If you broadcast RF energy, no matter the modulation protocol, It ends up in the RF spectrum. No new news here. You can rig a spectrum analyzer and see the carrier frequency and note where it is and see the sidebands and everything else but the analog equipment to do this is super expensive. Now we have SDR, all the difficult stuff is done in software. The expensive hardware dissolves into algorithms and fancy math equations. Of course, you still need and antenna, RF filter, and maybe a bit of amplification as a frontend before you hit a ADC (analog to digital converter). Next, we send the computer a digital serial stream to an appropriate driver that provides the data for the whole freakin' RF band the frontend filter let in, and an appropriate app suitable for your operating system and you can display all this data in its glory. You can break into the business for about $20 and there is a lot of open-source software apps for SDR that is free.

Then heart of SDR is DSP. I know, another acronym. This time it means digital signal processing. Not much help is it. What it does is simuate a mixer, perform quadrature modulation or demodulation functions, or any number of filtering and frequency selections. The biggy is FFT. Great, what's that? Fast Fourier Transfer or turning a complex waveform into its fundamental component sine wave frequencies, in this case, so we can digitize it. Now the math kicks in and then it's a matter of display to the user and the demodulated signal can be heard as audio or as digital packet information. Other software can now go to work on the protocol and interpret the data. The slick thing is you can reverse this process and create a modulated carrier with your voice and boost it up to transmit to the world but you have to have a license for that. DX-ing or listening to long distance transmission is uninhibited and fun.

What you can do with this information? You don't need to know the math or the complex RF circuitry. You just buy a SDR dongle, plug in an antenna, download a compatible SDR app and you're in business of searching radio stations from Ham bands to satellites to learn what's going on out there in the electromagnetic ether.