One of the key areas of any radio system is that part where the signal is transfered from the transmitter to the receiver. This involves the use of antennas or aerials to radiate the signal as an electromagnetic wave, and then there is the way that the electromagntic wave travels or propagates between the transmitting antenna and the receiving one. Thus antennas and propagation are key areas for any radio system
Check out these tutorials on the radio spectrum and radio propagation.
Key tutorials about transmission lines - what they are and key aspects about the performance and operation of transmission lines.
Learn about antennas and how they work in these tutorials and descriptions for building and installing them.