The concept of HERF guns has been around on the Internet for a long time. I remember reading about people trying to DIY them in the late 90's on some early websites, along with other such fun topics as railguns.
Simple in concept, there are many hacky (and questionably executed) examples around (i.e. YouTube "HERF" guns which are just gutted microwaves pointed at things, but not really High Energy RF), but in reality trying to project sufficient energy at any real distance is quite difficult, thanks to the inverse square law as well as the difficulty of keeping RF energy focused in a tight beam so you're not wasting it everywhere except your target.
I wonder if a more distributed approach might be more effective - rather than trying to build one giant emitting device capable of throwing enough energy at a target potentially several km away, a group of (relatively) smaller devices all throwing energy at it might be more effective, much like killing cancer cells by many smaller doses from different angles rather than one singular large dose along one angle.
There are various hand held options already available on the market but I doubt they can really be effective against anything beyond garden variety quadcopter type drones at a few hundred feet, especially when they run off self-contained rechargeable batteries and aren't much bigger than a battle rifle...