When you think about building a radio transmitter circuit, the first thing that comes in mind is it requires too many analog components. But wait a minute, this guy demonstrates an AM transmission using a microcontroller. The interesting part is it uses a plant as an antenna for transmission. This project is based on Atmega324 microcontroller, internally clocked at 8 MHz, and it generates a modulated AM signal using internal timers and counters. It uses TC1 timer/counter to generate a PWM signal of approximately 540 KHz. This is the carrier frequency. You know that before AM transmission, the high frequency carrier signal is modulated with a low frequency message signal. In this case, the carrier frequency is modulated by varying the duty cycle of the output PWM from 5% to 50% using TC0 timer/counter.
The modulated output from the microcontroller has square waves. A simple RLC resonant circuit can suppress the higher frequency components and change the square waves into sine waves. Now it is ready for transmission. The author tested it with a plant as an antenna, and worked well. Visit the link below to see more details about the project.
For more detail: AM radio transmission using AVR using Atmega324 microcontroller