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Just to complicate things, most aurorareflected
signals are very weak; very little of
the original signal is reflected to anyone
receiver. The signal-to-noise ratio is often as
low as zero db.
Somewhat the same situation prevails
when the trail left by a falling meteor
provides the refracting ionization. In this
case, though, the warble is absent. Any
Doppler shift is usually constant. Signal
levels, however, are much lower because the
refracting volume is much lower.
Scatter techniques depend more upon
true reflection, of the same type that makes
an oncoming automobile's headlights visible
over the rim of a hill on a foggy night. The
original signal is reflected in all directions by
tiny discontinuities in the atmosphere (trope
scatter) and ionosphere (ionospheric scatter).
Scatter transmission provides the most
reliable and consistent form of long-distance
rf communication, but requires power levels
greater than those allowed the ham by law
to attain reasonable distances with high
reliability. At amateur power levels, distances
are so short,
that most scatter signals
are thought to be "ground wave" instead.
All of these effects are present with all
radio frequencies, but their effectiveness
varies with frequency. At moderate frequencies
(15 meters and below) they are
usually overpowered by "normal" skip transmission.
In the VHF range they are most
observable, and many VHF operators specialized
in using one or more of these
techniques. As the frequency goes on up, the
amount of refraction becomes too small to
return a usable signal level and the effect
again appears to disappear.
How Is A Signal Radiated? The subject of
just how an rf signal can be propagated is a
'most profound one, and virtually all the first
installment of our previous Advanced class
study course (March, 1968, issue) was devoted
to it.
A few minor modifications to the propagation
model we put together in answer to
our question "How Does Signal Reflection
Occur?" can, however, offer some additional
insight into the subject.
As we explored reflection and refraction,
we discovered that a conductor cannot
refract a wave but must reflect it. At that
point, we declared that the energy had no
place else to go and so all the incident wave
went back out as a reflection.
If, however, the conductor happens to be
serving as an antenna, that statement was
NOT RETURNEO
---- - ------
--
- - -- -..:;:---
Fig. 4 - Both single and multiple-hop propagation
are shown here. Wave launched at original
angle for shortest skip range hits earth at
moderately steep angle and is reflected back
toward ionosphere. There it is refracted again
and comes back down at double the original
distance. "Skip distance" is distance from
transmitter to the minimum t-hop point; this
region is sometimes called the "dead zone".
Waves launched at higher angles fail to return
(except for Pedersen waves, see Fig. 3). Those
launched at intermediate angles fall between
minimum and maximum l-hop ranges.
not fully correct. The energy does have some
place else to go - down the feedline.
In the case of a transmitting antenna,
energy is coming up the feedline instead. In
either case, the system is no longer in perfect
balance.
When the frequency of the wave which is
exciting the antenna is such that a standing
wave can develop on the antenna structure
itself, it does so. This standing wave can be
thought of as the re-radiated wave from all
the surface particles. However, since the
standing wave maintains a perfect phase
relationship with the radiated energy, it will
couple with the radiation field and permit a
much more efficient transfer of the energy
itself.
What's more, the fields of the standing
wave will induce a current inside the conductor
- where the radiated wave itself
cannot get because of reflection at the
surface. This action is what moves the
energy through the totally-reflecting boundary
of the conductor.
If we're trying to radiate a signal rather
than receive one, we begin by pumping
energy into the antenna conductor at a
frequency at which the antenna has electricl
resonance. This produces a standing wave
upon the antenna, and this standing wave is
accompanied by a magnetic field which is
directly ' associated with current flow in the
conductor.
The variation in the magnetic field is
accompanied by a variation in the electric
field established between the ends of the
antenna conductor, and the phase relationships
between the magnetic and electric
fields which result are such that the "wave"
which they define is a travelling wave rather
than a standing wave.
108 73 MAGAZINE