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You should be more accurate in your questions.
Arduino, as far as I know, has no radio on board. What signal power do you want to save?
At which rate? Describe the whole system first (arduino? Which one? Radio extension board?
Which one? Bluetooth? Other???) Then where do you want to save? In an SD card? In the internal flash? How long?
Edit: I forgot this: what is the relation with the spectrum analyzer of the title?