Can you make a microwave oven by taking a microwave generator and attaching one lead to one side of a metal box, and the other lead to the other side of the metal box, and putting food in the box and flipping the switch on? If this is possible, can you take a can of food and put two leads across it to heat the food? At such high frequencies, the can, can be modeled as a capacitor whose plates are the top and bottom of the can, and the sides of the can, can be modeled as inductors in parallel with the capacitor. So it's an ordinary LC circuit whose resonant frequency is also the frequency of the electromagnetic fields inside the can/cavity (the capacitor has its voltage and hence electric fields changing at the frequency of the microwave generator, as do the magnetic fields from the inductor sides). Or do you have to feed a microwave cavity by a waveguide? Why is there a waveguide at all - can't you just put the antenna inside the cavity, with direction the same as the electric field of the preferred standing wave patterns? Microwaves have small wavelength, so the antennas are short enough to not take any space. Also, with a waveguide don't you have to match the impedance at the end of the cavity, so as to not have waves reflected back, while you don't have that problem if the fields are antenna fed without guidance? Also, can one view a waveguide as basically just saying that you don't want to deal with having to have a return wire to your generator, so you're just going to beam alternating voltages with an antenna, and you deliver this voltage to a load by connecting the load to a receiving antenna? For some reason I have hard time believing this is efficient - can you really beam large amounts of energy like this to efficiently power a neighborhood, or are these things only used for communications and not for power?