BASIC YAGI ANTENNA DESIGN FOR THE
non-technical, no theory user information for
Yagi Antenna Design
and How to Understand basic Yagi's
Condensed and re-edited from various
article is not designed to give you construction details for building a
Yagi. It is designed to give you a better basic understanding of how the
Yagi is designed and the basic functions of each of its
DESCRIPTION OF A YAGI
The word "Yagi" is used to
describe a type of antenna and
is credited to very famous Japanese antenna experts by the names
of Yagi and Uda! Most hams refer to this type of antenna as the
"Yagi" rather than use both men's
They discovered that by adding "elements"of various lengths
and spacings in front of and behind a dipole antenna that the
performance and effectiveness of the dipole could be greatly
increased and the pattern of the dipole rf energy could be "beamed" or
focused in one direction, with the resulting "effect" of making it appear
that the transmitter was running lots more power than it actually was,
yielding much stronger signals both on receive and
The Yagi antenna's overall basic
design consists of a "resonant" fed dipole (the fed
dipole is the driven element and in the picture
above and the second from the left side ), with one or more
elements are called the "reflector"and the "directors."
is on the far left in the picture above and the directors are all of the
elements starting from the third element from the left and continuing to
the right side of the picture.
horizontal section between all of the elements in the Yagi is called the
From experimentation, they determined that the "effect"
of their designs created much more "powerful" antennas compared to
the standard dipole by just adding a few more elements to it.
learned that by changing the space between the elements, and the
element lengths, that they could "tune" it to get various
results depending on what they wanted it to do. They found that they
could change the forward "gain" of it and also that they could change
the way it performed in other aspects.
ELEMENTS OF A YAGI
THE DRIVEN ELEMENT
The driven element of a Yagi is the feed point
where the feed line is attached from the transmitter to the Yagi to
perform the transfer of power from the transmitter to the
A dipole driven element will be "resonant" when its electrical
length is 1/2 of the wavelength of the frequency applied to its feed
The feed point in the picture above is on the center of the
The director/s is the shortest of
the parasitic elements and this end of the Yagi is aimed at the receiving
station. It is resonant slightly higher in frequency than the driven
element, and its length will be about 5% shorter, progressively than the
driven element. The director/s length/s can vary, depending upon the
director spacing, the number of directors used in the antenna, the desired
pattern, pattern bandwidth and element diameter. The number of directors
that can be used are determined by the physical size (length) of the
supporting boom needed by your design.
The director/s are used to
provide the antenna with directional pattern and gain.
The amount of gain is directly proportional to the length of
the antenna array and not by the number of directors used. The
spacing of the directors can range from .1 wavelength to .5 wavelength or
more and will depend largely upon the design specifications of the
The reflector is the element that is
placed at the rear of the driven element (The dipole). It's resonant
frequency is lower, and its length is approximately 5% longer than the
driven element. It's length will vary depending on the spacing and the
element diameter. The spacing of the reflector will be between .1
wavelength and .25 wavelength. It's spacing will depend upon the gain,
bandwidth, F/B ratio, and sidelobe pattern requirements of the final
BANDWIDTH AND IMPEDANCE
The impedance of an
element is its value of pure resistance at the feed point plus any
reactance (capacitive or inductive) that is present at that feed point. Of
primary importance here is the impedance of the driven element, the point
on the antenna where the transfer of rf from the feedline takes
Maximum energy transfer of rf at the design
frequency occurs when the impedance of the feed point is equal to the
impedance of the feedline. In most antenna designs, the feedline
impedance will be 50 ohms, but usually the feed point impedance of the
Yagi is rarely 50 ohms. In most cases it can vary from approximately 40
ohms to around 10 ohms, depending upon the number of elements, their
spacing and the antenna's pattern bandwidth. If the feedline
impedance does not equal the feed point impedance, the driven element
cannot transfer the rf energy effectively from the transmitter, thus
reflecting it back to the feedline resulting in a Standing Wave Ratio.
Because of this, impedance matching devices are highly recommended for
getting the best antenna performance.
The impedance bandwidth of the
driven element is the range of frequencies above and below the center
design frequency of the antenna that the driven element's feed point will
accept maximum power (rf), from the feedline.
The design goal is
to have the reactance at the center design frequency of the Yagi = (0),,,
(j + 0).
The impedance matching device will now operate at it's optimum
bandwidth. Wide element spacing, large element diameter, wide pattern
bandwidth, and low "Q" matching systems will all add to a wider impedance
ABOUT ANTENNA PATTERNS
antenna's radiation pattern or polar plot as it is sometimes called plays
a major role in the overall performance of the Yagi antenna.
directional gain, front-to-back ratio, beamwidth, and unwanted (or wanted)
sidelobes combine to form the overall radiation pattern. The antenna's
radiation pattern bandwidth is the range of frequencies above and below
the design frequency in which the pattern remains consistent.
amount of variation from the antenna's design specification goals that can
be tolerated is subjective, and limits put into the design are mainly a
matter of choice of the designer. "In other words.....trade
Equal spaced, equal length directors may
give higher gain at a particular frequency, but the bandwidth is more
narrow and larger sidelobe levels are created.
Wide spacing will
increase the bandwidth, but the sidelobes become large.
varying both the spacing and director lengths the pattern and the pattern
bandwidth may be more controlled.
More directors within a given boom
length won't increase the gain by any great amount, but will give you
better control of the antenna's pattern over a wider range of frequencies
in the band of design.
If you reduce the length of each succeeding
director by a set factor (%), AND increase the spacing of each succeeding
director by another factor, a very clean pattern with good pattern
bandwidth can be obtained.
The TRADE OFF......will be a small loss in
the optimum forward gain (10% to 15%).
In a nutshell......when you make a change to one part of the
antenna, this changes the performance of another part.....all changes
interact with each other and the final performance!
With highest forward gain design, the main lobe
becomes narrower in both the elevation and azimuth planes, and a backlobe
is always present. When you design "out" the backlobe, the pattern gets
wider and the forward gain goes down. In some cases, the sidelobes become
A WELL FED YAGI IS A HAPPY
There are many ways to feed the Yagi, but they can
be condensed into two main categories:
The balanced feed and unbalanced
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.
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 to adjust.
The Unbalanced feed system:
Another method (for
low impedance feed points) 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 feed point 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 feed point impedance and the feedline
For example, two 75 ohm sections paralleled will equal 37.5
ohms and will match a 25 ohm feed point to a 50 ohm feedline with a 1.0 to
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 feed point without any loss of
DESIGNING AND BUILDING A YAGI FROM SCRATCH
USING THE COMPUTER!
W7RAI has a very nice DOS based Yagi design program that
can be downloaded for FREE!
It will save you all that tedious math and
the frustrations involved in designing a Yagi.
It will "design" a
multielement Yagi for frequencies up to 999mhz. You simply download it
from the link below, unzip, install it and run. You can tweak many of the
variables in it to design a Yagi that will perform well for you. There is
also a program within it that will design a gamma
Run QYUTIL.EXE in the QY4 PROGRAM from WA7RAI for the
gamma match construction details.
See more about the QY4 ANTENNA PROGRAM
SEE A 6 ELEMENT 2 METER SSB (144.250)
Further information on antenna design and feed systems, see 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.
Thanks to all who make Ham Radio antenna design
and experimentation a fantastic sub-hobby to Amateur
The ARRL Antenna Book!
EXCELLENT SOURCE OF ANTENNA THEORY, DESIGN AND
PROJECTS! EXPERIMENT! EXPERIMENT! EXPERIMENT!
Back to the Antenna Design
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