Effect of RF frequency on S2C

I am using Xbee S2C.
I have tried to check effect of other RF devices on Xbee S2C using walky-talky of frequency range 100Mhz- 512Mhz.
when walky - talky is taken near to S2C < 20cm , receive signal strength (measured using remote AT command DB) reduces abruptly.
please guide is there are any special instruction to be followed for PCB layout ,
Is there any document saying effect of other frequencies on S2C module

Design notes for PCB layout are mentioned in product manual. Check them out: