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