Terahertz Radiation Generation Let me start by sharing some of what I've read so far: 
Why is Terahertz radiation so hard to generate?
https://spectrum.ieee.org/aerospace/military/the-truth-about-terahertz
I've done some reading into the different methods of producing the radiation, it seems like the nanotube/graphene antennas can't actually get the range you would want, only sub-terahertz. I read in the second link that the best method required it to be cooled down to <200k, but did not mention which method would work with this. I also have a good grasp of the limitations once you are actually able to produce it because of dampening from atmospheric moisture, though that's neither here nor there.
My question really boils down to this: What is preventing someone from simply building a circuit that can output a thz signal, and hooking it up to an antenna? Bonus: what would such a circuit look like?
 A: There are generally 2 ways to generate THz radiation: RF and laser.
I'm not a electronic engineer but I'd imagine RF oscillator in terahertz range is beyond our currently technology. In you motherboard there are DIMM slots for RAM sticks, which runs at about 2GHz. DIMM slots are through-hole component, which has little solder leads on the back side of the motherboard. The length of those leads can make a difference whether you can boot your computer or not. And that's just 2GHz. Now imagine a 1000GHz circuit.Have you ever seen an IC with 1000GHz bandwidth ?
So the most RF source are based-on cavity oscillation like magnetron.
On the other hand, there are loads of method to generate terahertz with pulsed laser:

*

*Optical rectification with non-linear crystal.(require fs laser)

*Non-linear air-plasma emission.(require fs laser)

*Photoconductive antenna emission.(require fs laser)

*Free-electron laser.

*Carefully designed quantum cascade laser chip.

Non of those method are cheap and they are all in the investigation stage.
