2.4 GHz Annihilation Device, could be lurking in the darkness
When: Early October 2015
Where: Milton Keynes Stadium
Why: Rugby World Cup Sporting event
How: I was using a Cisco 2600I autonomous access point, operating in Spectrum Connect mode.
What: I had the privilege of being present at the Milton Keynes Stadium during the Rugby World Cup in a professional capacity.
At one point I was asked to perform spectrum analysis in and around the Outside Broadcast compound, located in the car park. One of the Access Points in this area had reported a jamming device and I was sent out to investigate further.
I was unable to find anything worthy of note in and around the Outdoor Broadcast compound. However as I made my back towards the engineering office, I began to detect RF activity with 100 % duty cycle with a fast Fourier transform that I did not recognise on and around channel 6.
- The screenshots have been taken after the event from playing back the recorded file
- If you would like to view the the capture file yourself, it can be downloaded here - 10.06.15 15.19 Capture.ccf
- Signal Strength Swept Spectrogram Colour Scale: -30 to -95 dBm
- Duty Cycle Swept Spectrogram Colour Scale: 0% to 99%
I ventured further into the arena and the interference signal strength increased!
I was now in the Arena (this is not the stadium), a large event space attached to the stadium, there was a stage, musicians setting up equipment, food and drink vendors dotted around the large space. I moved around attempting to identify the source of this exceptionally powerful transmission.
Making my way onto to the stage, I began to query the AV technician about any equipment he might have that could transmit in the 2.4 GHz band.
He had nothing he was aware of transmitting in the 2.4 GHz band.The reported signal strength peaked around -16 dBm!!!
The big stumbling block for me at this point was the omnidirectional antennas in the AP, I could not identify a direction that the interference was coming from.
Around this time I spotted a gentleman with a large hand-held piece of equipment that caught my eye. I wandered over and began talking to him.
As it turned out, this gentleman was working for Ofcom, (the communications regulator in the UK). He was NOT the source of the interference but rather a man with a more accurate tool than I had. You can see the highly directional antenna in the picture, though it was not attached to the tool when I took this photo.
With his help we confirmed that the RF activity I was detecting was real, his equipment could see it too. We identified that the interference was coming from the ceiling of the arena.
When looking up at the ceiling I could see two Ruckus APs fixed to the lighting grid and many powerful downlights. The ceiling was around 8 to 10 metres high. It did not seem feasible that the wifi equipment had become faulty and started pumping out this RF because of the power levels involved.
I made contact with the venue manager and began to explain what we were seeing, he nodded and explained to us that the lighting in the arena was causing this. To test this without plunging the arena into darkness we switched off half of the lights, this produced a noticeable result.
In discussion with the venue manager I discovered the following:The lights in question were Ceravision High Bay
- These lamps are controlled using RF, I think this is a proprietary 900 MHz signal, though I may be wrong
- They use a RF microwave source in the form of a magnetron
As discovered they output very powerful RF energy in the 2.4 GHz band. The venue manager explained that they had been intentionally tuned to channel 6, in an attempt to permit communication on channels 1 and 11. This is the most powerful non-Wi-Fi interference I have discovered so far.