rf - Free and Open Source Software

----- ----
o
F[EO------- - - ---- - - - -,
0 -
-,
-
-
FH O
8 -4 NAv ELEN GTH T E R~INA T E D II,,,, T[,,,"I'"
- .
-
•
1
Fig. 5 - Difference in current distribution
between reso nant antenna (A) and terminated
or travell ing-wave antenna (B) creates differences
in radiation patterns. On resonant
antenna, current goes in different directions at
different points to create standing wave. On
terminated antenna, current flo w is all one-way.
f rom feedpclnt t o termination. Both antennas
are shown as being "current-fed" at maximum
current points.
individual particles in refraction. Those
which add up instead of cancelling become
travelling waves leaving the antenna.
In the resonant antenna, the separate
portions of the conductor are not necessarily
in phase with each other because the to tal
reflection at the ends of the antenna intrao
duces a 180 phase change, and they most
certainly are not of equal strength as radiators
since the current is not constant.
Mutual interference still operates to cancel
out most of the fields and leave a radiating
t ravelling wave - but the pattern is
different.
The most noticeable difference is that the
travelling-wave antenna is unidirectional
while the standing-wave antenna is not. This
is because the curre nt in the travelling-wave
antenna is flowing only one way, while in
the standing-wave antenna current is flowing
in both directions (out and back) at the
same time to create the standing wave.
Fig. 6 compares the directional patterns
for a terminated long-wire antenna (a travelling-wave
type) and for a resonant long-wire
of the same length.
A key point to keep in mind concerning
signal rad iation is that each individual small
part of any radiating structure, such as an
antenna, radiates with equal strength in all
directions. Its radiation pattern is essentially
a perfect sphere.
However, any radiating conductor which
has any length at all must be composed of
many such small parts, and each of them is
radiating in slightly different phase from all
the rest sin ce the exciting energy takes at
least a little time to get from one to another,
and phasing is time delay.
The result is that any possible (as
opposed to theoretical) antenna must have
some type o f radiation pattern, which is the
result of the interference pattern created by
the individual spherical patterns of its individual
parts. That's why we looked at
refraction and reflection first; the exact
same principle is involved in the creation of
the radiation pattern for any antenna, and as
we shall discover shortly is also involved in
our efforts to concentrate a signal in a
desired direction.
Fig. 6 - A - Radiation pattern of terminated
antenna fo ur wavelengths long is unidirectio nal
in general d irection of the wire, bu t has a null
directly off the wire's end. The pattern 's main
lobes make 26" angle with wire. Pattern is
symmetrical in three dimensions; consider this a
cross-section view of it looking down fro m top.
Fig. 6 - B - Resonant antenna of same 4,wavelength
length has this type of pattern; it's like
the terminated antenna's pattern with a mirror
image superimposed on it . Result has main
lo bes in both di rections, st ill with 26° angle a nd
symmetrica l shape. Bid irectional cu rrent flow
(Fig. 5 ) is d irectly responsible for this bidirectional
pattern.
Ho w Can a Signal Be Co ncentrated ? The
" isotropic" antenna, which doesn' t ex ist in
practice but is the basis of antenna theory ,
radiates any power applied to it with equal
strength in all directions. Its radiation pattern
is a perfect sphere.
110 71. M A~ A7 IN ~