QY4 ANTENNA DESIGN PROGRAM DOWNLOAD
This article below was written
by WA7RAI and will serve to introduce you to a computer program for
designing Yagi antennas that you can download.
Introduction to QUICK YAGI.
Yagi Designer / Analyzer / Optimizer
YAGI DESIGN BASICS
The yagi antenna has been with us for more than half a century now, but
only with the recent advent of the personal computer has its real potential
begun to be explored by radio amateurs. Hopefully, this software package
will contribute to continued exploration.
The following text contains some basic yagi design concepts aimed toward
the non-technical user who may need assistance in this area.
The yagi antenna's basic design is a "resonant" fed dipole (the fed dipole
will be referred to from here on as the driven element ), with one or more
parasitic elements. These parasitic elements are called the "reflector"
and the "director." A dipole will be "resonant" when its electrical length
is 1/2 of the wavelength of the frequency applied to its feedpoint.
A: THE DIRECTOR
The director is the shorter of the parasitic elements. It is resonant some-
what higher in frequency than the driven element, and its length will be
about 5% shorter. The director length can vary by a considerable amount,
depending upon the director spacing, the number of directors used in the
array, the desired pattern or pattern bandwidth, and element diameter. The
number of directors that may be used are limited only by the physical size
of the array. The parasitic director is used primarily to achieve direct-
ional gain. The amount of gain is directly proportional to the length of
the array and not by the number of directors used. The spacing of directors
can range from .1 wavelength to .5 wavelength or greater and will depend
largly upon the design criteria of the array.
B: THE REFLECTOR
The reflector is a parasitic element that is placed to the rear of the
driven element. Its resonant frequency is lower, and its length is approx.
5% longer than the driven element. Its length will vary with the spacing
and element diameter. The spacing of the reflector will be between .1 wave-
length and .25 wavelength. Spacing will depend upon the gain, bandwidth,
F/B ratio, and sidelobe level requirements of the array design.
The impedance of an element is its value of pure resistance at a feedpoint
plus any reactance, either capacitive or inductive, that is present at that
feedpoint. Our primary concern here is the impedance of the driven element,
as this is where the transfer of rf energy from the feedline takes place.
Maximum transfer of rf energy occurs when the impedance of the feedpoint is
equal to the impedance of the feedline. In most cases, feedline impedance is
50 ohms, but rarely is the feedpoint impedance of a yagi 50 ohms. In most
cases it will vary from approximately 40 ohms to around 10 ohms, depending
upon the number of elements and the pattern bandwidth. If the feedline
impedance does not closely match the feedpoint impedance, the driven element
cannot effectively absorb the transmitted energy, thus reflecting it back to
the feedline (SWR). For this reason, impedance matching devices are highly
recommended for optimum performance.
The impedance bandwidth is the range of frequencies above and below the
center design frequency that the driven element's feedpoint will effectively
accept power from the feedline. It is desirable to have the reactance at
the center design frequency of the yagi be nil (j +- 0). This will allow
the impedance matching device to operate at its optimum bandwidth. Also wide
element spacing, large element diameter, wide pattern bandwidth, and low "Q"
matching systems will all contribute to a wider impedance bandwidth.
The radiation pattern plays a major role in the performance of the yagi
antenna. The directional gain, front-to-back ratio, beamwidth, and unwanted
sidelobes combine to describe the radiation pattern. The radiation pattern
bandwidth is the range of frequencies above and below the design frequency
in which the radiation pattern remains consistent. The degree of non-
consistency that can be tolerated is subjective, and limits such as minimum
front-to-back ratio and sidelobe levels are mainly a matter of choice.
Equal spaced / equal length directors may give higher gain at a particular
frequency, but the bandwith is narrow and unacceptable sidelobe levels are
common, and while wide spacing will increase the bandwidth, the sidelobes
become quite large.
By varying both the spacing and director lengths (many successful combin-
ations are possible) the pattern and the pattern bandwidth may be controlled.
More directors within a given boomlength will not increase the gain by any
large measure, but will allow better control of the pattern over a wider
By reducing the length of each succedent director by a set factor, while in-
creasing the spacing of each succedent director by another factor, a very
clean pattern with a good pattern bandwidth can be obtained. The trade off
will be a small reduction in the optimum forward gain (10% to 15%).
III GAIN vs FRONT-TO-BACK RATIO
The subject of gain vs front-to-back ratio can be related to the adage about
"having your cake and eating it too," which is to say that both cannot exist
at the same time. At the point of highest forward gain the main lobe becomes
narrower in both the elevation and azimuth planes, and a backlobe is always
present. When this backlobe is suppressed, the pattern becomes wider and the
forward gain decreases. In some cases, the sidelobes become quite large.
IV FEEDING THE YAGI
There are a variety of ways to feed the yagi, but they may be condensed into
two basic categories; the balanced feed and unbalanced feed.
The balanced feed system may give a broader impedance bandwidth, but the main
problem is that the driven element must in most cases be split in the center
and insulated from the boom. Construction considerations aside, it is the
better of the feed systems. Meeting the requirements of a balanced matching
system is usually the main problem, but there are many methods available.
One method is to not split the driven element and use a "T" match, which can
be described as two gamma matches on each side of the center of the element,
fed with a 1:1 balun at the center. The main drawback is that it's difficult
Another method (for low impedance feedpoints) uses a split element insulated
from the boom, and is fed with a "down-step 4:1 balun" made by combining two
1/4 wavelength sections of coaxial feedline in parallel, attaching an equal
length of insulated wire to the outside of these sections, and connecting it
to the center conductors at the feedpoint end and to the shields at the feed-
line end. The impedance of this type of "balun" should be at or near the mid-
point value between the feedpoint impedance and the feedline impedance. For
example, two 75 ohm sections paralleled will equal 37.5 ohms and will match a
25 ohm feedpoint to a 50 ohm feedline with a 1.0 to 1 SWR.
The most common method in use by hams today is the gamma match. It will
provide an easy and sure method of matching to the feedpoint without any
loss of bandwidth. Run QYUTIL.EXE for gamma match construction details.
Further information on antenna design and feed systems may be found in The
Radio Amateurs Handbook, The ARRL Antenna Handbook, Dr. J.L. Lawson's Yagi
Antenna Design (ARRL), or Bill Orr's Radio Engineer's Handbook, to name
only a few.
(c) 1990,94 by RAI Enterprises All rights reserved Download the zip file here