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Re: radio propagation



on 17/4/01 1:31 PM, Ben Crowell at crowell01@LIGHTANDMATTER.COM wrote:

I was looking through some of my father's ham radio books
recently, and came across something I didn't understand. etc, etc

For propagation in the ionosphere one can, as in more conventional optics,
use wave theory (physical optics) or, under appropriate conditions, ray
theory (geometric optics). For wave theory it is necessary to use a complex
index of refraction but for many situations, as the existence of a "skip"
distance, which is Ben's main question, the easier ray propagation can be
used. The key factor for successful use of ray theory is that there should
be little change in the properties of the medium in the space of a (vacuum)
wavelength.

The vertical distribution of electron density in an ionospheric layer can be
described as varying as a parabolic function with height (Chapman layer), so
that the refractive index increases to a maximum and then decreases. If a
radio signal is launched vertically into the ionosphere, whether or not it
is transmitted depends on the frequency of the wave. If the frequency is
high enough it will penetrate. In the probing instrument known as an
ionosonde, the transmitted frequency is increased and the reflected signal
monitored .so that the frequency at which penetration occurs -the critical
frequency - is measured. This critical frequency is a measure of the
maximum electron density in the layer. By a little manipulation, the "true"
height of the maximum of the layer can also be got from the ionosonde
record.

The critical frequency is important in the determination of the skip
distance. So far I have been describing the situation for vertical
incidence. Let's move to oblique incidence of the transmitted wave. If the
transmission frequency is less than the critical frequency, then there is no
minimum skip distance; any range can be attained and there is a one-to-one
correspondence between angle and range.

If the frequency is greater than the critical frequency, then at steep
angles the rays penetrate the ionosphere - no reflection.

As the steepness decreases, we reach the situation where reflection occurs.
If the angle of incidence is theta, then when theta increases to reach a
value such that (incident frequency) / (critical frequency) cos (theta)
equals unity, refection occurs. At this angle, the refracted ray travels
parallel to the layer maximum and the propagation distance is infinity.

As theta increases, the propagation distance decreases to a minimum and then
increases until it becomes infinite again at theta equals 90 degrees.

So, above the critical frequency, for small angles of incidence there is no
reflection; above that angle there are two rays which lead to attainable
distances of propagation. -at the minimum distance the two rays merge into
one. The upper ray is called the Pedersen ray and the low ray is called -
wait for it - the low ray. Not surprisingly there is a lot of attenuation
with the Pedersen wave.

Of course there is a ground wave propagated and much radio reception of AM
signals is within the skip zone and so by ground wave propagation.

Brian McInnes