Tropospheric EM propagationAt frequencies above about 30 MHz, the lower atmosphere bends radio waves towardsthe surface (Fig. 27-3). Tropospheric banding or tropo occurs because the index ofrefraction of air, with respect to EM waves, decreases with altitude. The effect is simi-lar to sound waves hugging the surface of a lake in the early morning, letting you hear aconversation a mile away. Tropo makes it possible to communicate for hundreds ofmiles when the ionosphere will not return waves to the earth.502 Data reception27-3The lower atmosphere bends EM waves toward the surface.Another type of tropospheric propagation is called ducting. It takes place whenEM waves are trapped in a layer of cool, dense air sandwiched between two layers ofwarmer air. Like bending, ducting occurs almost entirely at frequencies above 30 MHz.Still another tropospheric-propagation mode is troposcatter. This takes place be-cause air molecules, dust grains, and water droplets scatter some of the EM field at veryhigh and ultra-high frequencies (above 30 MHz).Exotic modes of EM propagationRadio waves can bounce off the aurora (northern and southern lights). This is auroralpropagation, and it occurs at frequencies from roughly 15 to 250 MHz. It can take placebetween stations separated by up to about 2000 miles.Meteors entering the upper atmosphere produce ionized trails that persist for sev-eral seconds up to about a minute; these ions reflect EM waves and cause meteor-scat-ter propagation. This mode allows communication for hundreds of miles atfrequencies from 20 to 150 MHz.The moon, like the earth, reflects EM fields. This makes it possible to communicatevia earth-moon-earth (EME), also called moonbounce. High-powered transmitters,sophisticated antenna systems, and sensitive receivers are needed for EME. Most EMEis done by radio hams at frequencies from 50 MHz to over 2 GHz.Receiver specificationsAny communications receiver, whether analog or digital, audio or video, must do certainbasic things well.