AM: still used by several dozen pubcasters
AM radio uses the low frequencies where radio began, which have much longer wavelengths. While FM antennas are relatively small and mounted high on towers, AM’s longer wavelengths use the entire tower as an antenna, along with a network of underground wires that typically surround the tower in a circle as wide as the tower is high. Experts in the black art of AM facility design recommend that AM towers’ height be a precise fraction, such as one-quarter, of the station’s wavelength so that the tower will resonate with the frequency. Stations with lower frequencies tend to have higher towers. Old-timer KOAC in Corvallis, Ore.