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stage o f the game, it's a major problem!
That's what shielding is all about.
The antenna is the bridge between conduction
and radiation. At the transmitter,
the energy is conducted to the antenna, and
radiated from it. At the receiver, the antenna
picks up the radiated energy, and the energy
it receives is cond uc te d into the receiving
equipment.
So long as our rf energy is being conducted,
it follows most of the normal rules
which apply to de and low-frequency ac
(with a fe w exceptions such as skin effec ts).
When it is radiated, the special rules which
apply to radiated energy get into the ac t.
It might appear most logical to move
from here directly into our examination of
the bridge between conduction and radiation,
the ante nna. Ho wever, action of many
types of ante nnas involves the rul es of
radiated energy rather than those of cond uction,
and so is easier to comprehend wit h a
knowledge of these rules. For that reason
we'll examine the rules of radiated energy
next. Then we'll move on to look at the
antenna situation.
Ho w Does Signal R eflection Occur? Reflectio
n of a signal is just one of two effects
which occur when radiated energy meets
anything. To fi nd out how a signal is
reflected, we must examine the way in
wh ich radiated ene rgy interacts with anything
it meets.
It's easiest to understand by keeping in
mind that light is also radiated energy; any
rule followed by an rf wave must also be
followed by light, and any rule obeyed by a
light beam must also be obeyed by rt:
It's also important to keep in mind that
the rules which determine action of waves,
while simple enough in themselves, are at the
very heart of all modern physics. Most
engineering textbooks make no effort to
explain the rules - they merely state that
the rules are followed.
One volume which does attempt to explain
them in detail (Fields and Waves in
Modern Radio, by Ramo and Whinnery)
makes generous use of matrix algebra and
differential eq uations derived from Maxwell's
Equations to present the explanation.
But we're not afraid to take a chance on
oversimplifying a complex subject in the
interests of getting the main pa rt of the idea
across; we may make a few minor errors
along the way but in gene ral the following
explanation is how it works.
And you won't find the slightest trace of
mathematics in it, either.
A word of warni ng is in o rde r, however.
While the main idea is presented accurately,
don't get into any arguments with physicists
and cite this material as your reference. It
may not be all that accurate; in case of
conflict, believe the physicist!
If you're still with us, then, let's dive
right into just ho w "wave mechanics" and
"quantum theory" describe the interaction
of radiated energy and matter.
While nobody yet knows exactly what a
"wave" of radiated energy amounts to or
just how it manages to get from here to
there, a number of ideas and concepts (the
big brains call the "models") have . been
developed - and most of them seem to fit at
least parts of the needs pretty accurately.
One of these ideas, which is the basis of
q uantu m theory , is that a wave consists of
minute packets of energy called "photons"
and that the amount of energy per packet is
related to the frequency of the wave . The
higher the frequency, the more energy per
packet.
In this scheme of things, a light wave
packs more punch than does a radio wave,
and an X-ray has more punch than either.
The effects which we observe in wave s,
such as those of reflection, refraction, diffusion,
or scattering, occur only at the boundaries
whe re the wave moves from one substance
to anothe r. So long as a wave is
travelling in a single medium, whether that
medium is air, a sheet of plastic, g1-,SS, or the
unknown substance today's scientists call
merely "space" and the learned men of an
ea rlier era knew as the " aether", it can
produce no observable effect!
At the boundary which separates o ne
medium from another, though, one major
effect occurs. This effect shows up as two
distinct phe nomena - and it's only because
of them that we can tell that waves exist.
The effect which occurs is an interaction
between the wave's energy and the particles
which make up the medium; normally these
particles are atoms, but so metimes they are
molecules and in a very special case they
include electrons as well.
The particular type of interaction which
occurs depends upon the relationship between
the frequency of the wave and the
serf-resonant frequency of the particles involved.
Eac h of the particles of atomic or
molecular size does have a self-reso nant
frequen cy, and it's most co nvenient to think
of them as being tiny tank circuits exposed
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