Technical Information


Propagation Generally

Propagation is the ability of radio waves to travel beyond line of sight. Generally, this means through either ground wave, curving over the surface of the Earth, or sky wave, "skipping" up and down between the Earth's surface and the ionosphere. Skipping may manifest itself by the waves bending as they encounter different layers of the ionosphere (most likely on the lower frequencies) or by other factors, such as meteor scatter, tropospheric scatter, or troposhperic ducting (more common on higher frequencies). With respect to ionospheric propagation, the longer the wavelength, the greater the bending for a given degree of ionization and the greater the absorption as the wave moves through an ionized layer; absorption also increases with intensity of ionization and density of the atmosphere. The better the "propagation" the more likely one is to hear — and contact — distant stations.

Refractivity and Absorption

The ability of the ionosphere to reflect (or bend) a radio wave is referred-to as the "refractive index," which measures the ability of a substance (say, for example, the ionosphere) to bend a light or radio wave: with a low or zero index the wave passes through the substance with no or little bending, while a substance with a higher index bends the wave more as it enters until the wave does not enter at all and is instead reflected. This index is often combined with the second element of propagation, absorption, in the "complex index of refraction." This incorporates the substance's tendency to absorb the radio wave. So there are the two elements.

In general, as the radio frequency goes up, absorption and refraction go down. At broadcast AM and amateur 160 meter frequencies, absorption and refraction are very high. During daylight hours, the radio wave travels along the ground, reflected by the lowest levels of the ionosphere; at night, the wave may travel many thousands of miles, in multiple paths, reflected from the higher layers (and these multiple waves make themselves heard through the fading in-and-out of the signal as various paths, arrive at the receiver going in-and-out of phase. By the traditional shortwave bands and the 80 and 40 meter amateur frequencies, daytime refractivity has dropped to the point that the wave is fully absorbed before it does any degree of bending, but with the lifting of the ionosphere at night the sky wave can develop. Twenty and ten meters can produce a sky wave during the day as absorption has dropped low enough for the wave to not be absorbed before the wave can bend.

It is important to note that low frequencies bend but are attenuated heavily, while higher frequencies are less attenuated but are also less prone to bending; therefore, use of the highest frequency that will propagate (i.e., bend) will provide the best signal due to its lessened absorption.

The most effective means of communications is to use the highest frequency that will propagate while not being absorbed which is known as the "Maximum Usable Frequency" (MUF). This provides for the highest frequency that will reflect with the least amount of absorption (higher frequencies will not produce a sky wave, while lower frequencies are more highly absorbed). This principle will maximize propagation while minimizing signal loss due to absorption.

There are three principal layers of the ionosphere: the D-layer is the lowest, almost totally absorbing the low bands during the day when its ionization is proportional to the height of the sun — only higher angles of radiation can pass through this layer to the higher layers; the E-layer, at about 70 miles, only holds ionization when in continuous sunlight — being greatest at local noon, giving maximum single-hop distances of about 1250 miles — and disipates nearly totally with sunset; the F-layer, at approximately 175 miles at night, slowly decreases in ionization after sundown, reaching its minimum just before sunrise — when it has split into two layers at about 140 and 200 miles — it gives maximum single-hops of about 2500 miles.

Here follows a description of how these elements manifest themselves during the course of a typical day.

The D-layer, the lowest level of the ionosphere, is heavily ionized during daylight hours; most so directly under the sun. The E-layer is also heavily ionized during daylight. On the low bands, such as the a.m. broadcast and the 160 and 80 meter amateur bands, signals beyond the local area (the ground wave) are totally (more or less) attenuated as they strike the D-layer: there is local reception only. At sunset, the D- and E-layers dissipate (pretty-much immediately — sometimes even before sunset); The F-layer holds and propagates on these frequencies now that the waves can reach it, freed of the absorption problems of the lower layers. Although the F-layer splits (into F1 and F2) and dissipates overnight, it will hold on these low frequencies all night long, continuing propagation until the E- and D-layers reform after sunrise.

The higher HF bands, the 40, 20, 15, and 10 meter amateur bands, for example, will not be absorbed by the D- and E-layers during the daylight hours even though these layers are heavily ionized: the F-layer (and the E-layer) propagate. At sunset, ionization decreases across all layers: the highest frequencies drop first and cease propagation. The F-layer continues dissipating overnight, with propagation degrading into the lower bands. The highest frequency that will propagate continues to drop until sunrise and the reformation of ionization.

Ionospheric propagation based on the routine daily cycle virtually ceases above the 6 meter (50 MHz) amateur band; however, there are modes that occur beyond the daily cycle (see below) that may extend propagation into the GHz range.

Propagation on the frequencies at and below the a.m. broadcast band are mostly of the nature of ground waves, which, on those frequencies, will follow the curve of the Earth. These frequencies comprise some maritime stations, aeronautical beacons, and long-range navigational and military communications facilities, not generally of use to the amateur.

Gray Line

An interesting element to this pattern of propagation is called "gray line." The gray line, also sometimes referred to as the terminator, is the line of sunset/sunrise around the world. Along the gray line, extremely long signal paths are possible. One major reason for this is that the D-layer, which — as noted above — absorbs HF signals, disappears rapidly on the sunset side of the gray line, and it has not yet built upon the sunrise side; meanwhile, the F-layer is heavilly propagating. The gray line is not a simple north/south line running around the globe from pole to pole across the equator; rather, it presents numerous east/west possibilities; for example, in mid-summer, it is possible to have a gray-line path from North America to Europe, specifically, from evening in central North America to morning in eastern Europe. This is in fact logical, as North America is about six hours ahead of Europe, so late dusk in Chicago (about 9:00 p.m.) would be early morning twilight in Rome (4:00 a.m.).

Critical Hours

While not Gray Line per se, this is a somewhat similar phenomenon. The FCC regulation Section 73.187, "Limitation on daytime radiation" (also called the "critical hours" rule) [47 CFR 73.187], addresses the issue of AM broadcast-band stations with daytime-only licenses operating in the two hours immediately following sunrise and immediately preceding sunset. The FCC states that

For AM broadcast stations, the term critical hours refers to the time periods of sunrise to two hours after sunrise, and two hours before sunset to sunset. During these periods, the ionosphere has commenced its transition from daytime to nighttime conditions (or vice versa), resulting in greater coverage than would be expected from a daytime-only analysis. But because the transmitting station operates with its daytime power between sunrise and sunset, the extended skywave signal can be strong enough to interfere with other stations. This [is referred to as] daytime skywave phenomenon[.]

See AM Broadcast Critical Hours Calculations -- Section 73.187 (retrieved 3/2014).

This would correspond to the time immediately subsequent to or immediately prior to gray line periods in the morning and evening, respectively (one infers that gray line indicates that the sun is still below the horizon, while "critical hours" refers to the times when the sun is just above the horizon). This is, in effect, the trailing edge of the gray line in the morning and the leading edge of the gray line at night.


NVIS, "Near Vertical Incidence Skywave," is used to bounce an HF wave into the area between the ground wave and the normal first-bounce skywave. This is particularly useful in areas where VHF line of sight is not possible (e.g., mountainous areas) but are short of the first HF skywave. The signal is transmitted nearly straight up from a near-to-the ground dipole, possibly supplemented by a reflector-element between the dipole and the ground. "Near-to-the-ground" is typically less than 1/8th wavelength above the ground, but greater than about six feet to prevent the ground from absorbing the signal (so 1/8th of 40 meters is 5 meters, about 15 feet).

In the days before mobile military VHF operations, this was often the only means possible for field units to communicate with nearby (but out of ground wave) units.

Ducting, Sporadic E, Meteor Scatter, Ionospheric Ducting, and Tropospheric Scatter.

Unusual circumstances may allow for atypical propagation, often allowing for frequencies not expected to propagate to do so. While not always applicable to Amateur operations, these occasionally create uncommon DX possibilities. With the exception of ionospheric ducting, these modes apply primarily to VHF, UHF, or both.


Often weather related, ducting occurs when adjoining air masses in the troposphere form what is, in effect, a wave guide, and "duct" (i.e., bend) signals throughout the VHF and UHF ranges. A typical cause of ducting is a temperature inversion, where a layer of warm air overlies cooler air below: this boundary layer becomes the waveguide, the top of the duct. Paths of up to 1000 miles may occur, and may persist for hours, days, weeks, or even longer.

Sporadic E

As the name implies, this is a "sporadic" event where scattered dense patches of heavily ionized clouds form in the lower E layer, typically offering propagation in the 25 to 100 MHz range, but known to extend throughout the VHF range. Sporadic E follows seasonal patterns, typically, in temperate regions, in the late spring, early summer, and early winter in mid morning and early evening, but can occur at any time (or not occur; thus, "sporadic").

Meteor Scatter

Unpredictable at best, meteor scatter relies on the ionization created by meteors entering the atmosphere, reflecting signals, which is — predictably — rather rare. The ionization is of relatively short duration, but can provide paths in the 30 to 50 MHz range. As meteor activity is generally unpredictable (except for periods of specific meteor showers), this means of propagation typically relies on continually attempting a transmission (for example, monitoring a path by repeatedly transmitting a test signal, then transmitting data in a burst when the test signal indicates the path as active). Software, such as K1JT, would typically be used to conduct such transmissions.

Ionospheric Ducting

Somewhat arcane and still largely unexplored, ionospheric ducting (with two characteristic elements of chordal hopping and inter-layer ducting) involves ionospheric modes that do not skip back to the earth's surface.

Chordal hopping involves the wave skipping along the bottom of the F layer without ever being bent enough to leave the layer's immediate surface area, so that the wave — travelling in a straight line — continually encounters the F layer. Inter-layer ducting occurs when the wave skips between the E and F layers. Without the earth-return there is less energy loss over a given path than paths that return to the earth.

(See, for example, Marcel H. De Canck (ON5AU), Ionosphere Properties and Behaviors — Part 3 [retrieved 6/2014]).

Tropospheric Scatter

At the upper ranges of UHF (2 to 3 GHz), using high power, multiple simultaneous paths, and very sensitive antennas, tropospheric scatter permits one-hop communications over hundreds of miles, although signal loss is extremely high. Paths of over 1000 miles are possible (e.g., Kaua'i to Midway, 1300 miles). This is not an amateur mode, but had multiple applications prior to the advent of more modern and reliable systems. [Author's note: I cannot think of any place where this system is still in use.]

An installation would consist of a transmitter in the range of 10 kilowatts; a transmit antenna (or antenna system) aimed at the tropopause (the boundary layer between the troposphere and the stratosphere) mid-way between the transmitting and receiving stations; and a receive antenna with 40 to 60 dB gain (and a very sensitive receiver). A 10 kilowatt transmitted signal could deliver 0.00001 milliwatts (0.01 microwatts) at the receiving antenna. These are dedicated facilities with large fixed antennas.

Military systems, which were decommissioned in the mid 1980s with the advent of satellite-based communications systems, typically utilized frequencies around 2 GHz and were reported to be able to maintain over 99% reliability over the signal path.

Attenuation and Propagation Chart

Frequency in MHz:
a.m. broadcast 1.8 3.5 7.0 14.0 21 28 50 . . .
Wavelength in meters:
600-200 160 80 40 20 15 10 6 . . .
This depiction of the relationship among frequency (wavelength), attenuation, and propagation shows that lower frequencies propagate (ground or sky wave) but are attenuated; higher frequencies are not attenuated but do not propagate, either; best choices are in the middle range of about 20 meters or otherwise the maximum usable frequency (MUF), which offers the most propagation with the least attenuation.
very heavily attenuated . . . . .
less attenuated . . .
. low attenuation . .
. . little to no attenuation . . .
heavily propagated . . . . . . .
. rapidly decreasing . . . .
ground wave sky wave . . . . .
. . . .
these frequencies work mostly in the evenings . . . day & night . . .
. these may propagate during the day. . .
. . .

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Revised: 9 June 2014
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