Why do 1930s airplanes have circular antennas on top?

That's an “RDF loop”, which stands for Radio Direction Finder. You could use it to home in on a variety of radio sources and calculate the directional bearing of the aircraft to the radio source for navigation purposes (even commercial radio stations, if you knew the approximate location of their antennas, were fair game). read more

Using such a shielded loop antenna when rotated (by motor or hand crank), you would experience 2 bearings relative to the aircraft where the signal nulled out and disappeared - about 3 degrees wide - so you knew that the radio station was 90 or 270 degrees from your aircraft heading. read more

You have to be able to scan side to side at least a little bit, even for geosynchronous satellites. The satellite is not always directly overhead, and airplanes do bank occasionally, after all. There are a few designs in the works for nearly-flat antennas, but this is not really a big issue for large airliners. read more

If your plane is equipped with VOR receiving capability you will have a VOR antenna as well which is generally mounted on the tail. You will also have a UHF antenna for your transponder and DME, its located on the bottom of the plane . If equipped with an ELT you will also have an ELT antenna, generally on top. read more