Author's completed computer optimized 4 element Yagi
This article contains discussion of all the different antenna principles
previously described elsewhere on this website. A thorough reading and
understanding of the
other sections are necessary to comprehend all the terms used on this
page. It is not necessary to understand all the terms and theories
described to build and enjoy
this 4 element Yagi however. This particular section is geared towards
the "freeband" CB operator - that is one who uses CB channels above
(or below) the standard 40 CB channels. These CB "channels" are not
legal for use in the United States - but they are quite popular any
The frequencies mentioned in this article are legal CB channels in some
countries, check your local laws and act responsibly. This article
shines little light on finding
cheap aluminum or alternative methods of construction materials for Yagi
antennas. It is also devoid of information on the physical design of
Yagis. Sad but true,
useable aluminum tubing in small quantities is expensive. And the
physical design of Yagis is a subject beyond my limitations! What this
do is provide a truly optimized 4 element Yagi design for the 11 meter
"freeband" DX operator. Careful consideration was given the operating
frequency and "rejection" needs of most 11 meter freeband DXers (CB
operators who communicate with other CB operators more than 150 miles
Over the past 5 years, I have gotten many emails regarding the Yagi .
antenna. They seem to be more popular than the
other types of beams. I have built my share of Yagi antennas, but in the
last five years (since 1997) there has been tremendous amounts of study
done on the Yagi antenna using computer modeling. Since the desktop
computer has become so powerful, serious computer modeling of
antennas has been put in the reach of many amateur antenna experimenters
and this has lead to very optimized versions of the Yagi antenna. These
optimized Yagi's feature much "cleaner" patterns - this means the
overall forward gain hasn't increased significantly, but the Front to
Front to Rear ratios have really been improved. So trading your old
1970s Hy-Gain Yagi for a brand spanking new computer optimized Yagi
won't yield you a new monster signal - but will certainly increase your
ability to reject signals coming from unwanted directions.
Most times this is more important that forward gain itself because the
CB bands are so crowded that being able to reduce some offending
signal is more important (than forward gain).
A few words on computer modeling of antennas. Most amateur antenna modelers (like me) are using Numerical Electromagnetics
Code or NEC-2 for short. A complete history of NEC-2 may be found at http://www.nec2.org/.
NEC-2 is basically a computer program (or application if
you like) that is able to simulate the electromagnetic response of
antennas and other metal structures on the computer. This means you are
able to build a antenna virtually on the computer screen and see its exact
performance without even building the antenna. Imagine
being able to change antenna length with a few clicks of the mouse and
see its effect immediately! NEC-2 has a few limitations but is
considered highly accurate if the antenna is built exactly the way it was specified on the computer. By accurate I mean the computer predicted performance
of the modeled antenna closely matches the performance of the antenna if it was constructed and tested "in the real world".
The study of Yagi antennas on the
computer has produced some outstanding new Yagi designs and shed some
light on the key dimensions of Yagi performance. In the past, gain was
with the number of elements a Yagi had. Computer modeling has shown that
boom length is the real factor that determines gain. For instance,
merely shoving more
elements on a boom will have little to no effect on gain. The boom
length must be allowed to expand. A classic example of this is you find
many CB antenna that
place too many elements on a given boom length. For instance, you often
find makers of CB antennas cramming 5 elements onto a 20 foot boom. If
you study this
design on the computer, you'll find that 4 elements on the 20 foot boom
will work the same, if not better. Knowing this,
it becomes obvious that it is more important to consider the length of the boom,
rather than the number of elements a Yagi has, to gauge performance.
Front to Back has really improved as well. Typical
Front to Back ratios on the the "cut and try it" Yagi were about 15 dB -
20 dB (max) in the real world. A carefully designed antenna today
can see as much as 30 dB - 35 dB of "rejection" (CBer term for "Front to
Back ratio"). Some Yagis antennas have "corner nulls" (the back corners
of the beam
pattern) that can reach numbers as high as -70 dB! Computer modeling
also puts to rest the idea that Front to Back (F/B) ratio increases with
every element you add (commonly advertised by antenna manufacturers). In
fact - you can tune a 3 element Yagi to have as much Front to Back
as a 8 element Yagi! Gain usually goes up with a longer boom, but the
Front to Back ratio can be tuned up to about 30 dB on almost any
These days the CB band is really crowded. There are many guys out there
just jamming the channels - particularly 27.555Mhz (USB). The best
thing a serious DXer can do is to build a antenna with a very high Front
to Back (F/B) or Front to Rear (F/R) ratio to help knock down
these annoying station's signals (besides completely ignoring them).
This also serves to reduce multipath fading that occurs during a
DX contact. I set out to design
a antenna with the "cleanest" pattern, with the best F/R ratio I could
develop and cover from 27.405Mhz - 27.855Mhz with
with an optimum performance emphasis on 27.555Mhz . Most of the
offending stations I hear (on 27.555Mhz) come from due West, therefore I
designed a 4 element Yagi to have the greatest "rejection" to the west
when I have the antenna pointed towards the North East (see figure 1).
initial design was based on a 4 element Yagi for 24Mhz (12 Meter Ham
Band) from the 19th edition of the ARRL Antenna Book
(which also contains great detail on the physical construction of Yagi antennas). I remodeled this
antenna for 27.555Mhz using NEC-2, specifically using the computer software "NEC Win Plus", by Nittany-Scientific and
Multi-NEC, by Dan Maguire (Amateur Radio Operator, AC6LA).
Figure 1 Coming Soon...
The boom length is short - just under 14 feet long, which is short
compared to the common length of 16 feet most used for traditional 4
element Yagi for CB. To make construction simple, I ordered up a MaCo 4
element Yagi M104C
. For around 160$US (direct from MaCo), it represents a fair value.
You are getting everything you need, including the gamma matching
section, which is a pain to homebrew yourself. The stock MaCo 4
element is a pretty average performer out of the box. With around 9 dBi
of gain and a F/B around 20 dB, its performance is very typical - see
Figure 2 - The radiation pattern (in free space) of the stock MaCo 4 element Yagi on Channel 20
Frequently the freeband operator takes a beam antenna made for the
regular CB channels (26.965Mhz - 27.405Mhz) and uses the dimensions
straight from the owners
manual. Then the gamma match is adjusted for the best SWR on the
frequency of interest. This results in very poor
"rejection", because the beam antenna is being operated so far off its
designed center frequency (see Figure 3). This is one reason so many
people have resorted to
money making snake oil fixes on their beams, such as extra "rejection wires".
The correct fix is to rescale (actually remodeled) the antenna design
computer for the new center design frequency to insure a useable F/B
ratio over the new operating range. Figure 4 shows the performance data
for the M104C operating
on the "freeband".
Figure 3 - The radiation pattern (in free space) of the stock MaCo 4
element Yagi operating on 27.555Mhz. Since the antenna is
being operated so far off its center design frequency (27.205Mhz,
Channel 20) - the performance, primarily the "rejection", suffers
greatly. See the text for more detail.
Figure 4 - Gain, Front to Back and Front to Rear ratio of the stock MaCo 4 element Yagi from 27.405Mhz - 27.855Mhz.
Gain is up slightly, but the Front to Back ratio could be greatly improved.
With some adjustment of the boom length, element spacing and element
length this antenna is transformed into a very potent performer, due
to its new found ability to "reject" signals from unwanted directions
better. See new radiation pattern in Figure 5.
My NEC-2 computer optimized antenna, that I call the "Freeband 4",
sacrifices about .5 dB over the 16 foot boom length.
It is about 1 dB down over a 4 element Yagi on a 20 foot boom. The loss
in forward gain is barely perceptible signal wise (remember 1db is
roughly only 1/6th of 1
S Unit). The increase in rear signal rejection is amazing! "Back corner"
rejection in the real world lives up to the computer predicted values -
I can reduce my
neighbors 40 dB over S9 signal (that is "way, way in the red" on the
signal meter) down to nothing! The shorter boom length is a bonus too -
most antenna mounts
and rotors can handle a 13 foot - 14 foot boom. Its a mere 4 foot longer
than the common 9 foot boom length used for 3 element Yagis. This
about 1 dB of gain over the tradition 3 element Yagi on a 9 foot boom
and as previously noted, probably has a much more usable rear rejection.
Figure 5 - The radiation pattern (in free space) of my computer optimized 4 element Yagi. Notice the reduction in the
size of the rear lobe as compared to Figure 2.
Figure 6 - Gain, Front to Back and Front to Rear ratio of the my computer optimized 4 element Yagi from 27.405Mhz - 27.855Mhz.
I went for the deepest "corner nulls", which occur on 27.555Mhz. F/B peaks slightly below this frequency.
The taper schedule of this antenna must be followed exactly for the
performance to match the stated figures - "close enough" will yield
disappointing results. This is the biggest mistake made by computer
modelers - not accurately building the model as entered
on the computer. Also limitations in NEC-2 must be accounted for. This
antenna can not simply be lengthened for regular CB use
(26.965Mhz - 27.405Mhz) - the carefully designed pattern will be lost. I
haven't reworked the design for the standard CB channels yet. Drop
me an email if you are interested. See Figure 7 for the dimensions.
Figure 7 - The dimensions for a optimized 4 element Yagi, optimized
for 27.555Mhz. Performance is excellent from 27.405Mhz - 27.855Mhz.
The 2:1 Bandwidth with the gamma match tuned for a 1.0:1 SWR @ 27.555Mhz
is over 1Mhz. SWR is under 2:1 from 27.100Mhz - 28.200Mhz (real world
measured SWR made with MFJ-259B SWR Analyzer).
This antenna was design to be operated in the horizontal position
(producing horizontal polarization). All the above
paragraphs describing antenna performance are strictly referring to this
design in the horizontal position. This is the most common
polarization used by DXers . The pattern of this design in the vertical
position is shown in Figure 8. I display this pattern here just to show
the difference that occurs if the Yagi is operated in the vertical
position. No attempt whatsoever was made to optimize this antenna in the
position. You can see there is a *huge* change in the pattern when the
antenna is operated in the vertical position - this fact is rarely
pointed out in
books on antennas for DXing because they all assume the horizontal
position will be used. This is not always true on the
11 meter CB band. The 4 element Yagi in the horizontal position does
provide better all around F/R ("rejection") than it does
in the vertical position. Again, I wouldn't recommend you switch to my
design over your current design if you need to use it in the
vertical position - I can't promise better performance.
Figure 8 - The radiation pattern (in free space) of the optimized 4
element yagi in the vertical position. The pattern is
very different compared to the antenna in the horizontal position. The
overall rear rejection (Front to Rear ratio) is much lower in
the vertical position (this happens to all Yagi's operated in the
A final word about computer modeling of antennas. If you understand and
enjoy building antennas, computer modeling is the
next logical step for the experimenter to take. There are quite a few
references out there you should check out, the subject is covered
well on other websites, such as http://www.cebik.com/.