[Discuss-gnuradio] peaks without antenna

Hi, 

I was running uhd_fft.py on USRP2 with rfx900 doughter-board. The parameters that I am using are  850 Mhz center frecuency, sample rate 10 Mhz. I got a peak every 500 KHz even though no antenna is connected.

I dont know what is wrong, please help me

thank you

Julio