enough atoms are ionized in this way,
both the permittivity and permeability
of the ionosphere change enough that
it can refract a radio wave. If the wave
is bent enough to return to Earth, that
is sky-wave or “skip” propagation.
Ionization is created and
disappears each day as the Sun
illuminates different parts of the Earth,
forming into several distinct layers or
regions: D, E, and F as shown in Figure
5. (The F layer can further split into the
F1 and F2 layers during hours of peak
illumination.) The D layer is the lowest
at around 30-50 miles, with the E layer
just above it; the F layers are found at
Each layer has different properties
and creates different effects as
experienced on the ground. (There are
also seasonal effects due to the tilt of
the Earth’s axis.)
The lowest layer (D) acts mostly as
an absorber of radio energy. It is the D
layer that soaks up AM broadcast
signals during daylight and limits
reception to ground wave range.
Immediately after sunset, the free
electrons recombine with the ionized
atoms and the D layer disappears.
AM signals can then be bent back to Earth by higher
layers and long distance sky-wave reception becomes
possible after sunset.
There is a thriving community of “broadcast DXers”
The E layer — next highest in the
stack — is a bit of a mysterious place.
Too high to be dense enough for
absorbing RF and too low to create
reliable long distance communication,
its main claim to fame is for reflecting
regions called sporadic E or Es clouds.
These patchy regions of ionization are
thought to be created by wind shear,
compressing metallic ions and dust
particles into thin layers.
When ionized by solar UV, these
thin layers reflect signals back to Earth,
enabling contacts to be made over an
average of 1,200 miles. While your FM
receiver is on, you may hear stations
appear very quickly with good signal
levels, last for a while, then fade just as
quickly — that’s probably due to Es.
Hams operating at 50 MHz enjoy
“E skip,” dubbing those frequencies the
“Magic Band” for the highly variable —
but exciting — propagation far beyond
the usual range.
Finally, the F layers are where the
real long distance action is for
frequencies below 30 MHz — the
traditional shortwave bands. In the highly rarified upper
reaches of the atmosphere, gas density is very low and
the ionized regions may last all day before recombining at
July 2016 13
With the ionosphere so dependent on solar UV to
exist, it is strongly affected by events on the Sun. The most
significant long-term relationship is driven by the 11 year
( 10. 7 years, actually) sunspot cycle in which the number of
spots waxes and wanes. More spots equals more solar flux
equals better HF propagation equals happy hams and
shortwave broadcast listeners!
In the short-term, the Sun is a highly variable source
of propagation excitement. Solar flares and coronal mass
ejections occur nearly every day at scales from nearly
indetectable to massive. Coronal holes and the solar wind
send charged particles streaming away from the Sun.
When they encounter the Earth's geomagnetic field,
the fun really begins, creating the auroras and other
interesting phenomena that can enhance, disrupt, or even
shut down HF propagation entirely.
Numerous websites are dedicated to watching the
Sun and its effects here on (or rather, above) the Earth.
FIGURE 6. The ability of the ionosphere to return signals to Earth
depends on the signal frequency and the vertical angle at which the
signal encounters the ionosphere. At a sufficiently high vertical angle,
the signal cannot be bent enough and is lost to space. The critical
angle at a given frequency is the highest vertical angle at which a radio
signal of that frequency can be returned to Earth.
Graphic courtesy of the American Radio Relay League.
FIGURE 5. The ionosphere forms daily
at heights of 30 to 260 miles. Due to
variations in density, several different
layers are formed, each having a different
effect on radio waves. Graphic courtesy of the
American Radio Relay League.