The Amateur Radio Community lots of electronic and Radio frequency resources going back to Radio's start. Lots of innovation and cheap tricks. The EM Drives are specially shaped closed cavity resonators. Normal cavity resonators are widely used as the oscillatory systems of generators (klystrons, magnetrons), as filters, frequency standards and measuring circuits.The Amateur resources tend to be more plain language. They also cover all the electronics as well.
There are safety, interference, shielding and grounding issues. These high power Radio frequency systems can cause interference/damage to other devices.