Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/higyagi/hgants.txt

Text File | 1996-06-30 | 6KB | 129 lines

   A NEW CLASS OF DIRECTIVE ANTENNAS
       R. P. Haviland, W4MB

   Improve Yagi performance
   with curved 1.5 wavelength elements

    In the May. 1983 issue of Transactions on Anten-
nas and Propagation, Chang and Cheng introduced
a new class of antennas that appear to offer much
promise for VHF use. Based on concepts developed
earlier by F. M. Landstorfer,2 these antennas feature
curved elements, each longer than a wavelength and
shaped to compensate for the reversals in phase that
occur each half wavelength along an element.

     With the 1.5 wavelength elements in the classic
reflector-driven-director configuration used in the
original experiments Landstorfer claimed gains of 11.5
dBi. The same gain in a conventional Yagi using
straight half-wave elements would require about nine
elements and a much longer boom. While the new
design requires greater width, the combination of gain,
short boom length, and mounting simplicity form the
attractive features of the design.

 Principle of Operation

     The general concept and plan of these antennas is
shown in fig. 1. The center part of the elements
resembles a V radiator. The phase center of the V
radiation lies along the center axis, and some distance
from the apex of the V. A wave radiated from this sec-
tion will arrive at the other element parts after a time
a delay that corresponds to a phase rotation. As a result,
even though the outer sections are out of phase with
respect to the center section, the delayed wave will
be at least partly in phase with the waves radiated by
the outer sections. This addition of wave components
accounts for the increase in gain over a conventional
straight-element Yagi.

     The design problem presented by these antennas
is to determine the shaping of the elements for max-
imum gain. This subject was addressed by Chang and
Cheng in their article.' They approximate the current
distributions on the array elements by the method of
moments, dividing each element into 22 sections and
analytically determining the shaping for maximum
gain. The computations are extensive, involving a 63
by 63 complex matrix manipulation (a solution requires
approximately 40 minutes of DEC-10 computer time).
The problem is far beyond the capability of home
computers.

     Fortunately, Chang and Cheng have summarized
their results in such a form that makes it possible to
duplicate their optimized design for a three-element
Yagi array. For convenience, the results have been ar-
ranged as a computer program, fig. 2, written in
Simon's BASIC for the Commodore 64. The program
is written for easy translation to other versions of
BASIC; only the graphic generation section may re-
quire a complete rewrite.

     The program first determines whether hard copy is
needed, then requests its only input, the design fre-
quency. Element length and diameter are then out-
putted, followed by a table of X, Y values that define
the center-line position of each element. The feed-
point, or center of the radiator is taken as the coordi-
nate origin. Figure 3A shows the screen presentation
(the ending O's indicate that the end of the element
has been passed). Pressing the space bar produces
a plot of the lines defining the element centers, as
shown in fig 3B. Pressing the space bar again either
initiates a hard copy or terminates the program.

     The general resemblance of this type of antenna to
a conventional Yagi is apparent in the figures. The ele-
ment shaping causes a taper towards the forward
direction, even though the elements are the same
length. And the deep V of the director gives an effec-
tive wide spacing for the director.

     The performance of this optimized design is very
good. According to Chang and Cheng,l gain calcu-
lates to be 11.8 dBi. Beamwidth is 32 degrees in the
element plane, and 62 degrees at right angles to it.
Front-to-back ratio is just less than 15 dB in both
planes. Feed impedance of the 3/2 wavelength
radiator is calculated to be 14 + j33 ohms.

     It should be noted that the design values are opti-
mum only for the element diameter given. This was
arbitrarily set at 0.01 wavelength by Chang and Cheng.
Performance should not be greatly affected by a rea-
sonable change in element diameter.

     Because of the complexity of the required calcula-
tions, and the many hours of mainframe computer
time necessary to perform them, it is unlikely that there
will be much further analysis of the type. Further work
will have to be experimental. None has been attemp-
ted by the author, but it would seem that additional
gain could be secured by placing additional directors
of similar shapes in front of the present single direc-
tor, using appropriate spacings. It would also seem
that any of the common matching methods would be
usable. Stacking spacing rules of high-gain Yagi type
would appear necessary.

Conclusion
     Those who do not have a computer available, or
who wish to avoid the tedium of typing in the program,
can use these results by simple frequency scaling. All
table dimensions should be multiplied by the ratio,
147/new frequency, since the table was calculated for
147 MHz. Element diameter and length vary in the
same way.

            References

1. Chang-Hong Liang and David K. Cheng. "Directivity Optimization for Yagi-
Uda Arrays of Shaped Dipoles." IEEE Transactions on Antennas and
Propagation AP-31, Volume 31. No. 3. May, 1983, pages 522-525.
2. F. M. Landstorfer. "A New Type of Directional Antenna." Antennas and
Propagation Society International Symposium Digest. IEEE. 1976. pages
169-172.