rf - Free and Open Source Software

and direction usually undergo change. Speed
may either increase or decrease; the change
of direction usually depends upon what
happens to the speed.
If the refracting medium has characteristics
which change gradually wit hin the
material, the speed and direction of the
refracted wave will also change gradually as
the wave proceeds in the material.
The ionized layer is such a medium; its
makeup changes - both from minute to
minute (and other periodic changes) and at
various points within the layer at the same
time.
Thus a wave transmitted from the earth
will be bent or refracted only sligh tly as it
enters the ionized layer, but the deeper it
penetrates into the layer the more its direction
is changed. When the original direction
has been changed enough to turn it around a
corner, the wave is moving out of the
material rather than in, and then the change
in direction becomes less the farther it
travels.
Eventually the wave will co me back out
of the layer, provided that the refraction
doesn't just happen to trap it completely
within the layer and bend it only enough to
keep it trapped. Even if this should happen
at some spot, there arc enough irregularities
in the layers that the energy would escape
elswhere - and such an action may be at
least partially responsible for some types of
fading.
As Fig. 2 shows, when the wave emerges
from the layer there is no way at all you can
_.- ---
---_.~----
""
/ ,
/ -,
/
.>
,
--
-- , "
.
..
,
/ "
,,~- --- ---------
~--
Fig. 2 - Refraction of radio wave in ionosphere
IS cause of apparent "reflection" of skip signals
as shown here. Since ionization level changes
gradually within an ionized layer. angle of
refraction is continually changing. This bends
wave back in new d irection, making it appear to
have been reflected from a su rface at somewhat
greater height (dashed line). Wave reaching
layer at shallow angle (dotted) does not penetrate
so deeply as one hitting at sharp angle
(solid); therefore it is bent less and so retu rns to
earth at greater range than difference of angles
alone would indicate.
determine that it wasn't simply reflected
from a sharp surface at a somewhat greater
height. This fictional reflecting surface's
height is what is referred to as the "virtual
height" of the skip layers.
The reason we know it works by refraction
rather than reflection is that the virtual
height of a layer appears to change with the
angle at which energy hits it. The sha llower
the angle, the lower the virtual height. You
can see from the dotted-line example in fig.
2 that this would be expected wit h refraction,
but not wit h reflection.
This mechanism in the ionisphere indicates
that the angle at which the signal will
be "reflected" depends critically upo n the
angle at which the signal arrives, and also
upon the condition of the ionized layer at
that particular time. High-frequency signals
packing more punch per photon, bore right
on through much more readily than do those
of lower frequency - so that as you keep
going u p in frequency. you find a point at
which the signal simply doesn't come back
down. Instead, it bores on out headed
toward outer space.
The angle at which the signal hits the
layer depends, in turn, upon the act ual angle
at which the wave leaves the transmitting
antenna. This depends upon the antenna
design, its height above electrical ground,
and the nature of the ground surface within
several wavelengths of the antenna site. The
lower the angle at which the signal leaves,
the more shallow will be the angle at which
it hits the refracting layer, and the greater
will be the distance covered before it returns
to earth.
Any substance which is capable of refracting
the wave can cause "reflection" by
refraction in this same manner. In addition
to the horizontal ionized layers which make
up the ionisphere, rf signals are frequently
"reflected" from the aurora borealis and
from the trails of ionization left behind by
meteors. At VHF, similar effects are caused
at the boundary between different layers of
air in the atmosphere.
Ho w Does Reflection Affect the Signal?
True reflection has virtually no effect upon
the signal, except that its phase changes
180 0 during the process of reflection. " Reflection"
by means of the refraction effect,
though, can affect a signal in many ways.
Reflection of VHF signals from the shimmering
veils of ionizati on which are known
to science as the aurora and to the general
public as " the Northern lights" offers several
examples of such effects.
The aurora is a rapidly moving affair. Its
106 73 MAGAZINE