I couldn't observe 2.4G Wi-Fi signal and didn't know why

GNU Radio
1

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!

2

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

3

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

4

On Sun, Nov 10, 2013 at 12:03 PM, Martin B. (CEL)
[email protected] 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