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known that almost any spaci ng (within
reason) can be used, or alternatively alm ost
any tuning of element lengths. Once a
spacing is chosen, then the tuning of the
eleme nts must be matched to it ; if element
tuning is chosen first , then th e spacing must
be adjusted to obtain maximum performance.
In comparison to the other three types of
beam antennas, parasitic arrays offer the
highest performance per unit size. On paper
at least, you can get any desired gain from a
physically small parasitic array if you just
use enough elements and tune and space
them properly. In practice, the gain really is
limited - but you can get a IG-time increase
in effective radiated power from an antenna
only a half wavelength wide and a wavelength
long, which is much more than any of
the other types of beam can provide. For
this reason many engineers call such designs
" super-gain" antennas.
B OOST
Fig. 10 - Simplified buildup of pattern for
rhombic antenna is sim ilar to that for terrninated
V; lobes aimed in same direction boost
each other and all the rest cancel. Cancellation
is more co mplete in rhombic.
but is nowhere near that to be ex pected
from either a driven array or a parasitic of
similar size - because each leg of the
rhombic needs to be at least four wavelengths
long to get the directive effect.
You can also get some directivity from an
unterminated V. This is essentially tw o
long-wires side by side. The cancellation
effect still works to take out part of each
long-wire's pattern, but the resulting beam is
bidirectional with its major lobe splitting the
V angie as shown in Fig. II.
BOOST
•
\
Fig. 9 - Buildup of radiation pattern for terminated
V antenna with each leg 4 wavelengths
long is shown. Each leg of V by itself has
pattern of terminated long-wire (Fig. 6) ; legs
are placed at proper angle to make main lobes
co inc ide in one direction, and ca ncel out to at
least some degree in aU others.
The travelling-wave antenna, such as the
rhombic or the terminated V, gets its gain by
a cancellation effect also. As Fig. 6 showed,
a travelling-wave antenna is inherently unidirectional
- but puts its power into a cone
rather than a beam. If two such antennas are
erected side by side to form a V as shown in
Fig. 9. their patterns can be made to can cel
each other out in most directions while they
add together in just one and form asingle
beam of radiation. This is the terminated V.
If the terminations at the wide end of the V
are removed and another pair of antennas is
put in their place, with terminations at the
narrow end [Eig. 10), you have the rhombic.
Gain of such an antenna is moderately high,
Fig. 11 - In unterminated V antenna, lobes in
both directions boost each other but side lo bes
cancel out. Result is bidirectional beam, sim ilar
to that of broadside or endfire pattern from
driven array.
The resulting pattern is similar to that
you get fro m a simple driven array (Fig. 8).
A parasitic array, on the other hand, concentrates
its power essentially in a single direction,
as does the terminated V or the
rhombic.
The subject of antennas and how they
work is one of the most im portant in ham
radio, because nowhere else can you get such
an improvement in your station's performance
for a comparable amount of effort.
We'll be going into it more in our next few
installments, but even then we will not be
able to cover it com pletely - the subject is
just too large.
A number of books are available at
various levels of technical knowledge. The
traditional authority on the subj ect is " Antennas",
by J ohn Kraus, W8JK, inventor of
the 8J K beam, the corner reflector, and the
helical beam. Terman's " Electronic and
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