250mW AM transmitter. Should get you 1km or so of coverage. Lets start first with a little history. AM was the very first modulation technique used to transmit human speech over the air by modulating the amplitude of a continuous high frequency carrier according to the modulating signal unlike FM which modulates the frequency of the carrier. The very first transmitters used a carbon mic in series before the antenna thus changing the amplitude according to the momentum resistance of the carbon mic. This method introduced a lot of power to the mic which was unacceptable. Later it was discovered that modulating the carrier in a mixer stage was far more convenient and caused less distortion. At first spark gap oscillators were used instead of sinusoidal oscillators but that caused so much noise that the audio was indistinguishable behind the background noise.
Now a little about the basic characteristics. AM usually has a very narrow frequency response. Its suitable for human voice (300-3400Hz) and usually has a very simple transmitter receiver circuit. AM can usually cover more range than FM without any extra power boost but is quite susceptible to electromagnetic interference from lightnings and other RF transmissions which induce spikes in the amplitude (which is what carries the information in AM) thus hard distorting the original soundwave. Also there is a modulation depth issue that needs to be attended to because at depth levels over 100% the signal gets reshaped and distortions spike to the roof. The demodulation aka detection can be a simple envelope detector revolving around a single diode and a few filters.
Advantages of AM over FM: larger coverage without any substantial amounts of power, usually requires less spacing than FM (9khz opposed to 75Khz) which means more efficient use of the electromagnetic resources. Simpler circuitry.
Disadvantages of AM over FM: easily susceptible to interference which leads to more noisy reception. Requires larger antennas because it operates in the lower end of the RF bandwidth.
