How does a radio transmitter work?

How does a radio transmitter work?

• i have built a simple low-power am radio transmitter for a science project from: http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p024.shtml i have got it working but I do not know how it works. if you can, please explain in relatively simple terms. thanks so much!

• Answer:

  The page you linked to is for AM (Amplitude Modulation) so I will describe AM transmission as simple as I can. The information you input to the circuit is modulated with a Carrier. Modulated means it is mixed/combined together, in simple terms. The Carrier is a higher frequency signal than your information. In this case the Carrier appears to be a 1MHz crystal. These two signals are seen on that page you linked too. The carrier is seen in green (1MHz) while the information is in red. The result of the modulation is a waveform (Blue output) with the carrier frequency (1MHz) and an "envelope" shape on the top and bottom of the waveform. The envelope is the name given to the shape on the top/bottom of the output waveform, hence the name Amplitude Modulation. If you look close, you can see that the information input to the circuit is maintained within that envelope shape! The top contains exactly the same info, while the bottom envelope has the inverse, or image, shape of the information. The math that explains the AM waveform is beyond the scope of this question so I'm not going to go into that. After the modulation occurs, the waveform can then be amplified (voltage) and sent out through an antenna.