# I couldn't observe 2.4G Wi-Fi signal and didn't know why **URL:** \ **Category:** GNU Radio\ **Created:** [November 10, 2013, 4:55am UTC](https://www.ruby-forum.com/t/i-couldnt-observe-2-4g-wi-fi-signal-and-didnt-know-why/237386 "2013-11-10T04:55:10Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [November 10, 2013, 4:55am UTC](https://www.ruby-forum.com/t/i-couldnt-observe-2-4g-wi-fi-signal-and-didnt-know-why/237386/1 "2013-11-10T04:55:10Z")
I know this may due to stupid reasons while I have no idea how to achieve it. Ubuntu 13.04(32-bit); gnuradio-3.6.5; GNU C++ version 4.7.3; Boost\_105300; UHD\_003.005.004-154-ge6a1de6d; USRP N210; RFX2400 There are two Wi-Fi APs in our lab with 802.11g and 802.11n standard. I use a simple software “wavemon” to get their information about channel and signal strength. I use four antennas for test. Two are official VERT2400, others are 2.4G antennas. I use the given example “uhd\_fft.grc” (a simple FFT block) hoping to observe frequency spectrum of Wi-Fi signal. No matter what antenna I set up (even without antenna) and what central frequency I tune, the FFT window only shows something like noise around -50–60dB on whole spectrum (sample rate=25M) which is unlike the 802.11 standard. There is low possibility of bad hardware I guess. I doubt that there may be some problems with my test methods and hope for some advice. Thanks! ---
**Author:** ![Howard\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/2bfe46/32.png) [@Howard\_He](https://www.ruby-forum.com/u/Howard_He)\ **Post date:** [November 10, 2013, 6:05pm UTC](https://www.ruby-forum.com/t/i-couldnt-observe-2-4g-wi-fi-signal-and-didnt-know-why/237386/2 "2013-11-10T18:05:23Z")
On Sun, Nov 10, 2013 at 11:53:41AM +0800, Howard He wrote: > I use four antennas for test. Two are official VERT2400, others are 2.4G > antennas. I use the given example “uhd\_fft.grc” (a simple FFT block) hoping to > observe frequency spectrum of Wi-Fi signal. No matter what antenna I set up > (even without antenna) and what central frequency I tune, the FFT window only > shows something like noise around -50–60dB on whole spectrum (sample rate=25M) > which is unlike the 802.11 standard. > > There is low possibility of bad hardware I guess. I doubt that there may be > some problems with my test methods and hope for some advice. Hi Howard, have you made sure the wifi stations are actually transmitting? Do you have a real spectrum analyzer which shows energy transmitted? And are you on the correct centre frequency? MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association ---
**Author:** ![Howard\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/2bfe46/32.png) [@Howard\_He](https://www.ruby-forum.com/u/Howard_He)\ **Post date:** [November 11, 2013, 12:49pm UTC](https://www.ruby-forum.com/t/i-couldnt-observe-2-4g-wi-fi-signal-and-didnt-know-why/237386/3 "2013-11-11T12:49:56Z")
Hi Howard, as for things that can cost you a lot of time to figure out, until you start all over again with calibration: Have you made sure that you attach the antenna to the correct RF port of the USRP, and connect that port to the correct connector on the daughterboard? Greetings, Marcus ---
**Author:** ![Howard\_He](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/2bfe46/32.png) [@Howard\_He](https://www.ruby-forum.com/u/Howard_He)\ **Post date:** [November 10, 2013, 6:35pm UTC](https://www.ruby-forum.com/t/i-couldnt-observe-2-4g-wi-fi-signal-and-didnt-know-why/237386/4 "2013-11-10T18:35:10Z")
On Sun, Nov 10, 2013 at 12:03 PM, Martin B. (CEL) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi Howard, > > have you made sure the wifi stations are actually transmitting? Do you > have a real spectrum analyzer which shows energy transmitted? > And are you on the correct centre frequency? > > MB Also remember that Wifi is bursty and can be fairly low power. You won’t see it constantly and will only see bursts of energy. If you do a peak/max hold you’ll start seeing something and a spectrogram/waterfall plot will show you the the signal bursts. Tom