Ionospheric Propagation explained

Below is a diagram of the earths upper atmosphere. The ionosphere is divided into several distinct layers as you can see.

Above 50 km to about 600 km (370 mi) is the ionosphere, notable for its 
effects on radio propagation. At these altitudes, atomic oxygen and nitrogen
predominate under very low pressure. High-energy solar UV and X-ray radiation 
ionize these gases, creating a broad region where ions are created in relative
abundance. The ionosphere is subdivided into distinctive D, E and F regions. 


Ionospheric Refraction
The refractive index of an ionospheric layer increases with the density of 
free-moving electrons. In the most dense regions of the F layer, that density 
can reach a trillion electrons per cubic meter (1012 e/m3). Even at this high
level, radio waves are refracted gradually over a considerable vertical 
distance, usually amounting to tens of km. Radio waves become useful for
terrestrial propagation only when they are refracted enough to bring them
back to Earth. See diagram below. 
 
Gradual refraction in the ionosphere allows radio signals to be
propagated long distances. It is often convenient to imagine the process as
a reflection with an imaginary reflection point at some virtual height above
the actual refracting region. 

The virtual height of an ionospheric layer is the equivalent altitude of a
reflection that would produce the same effect as the actual refraction.
The virtual height of any ionospheric layer can be determined using an
ionospheric sounder, or ionosonde, a sort of vertically oriented radar.
The ionosonde sends pulses that sweep over a wide frequency range, generally
from 2 MHz to 6 MHz or higher, straight up into the ionosphere.
The frequencies of any echoes are recorded against time and then plotted
as distance on an ionogram. 
The highest frequency that returns echoes at vertical incidence is known
as the vertical incidence or critical frequency. The critical frequency is
almost totally a function of ion density. The higher the ionization at a
particular altitude, the higher becomes the critical frequency.
Physicists are more apt to call this the plasma frequency, because
technically gases in the ionosphere are in a plasma, or partially ionized
state. F-layer critical frequencies commonly range from about 1 MHz to
as high as 15 MHz.
Back to 10m Repeater page
 Basics and early findings
The ionosphere plays a basic role in long-distance communication . 
Ionospheric effects are less apparent in the very high frequencies 
(30-300 MHz), but they persist at least through 432 MHz. As early as 1902,
Oliver Heaviside and Arthur E. Kennelly independently suggested the existence
of a layer in the upper atmosphere that could account for the long-distance
radio transmissions made the previous year by Guglielmo Marconi and others.
Edward Appleton confirmed the existence of the Kennelly-Heaviside layer 
during the early 1920s and used the letter E on his diagrams to designate
the electric waves that were apparently reflected from it.
In 1924, Appleton discovered two additional layers in the ionosphere, 
as he and Robert Watson-Watt named this atmospheric region, and noted
them with the letters D and F. Appleton was reluctant to alter this 
arbitrary nomenclature for fear of discovering yet other layers, so it 
has stuck to the present day. The basic physics of ionospheric propagation 
was largely worked out by the 1920s, yet both amateur and professional 
experimenters made further discoveries through the 1930s and 1940s. 
Sporadic E, aurora, meteor scatter and several types of field-aligned 
scattering were among additional ionospheric phenomena that required 
explanation. 

10m and the Ionosphere
The 10-m band is well known for extreme variations in characteristics 
and variety of propagation modes. During solar maxima, long-distance F2
propagation is so efficient that very low power can produce strong signals
halfway around the globe. DX is abundant with modest equipment. Under these 
conditions, the band is usually open from sunrise to a few hours past sunset.
During periods of moderate solar activity, 10 m usually opens only to low
and transequatorial latitudes around noon. During the solar minimum, there 
may be no F2 propagation at any time during the day or night. 
Sporadic E is fairly common on 10 m, especially May through August, 
although it may appear at any time. Short skip, as it is sometimes 
called on the HF bands, has little relation to the solar cycle and occurs
regardless of F-layer conditions. It provides single-hop communication
from 300 to 2300 km (190 to 1400 mi) and multiple-hop opportunities of
4500 km (2800 mi) and farther. 
Ten meters is a transitional band in that it also shares some of the 
propagation modes more characteristic of VHF. Meteor scatter, aurora,
auroral E and transequatorial spread-F provide the means of making contacts
out to 2300 km (1400 mi) and farther, but these modes often go unnoticed
at 28 MHz. Techniques similar to those used at VHF can be very effective
on 10 m, as signals are usually stronger and more persistent.
These exotic modes can be more fully exploited, especially during the solar
minimum when F2 DXing has waned. 

Back to 10m Repeater page