hi, I used an SDR TV dongle to make an SDR Tetra detector. I perform an fft on the data collected. It works fine for stuff like a key fob where it shows me a spike at a certain frequency but when I use it outside to see if there are other tetra users around (e.g ambulance going past) it shows me nothing. How can I diagnose what my problem is? Do I need to get a new antenna? Can processing power be an issue? thanks. I just need some guidance as I am stuck.