple Homematic devices
When **several different 868 MHz devices fail at the same time**, the problem is never the individual device. This is the classic sign of an elevated **noise floor** — an interferer is drowning out all regular signals in the band.
## What’s Going On?
Normal IoT devices transmit short bursts with less than 1% duty cycle. When a device gets stuck, it sometimes **transmits continuously** — instead of brief flashes, a constant signal at -60 to -65 dBFS. That’s enough to completely drown out the weak signals from radiator thermostats, blind actuators, and weather stations.
## The Tool: RTL-SDR Stick + SDR#
With a cheap DVB-T USB stick you can make the 868 MHz band **visible** — as a real-time waterfall diagram. You’ll instantly see whether someone is transmitting continuously and how strong the signal is.
**What you need:**
– RTL2832U stick with R820T2 tuner, e.g.
[RTL-SDR Blog V3 on Amazon](https://amzn.eu/d/0a14jTUp) — [check current price](https://amzn.eu/d/0a14jTUp)
– USB-A adapter if your laptop only has USB-C ports ([adapter on Amazon](https://amzn.to/4x8hEFt)) — the stick needs a real USB-A port
– [SDR# (AIRSPY SDR# Studio)](https://airspy.com/download/) — free
– Zadig (for the WinUSB driver)
**Setup:**
1. Plug in the stick and let Windows install its default driver
2. Open Zadig → select RTL2832U → install WinUSB driver
3. Open SDR# → Source: „RTL-SDR (USB)“
4. Frequency: **868,283,900 Hz** (868.28 MHz)
5. Sample Rate: **2.4 MSPS**, RF Gain: **-9.9 dB**
6. Hit Play → watch the waterfall
A **clean band** appears dark blue (noise floor -85 to -90 dBFS). Short bright flashes are normal Homematic bursts. **Persistent yellow or white stripes** mean: interferer active.
## Step-by-Step Diagnosis
### Step 1: Is the Source Inside Your House?
Before switching anything off: **take your laptop with the SDR stick and walk around the outside of your house** — all four sides. Watch the waterfall the whole time.
– **Interference equally strong outside and inside** → external interferer
(neighbour, street — report to the national telecoms regulator)
– **Interference noticeably stronger inside** → source is in the house
In my case the difference was unmistakable: almost clean outside, constant stripes inside. That confirmed — the interferer was in the house.
Next: **cut power to all 868 MHz devices.** Important: also **remove batteries** from battery-powered devices briefly — a stuck device keeps transmitting on battery too.
– **Waterfall turns dark blue** → confirmed: source was in the house ✓
– **Interference remains** → external interferer after all
### Step 2: Which Device Is It?
Switch devices back on **one at a time** and watch the waterfall after each:
– Band stays clean → this device is fine, switch on the next one
– **Persistent stripes appear** → this device is the culprit
Typical candidates in Homematic setups:
| Device | Suspicion level |
|——–|—————–|
| Homematic gateway / CCU | High — occasionally gets stuck |
| Wireless socket hub | Medium |
| Roller shutter controller (wireless) | Medium |
| Smart meter gateway (EMH Casa) | Low — signals too weak |
| Fritz!Box | No 868 MHz |
### What to Do Once You’ve Found the Culprit
**Short-term:** A power cycle is usually enough — the stuck device resets itself. That’s exactly what worked for me. Band clean, all devices reachable again.
**Medium-term:**
– Update the device’s firmware (often fixes the bugs that cause it to get stuck)
– Check RaspberryMatic CCU firmware version
– If it recurs: set up an ioBroker watchdog that detects communication failures and triggers an automatic restart
## Result
After power-cycling all 868 MHz devices:
– Waterfall completely dark blue — no interferer
– Weather station back to full range
– Garage remote working normally
– All Homematic devices reachable in ioBroker, no more `No ACK` messages
The SDR stick makes an otherwise invisible problem visible in seconds. Without it, I would have been blindly swapping individual devices or guessing for hours.
—
*SDR# waterfall with active interference: continuous stripes instead of short bursts*
*After restarting the device: band completely dark blue, interferer gone*
*Stick used: RTL2832U with R820T2 and 0.5 PPM TCXO —
[check current price on Amazon](https://amzn.eu/d/0a14jTUp)*
*(Affiliate link — no extra cost to you, I receive a small commission)*