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typical broadband arrangement covering
Channels 4 and 5. By changing
the length of the directors, reflector,
and driven elements it is possible to
cover other channels. As mentioned
before, each driven element is made
up of two different thicknesses of
tubing, spaced and centered to get
300 -ohm impedance match.
A somewhat different approach to
Fig. 10. Driven reflector
(quadrature dipole) array.
\
1t
Fig. 8. Zigzag
antenna.
the broadband yagi depends on spacing
and transposed transmission line
between the two driven elements for
good gain and impedance match. This
results in yagi performance over as
much as four low -band channels with
fairly constant gain.
The characteristics of the yagi antenna
show that its main advantages
are in apparent gain and narrow reception
angle. Its relatively narrow
bandwidth makes it usable only for a
few stations at best, and because of
its directivity it is only useful when
all the desired stations lie in the same
direction. One feature which is sometimes
desirable is the fact that it has
very little side and rear pick -up and
unwanted reflected signals can therefore
be eliminated. The gain of a
single yagi can be expected to range
between 8 and 10 db with approximately
2 db increase for each additional
unit in a stacked array. Unless
special means are included to provide a
300 -ohm impedance match, matching
stubs must be used. The author once
measured impedances of classic 3 -, 5 -,
and 10- element yagis and found them
to range from 26 to 8 ohms and all
the way to 1.8 ohms for a 5 element
double stacked array. This clearly
shows that unless proper impedance
match is obtained at the antenna, the
loss in power transfer can completely
cancel the apparent gain of the antenna.
Special Antenna Design
In addition to the antenna types
described, certain manufacturers have
developed still different varieties. Because
their use often is indicated by
local conditions and because of their
special features we shall here describe
some of the most recent types.
Zig -zag antenna: Shown in Fig. 8,
this antenna consists basically of a
dipole and a series of parasitic elements.
The length of the various
elements will determine the bandwidth
and directivity of the entire
unit. The particular model shown in
Fig. 8 is capable of gains similar to
a broadband yagi, but its directivity
is not as sharp. Stacking to achieve
more gain is not recommended, rather,
the gain depends on the total number
of elements used. Stacking is used,
however, to connect high- and low -
band arrays together. An impedance
of 300 ohms is obtained by locating
the transmission line connection at
the proper distance from the center
of the dipole elements.
Colinear array: Where very high
Fig. 11. Trombone
antenna.
gain is required on a single channel
and fairly good reception on several
others, the antenna array shown in
Fig. 9 will fill the need nicely. Depending
on the phasing and length of
harness of the four stacked dipoles
and their reflectors, apparent gains
up to 15 db can be obtained for a
single channel in the high band. The
gain and impedance match as well
as directivity on the other high -band
channels are not as high and an
average of about 6 db gain prevails
in the low band.
Trombone antenna: An extension of
the "V"-type antenna is found in the
type shown in Fig. 11. Effectively,
the antenna contains four driven elements,
tuned to the high portion of
the low band, as well as the high
band. Gains of this antenna vary
greatly with frequency with the highest
gain of 8 db around Channel 10.
One novel feature of this antenna is
its good performance on some u.h.f.
stations. This is due to the fact that
the major elements are a multiple
wavelength at those frequencies. Directivity
of the trombone antenna is
quite good and its 300 -ohm impedance
matches regular twin -lead without
difficulty.
Quadrature dipoles: Shown in Fig.
10 is a broadband, high -gain antenna
providing good directivity and impedance
match at all low channels. The
unusual feature of this array is the
fact that the reflector elements are
not passive, but are connected to the
colinear dipoles. This greatly increases
the gain of the system. The
upper dipole and reflector are tuned
to the high end of the band, while
the lower resonates near Channel 2,
providing the required broadband re-
sponse. Directivity of this arrangement
approaches that of a yagi, and
further gain can be obtained by stacking
several bays. Stacking bars, tuned
and located for 300 -ohm impedance,
are used to match the antenna to the
line.
Electronic beans, rotation: Most
readers are familiar with the use of
rotating motors to orient antennas
where weak signals are received from
different directions. In most of these
installations the antenna has a very
narrow reception angle and is rotated
accurately for best pictures. Occasionally
it is found that reception
from different directions does not require
such a narrow angle, especially
when no reflected or interfering signals
appear from side or rear locations.
For such installations an electronically
rotated antenna, one example
of which is shown in Fig. 12,
can be used. Basically a conical antenna,
this unit features three conical
parts displaced by 120 degrees, each
connected by a separate line to a
switch at the receiver. This permits
the viewer to select any two parts as
his conical antenna. Broadband reception
with good gain on all channels
and good impedance match are usual
features of conicals and this type is
no exception. One feature of this
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