Long distance reception of FM broadcast signals occurs in many different ways. Here's a look at the main propagation modes:

Sporadic E-skip (Es)

This signal propagation mode occurs when a FM signal strikes a highly ionized patch of the E layer of the atmosphere and the signal is reflected back down to Earth. As a general rule, Es will reflect a signal 500-1,500 miles. On rare occasions, Es "clouds" will line up to provide a double hop, producing reception of stations 2,000-3,000 miles away.

Es can come in with rapid fades, and you'll often find 2 or more stations fighting it out for control of the frequency. Often you'll have stations in as strong as any of your local stations, with full stereo. The stations you receive via E-skip will generally come from the same area, but beware...Es clouds tend to move around. For example, DXing from the Midwest,you might have a cloud move so that your opening starts out in the northeast and winds up in Florida over the course of several hours.

The best way to keep tabs on Es is to check your open TV channels between channels 2 and 6. These channels will be the first to be affected by Es, as they are lower in frequency than FM, which falls between channels 6 and 7. As the activity of the Es cloud increases, so will the chances of the opening moving on up to FM

What causes Es? There are several schools of thought on the issue. The radiation level of the sun and the chemical makeup of the atmosphere directly affect it. And current research shows that the electrical activity surrounding thunderstorms may have the most profound effect of all.

Tropo (Tr)

Tho a much more stable mode of propagation, tropospheric bending/ducting (Tr), for the most part, lacks the distance characteristics of Es. Openings by Tr can run from 200 to 1,000 miles, and sometimes even beyond.

Tropo is directly related to weather, with the influence of a high pressure area required for it to happen. With a temperature inversion, with warm air meeting cold, a low level conduit forms in the tropospheric layer of the atmosphere that causes FM signals to travel hundreds of miles. It's most likely in the morning and evening, with September and October often the best months. It's not unusual for tropo conditions to last for several days over a wide area.

Meteor Scatter (Ms)

This mode of propagation is often the most frustrating, as the signals are so short-lived. It can affect the entire FM band, but it's best to park on an open frequency and see what happens. Ms usually will bring in signals in the 200-1,200 mile range. Most meteors that enter Earth's atmosphere burn up, leaving a visible trail. FM signals reflect off this trail, giving DXers usually anywhere from 1-10 seconds of audio. Obviously, it requires a lot of patience! During the big meteor showers, like the Perseids in August, you'll hear several signal bursts each minute.

Auroral Scatter (Au)

FM signals can actually reflect off the aurora borealis, with catches possible in the 300-800 mile range (and sometimes beyond). It's far more likely to be accessible to northern DXers. If you hear about a solar flare or increased solar activity, be on the lookout for Au. Signals by this mode tend to be slightly distorted.