Hello, I have recently had a lab experiment as part of my physics degree, to measure RF signals in the AM band (starting from around 650 kHz) and got inspired to build my own one, and hopefully a small one. I've done some digging and the only problem I am having is the Antenna. It looked simple at first, in the lab it was a simple ferrite rod with wire wound around it, but all of a sudden I see many antenna designs with requirements of hundreds of meters for the same frequency ranges.
In short, I need help in at least getting some idea of how an antenna works and which should I go with. Knowing maxwell's equations already, I can either detect the Magnetic field, or use the Electric field. One thing that boggles my mind is what exactly do I need the antenna to do? what kind should I be looking for if I want to make it short?