• If you've ever wondered why DFS is REALLY necessary, take a look at this document ETSI TR 102 651 and the diagrams on pages 14 and 15.

    It is available at:

    I had never really appreciated how much trouble our relatively low powered WLAN devices could cause for RADAR.     It turns out RADAR antennas have gigantic Gain numbers, and a very large number of side lobes - which I wouldn't have guessed for such a directional antenna.

    Many documents list the relevant power levels and channel change rules, but I had never seen graphics that show the problem - and so clearly.

    Enjoy !

  • Thanks for sharing!

    I wrote a blog post on weather radars and DFS ( It didn't take me but a couple of days to get a screenshot of a interfering transmitter on a weather radar. It is so frequent. (And I didn't stare at the display for hours, I checked it occasionally.)

    The radars use magnetrons to produce the signal. That's why the frequency tends to shift slightly. Magnetrons aren't as accurate as crystal oscillators used in Wi-Fi. The signal is typically kilowatts, because the reflections of the rain droplets is so weak. I had an interesting chat with a lady in our national meteorological institute. She was very happy to provide me information if it could help reduce the interference. I promised to spread the word.

  • By Howard - edited: February 4, 2018

    I think the two sets of pictures together, i.e. yours and the ones in the ETSI paper, along with both sets of explanations make for a very compelling case for the DFS rules.

    As I said before, I'd never have guessed the real effects - probably due to my lack of much RADAR experience.

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