Propagation is something that every shortwave listener (SWL) around the
world can't get away from.
It is a topic not often brought up , and if it is , it is one that is
not widely understood by SWL'ers.
This article is an introduction to the basics of this extremely complex
subject and it is presented in a way that even the most newest SWL
should
understand.
Aspects of propagation that are not directly applicable to the
shortwave
spectrum or details on rare conditions are not explored deeply ,
leaving a
core of information that is applicable to all.
I will simplify some facts to make them easier to digest , and
hopefully at the end the reader will have a good foundation of what
propagation is and how it affects them.
gregW:-)
The Ionosphere and the Sunspot Cycle
The planet earth is surrounded by atmosphere.
At various heights above the Earth within the atmosphere are various
layers of gases.
The layer that has the most effect on shortwave listening (SWL) is made
up of gases whose electrons can become ionized when exposed to
radiation , it is
called the ionosphere.
As Ultraviolet and X-Ray radiation is emitted by the sun and travels to
earth on solar winds it affects the
electrons in this belt of ionosphere.
The amount of
ionization of the ionosphere varies with the
amount of radiation received from the sun.
During daylight hours the suns radiation is acting on the ionosphere ,
this is more effective in summer months than in winter times.
The majority of suns radiation activity varies depending on ¨The
Sunspot Cycle.¨
The Sunspot Cycle
Sunspot cycle's
lasts 9 to 14 years (averages 11) and during this
period
the sunspot numbers
rise and fall.
The ionosphere receives more radiation when there is more
sunspots
occurring on the surface of the sun.
Sunpots numbers vary from day to day , and yearly numbers vary within
this 11 year sunspot cycle.
Sunspot cycles also can vary in their maximum count from one cycle to
another.
At present (feb2006) we are at the lowest point of sunpsot activity in
this cycle.
Sunspots
The Layers and Structure of the Ionosphere
Often referred to as layers ,
they
are more accurately spoken of as regions , regions being made up of
groups of layers.
D Layer
The D layer is the layer closest to the surface of the Earth and is
at a
height of 50 km to 90 km and has mostly negative effects on radio waves.
It is responsible for absorption of HF
radio waves ,
particularly at about 10 MHz and below , with progressively smaller
absorption rates
as the frequency gets higher.
Absorption by the D layer is almost non existant at night and
greatest about midday , this is why higher frequencies are used for SWL
during the day.
This layer reduces its effect after sundown allowing lower frequency
signals to pass through it with less attenuation.
E Layer
At 90km to 120km above Earth is the E Layer.
Normally this layer only refracts radio waves having frequencies less
than 10 MHz.
When present the E Layer tries to absorb signals above 10 MHz.
At night time the E layer begins to disappear because the sun is no
longer ionizing it , this results in an increase in the height of the
layer.
The
increase in the height of the E layer increases the range and
which radio waves can travel by refraction
from the layer.
For most practical
purposes , the E layer is not very important for shortwave listeners.
Sporadic E Layer
The Sporadic E layer can reflect radio wave from 25 – 225 MHz.
Unusual conditions , often occuring during midday hours in the summer
months , create Sporadic-E events that may last for a few minutes or
several hours.
Communication using the sporadic E layer is not easily predictable ,
not reliable , and not used by international broadcasters on SW.
F Region
The F layer is 120km to 400km above the surface of the Earth and is the
most important layer in the ionosphere in terms of HF shortwave
listening.
At night the layer acts as one layer but during daylight hours it
divides into two layers, called F1 and F2.
These F layers are responsible for most of the skywave propagation of
radio waves , these layers easily refract radio signals on the side
of the Earth facing the sun.
The F2 region is the most important region for high frequency radio
propagation as:
• it is present 24 hours of the day;
• its high altitude allows the longest communication paths;
• it usually refracts the highest frequencies in the HF range.
The F region is
the most important group of layers for shortwave radio
propagation.
