David, below is some information on Yagi antennas that might be of interest
to you. -- Ron, K8HSY
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DIRECTIONAL BEAM ANTENNAS
The Monoband Yagi
Between 1926 to 1929, Shintaro Uda and Hidetsugu Yagi developed a beam
antenna that had sharp directivity and high gain. Later, work was done
primarily by Mr. Yagi and yagi was the name given to the antenna until
finally recognition was given to Mr. Uda. Its proper name is the Yagi-Uda
Array. Most hams call it a beam.
A monoband yagi is the name given to a yagi for a single band. The
performance of any commercially made monoband yagi is touted to have its
dimensions tuned for maximum performance. As you will see later, this is not
always the case. Monoband yagis being sold today are much improved over
older designs because of computer modeling programs available.
The yagi is made of two or more aluminum elements mounted on and
perpendicular to a boom. Hams use antenna rotors to turn the antenna in the
direction of the station they want to work. However, there are wire beams,
fixed in one direction, mainly on 80 meters, suspended between trees or
other supports.
Most high frequency beam antennas used by hams are in the horizontally
polarized configuration, which means the elements are parallel to the
ground. CB beam antennas and some two-meter beams are vertically polarized
with the elements at right angles to the ground. A 2-element yagi has a gain
around 3 to 4 dBd. A two-element yagi will have a driven element with either
a reflector or a director. The driven element is the only element receiving
power directly from the transmitter. The reflector and directors are called
parasitic elements because they receive power from the driven element by
inductive coupling. The 3-element yagi will have a gain of approximately 5
to 7 dBd or 7 to 9 dBi depending on its boom length. A three-element yagi
has one reflector, one driven element, and one director. Because the yagi
has a low radiation resistance, a matching system is located at the driven
element feed-point. The ratio of the radiation off the front compared to the
radiation off the back is called front-to-back ratio. Front-to-back ratio
and forward gain are factors to be considered in choosing a yagi design.
Both measurements are given in dB. All yagis have a good front-to-side
ratio, with the signal off the side being 50 dB below the front.
The reflector of a yagi is about 5% longer than the driven element. The
reflector, being longer, will have inductive reactance. The inductive
reactance shifts the phase of the re-radiated wave, which radiates and
combines with the driven elements wave and reinforces it in the direction
away from the reflector toward the driven element. A director is about 5%
shorter than the driven element. The director, being
shorter, has capacitive reactance, and this changes the phase of the
reradiated wave to reinforce the wave away from the driven element opposite
the reflector.
The gain of a yagi is derived from radiation being concentrated in one
direction at the expense of the other directions. One hundred watts fed into
a yagi with a gain of 6 dBd will have an apparent power of 400 Watts in the
main lobe. Because one hundred watts put into a yagi radiates only one
hundred Watts, and because that one hundred Watts of power is concentrated
in the main lobe, it is equal to the power from a dipole being fed with 400
Watts. This is referred to as effective radiated power or ERP, but a yagi
is not any more efficient than other antennas. Because of the Principal of
Reciprocity, an antenna having a 6 dBd gain on transmitting will also have a
6 dBd gain on receiving.
Adding more directors and increasing the boom length will increase the gain
of a yagi.
The front-to-back ratio ranges from 18 dB for a 2-element yagi to over 25 dB
for a multi-element yagi, provided the parasitic elements are carefully
tuned. The gain of a yagi is generally proportional to the boom length and
not necessarily the number of elements. Doubling the boom length, while
keeping the proper number of elements for that boom length, will add about
three more dB of gain. Tuning the yagi for maximum gain makes the bandwidth
very narrow, and it will have a poor front-to-back ratio. For these reasons,
we dont recommend tuning a yagi for maximum gain, because you will only
increase the gain by a fraction of a dB at the expense of front-to-back and
feed-point impedance.
Tuning the yagi for maximum front-to-back will help eliminate interference
coming from the rear of the antenna. The building of any yagi involves
compromise spacing and element tuning.
As you make the yagi larger by adding directors, the main radiation lobe
becomes narrower increasing the gain and ERP. The gain of a yagi with four
elements is about 7 to 8 dBd. You used to see 3 or 4 element yagis
advertised claiming a gain of more than 10 dB, but they never said if that
gain was referenced to an isotropic or a dipole. That gain also involves the
gain derived from signals reflected from the ground adding to the direct
wave. A more realistic gain figure is the "free space gain." Some
companies, who sell monoband yagis, inflate their gain figures. Beware!
Increased spacing of the elements will increase the gain of a yagi up to a
point. Increasing the spacing past that point will reduce the gain. The
spacing of a reflector or director needs to be in a range of 0.1 to 0.3
wavelengths. With a 3element yagi maximum gain occurs with both parasitic
elements spaced at about a quarter wavelength. Second and third directors
can have wider spacing. Most hams do not build yagis but buy them from the
many companies who sell them. Ham catalogs are full of pre-cut and tuned
yagis that come in boxes ready to be assembled in the back yard. Many of
these
are very good. However, there is a lot of satisfaction to be gained from
building your own.
In 1971, we purchased, a 15-meter monobander being sold by a reputable
company. Its performance was disappointing. It had only a 10-dB
front-to-back ratio. That design is no longer being sold. After reading some
books, we readjusted the antenna elements to some new dimensions and it
performed much better. This was the beginning of our yagi building. During
the last nearly 50 years, we built many yagis. During the period of 1979
until 1986, many multi-element yagis were constructed, gain measured,
formulas derived for spacing and element length, and the radiation patterns
plotted on graphs. In 1986, a computer program titled "Yagi" by Dean Straw,
N6BV, was bought. From that point on, that program was used to design and
set the element lengths to their proper values. Not much difference in
performance of the new designs was seen over what was previously used, but
tuning parasitic elements and running back and forth to the field strength
meter was eliminated. There are many better computer programs available
today for designing yagis and other antennas.
The largest yagis we built were a 4 element 20-meter yagi on a 38-foot boom,
a 5 element 15 meter one on a 27-foot boom, and a 5 element 10 meter beam on
a 24-foot boom. These are modest designs compared to some of the big
antennas used by contest stations. All these yagis were stacked one above
the other on a 20-foot mast coming out of the top of the tower. The 20 meter
one was on the bottom, next came the 15meter, and the 10-meter yagi was on
top. This method of stacking yagis for different bands one above the other
makes what is called a "Christmas tree array." These antennas worked well.
Since retiring and moving back home, we use pre-tuned directional antennas
because of the lack of a good place for an antenna range. Climbing is not
now an option because of age and infirmity.
If you make the reflector 5% longer than the driven element and the director
5% shorter than the driven element, you will be pretty much in the ballpark.
The beautiful part about a yagi is it will work reasonably well with the
element lengths only in the ballpark. By carefully tuning, you will get a
fraction of a dB more gain or a few more dB front-to-back, because the
spacing and diameter of parasitic elements affect the length required for
those elements. A yagi can be tuned for maximum forward gain, maximum
front-to-back ratio, or best impedance, but you can achieve only one of
these conditions at a time. Element tuning, at best, is a compromise.
Most hams who are yagi builders do not tune their antennas at all, but use
published dimensions for building them. Yagi builders who do tune, tune for
either gain or front-to-back and then match the driven element with a gamma
match, hairpin match, a series-resonant coax matching section, or a step
down balun. The feed-point of a properly tuned yagi is close to 25 ohms.
Since formulas for calculating yagi element lengths are readily available
from other sources, they will not be given here. Because yagi elements are
made from telescoping aluminum tubing, the elements will be tapered. The
diameter of the elements and the taper determine the lengths required for
tuning of the elements. A tapered element will resonate higher in frequency
than one not tapered. The formula to calculate the length of the tapered
elements is complicated, but there are computer programs to do that.
Trapped Multi-band Yagis
Some yagis have traps in the elements to make them into a multi-band beam.
Many of these commercially made antennas are available at ham radio stores
or directly from the manufacturers. In a 3-element, 3-band design, the
spacing on the booms is a compromise. A 3-band beam is known as a
"tribander." The spacing is close on 20 meters, optimum on 15 meters, and
wide on 10 meters. You cannot tune the trapped elements for maximum
performance on three bands simultaneously and have a good match on all those
bands. Since a good match is important to most hams, gain and front-to-back
ratio are sacrificed for a good match on triband beams. The inductors in the
traps load the elements in triband beams. Therefore, the elements are
shorter than
the elements of a 20-meter monobander. Regardless of the compromised design,
a triband-trapped beam is much better for working DX than a dipole. Many
hams have achieved working over 300 entities with tribanders having short
booms.
The radiation pattern from a yagi is at a lower angle than a dipole. This
gives the impression a yagi has much more gain than it does. A dipole has
unity gain, but that gain will be at a higher angle. The dipole puts out a
weaker signal at the low angles needed to work DX, and a yagi puts a strong
signal at low angles. In comparing a dipole to a yagi, the yagi may only
have a 4 dBd gain in its major lobe. The gain of the yagi at a low angle may
be 10 dB or so better than a dipole at that same lower angle. The gain of
any antenna is always measured in its major lobe, irrespective of where the
angle is in which the maximum radiation lobe occurs.
The front element is the director with traps for 10, 15, and 20 meters (it
takes two sets of traps to make the elements work three bands). Directly
behind it is the driven element with traps also for 10, 15, and 20 meters.
The rear element is trapped for 15 and 20 meters (a single set of traps
makes it work two bands). The entire lengths of the three longest elements
are resonant on 20 meters. The short element is a reflector for 10 meters.
Only the part of the antenna between the 10-meter reflector and the front
director is used on 10 meters.
Mosely builds trapped antennas that have two traps in one enclosure and you
can not determine the bands from the traps as you can on Hy-Gain and
Cushcraft beams.
Some triband beam models as the one above are built with longer booms so
they would have more gain on 20 meters, a good match on all bands, and
optimum 3-band performance. They achieve this by interlacing extra monoband
reflectors and directors on the boom placed between the 20-meter elements.
The extra elements have no effect on 20 meters or any band for which they
are not resonant. Some amateurs mistakenly think the extra elements work on
all bands, but they dont. The Cushcraft A-4 shown above is not a beam with
four working elements on any band. The old Hy-Gain TH6DXX and Mosley Classic
36 had six elements on the boom. They both had three trapped elements and
three monoband elements. They had three working elements on 20 meters, three
on 15 meters, and four elements on 10 meters. The trapped reflector worked
on 15 and 20-meters. The trapped driven element worked on all three bands.
The trapped director worked on 10 and 20 meters. On the boom was a resonant
reflector for 10 meters and one each resonant directors for 10 and 15meters.
When using one of them, we have often heard amateurs saying they were using
a six-element beam. This gave the other station the mistaken idea they were
working someone with an antenna with six working elements. Other beam
antennas interlace additional elements of different lengths to make the
tribander into a 5-bander covering 20, 17, 15, 12, and 10 meters. Hy-Gain
makes a 5-band yagi for 20
through 10 meters that has 11 trapped and monoband elements. It is the
Hy-Gain TH-11. Mosely makes a 6-bander that includes two elements for 40
meters. It is the Pro-67.
In order to achieve better SWR curves over a wide bandwidth, some triband
yagis have two driven elements spaced 3 to 5 feet apart. The front driven
element is shorter than the rear driven element. Both driven elements are
trapped. This double driven element scheme is called a log-cell. A log cell,
by itself, has a small gain and may slightly increase the overall gain of
the tribander. The KLM KT-34 and the
HY-Gain TH-7 are examples of this kind of antenna.
Is a monobander better than a tribander? We dont know if our tests can be
duplicated and no one else has ever said he has actually compared the two
antennas. It is "common knowledge" that traps have loss. Therefore, the ham
fraternity believes a monobander has to be better. From the tests we
performed here, we believe it is a myth a monobander is significantly better
than a tribander having an equal boom-
length. We believe the traps do not have enough loss to make enough
difference to matter. However, monobanders having very long booms and many
directors will outperform any tribander.
Having two towers, both having the same height and being 100 feet apart,
made it possible for us to do the experiment described here. The result is
useful information because it was made in a real world situation that would
be comparable to the average hams location. Both antenna element lengths
were set to Hy-Gain specifications. The constants were terrain, antenna
height, antenna boom length, frequency,
coax length, and power level. The only variable in the tests was the two
antennas being tested. The test was performed to see how much loss antenna
traps have. Had there been more than one variable, the tests would not have
been valid, because in any scientific experiment, the test is valid only
when one variable is being tested. In addition, more than one test has to be
made in order to average out the collected data errors. In this case, many
tests were made. On one tower was a 20-meter four-element Hy-Gain 204-BA
monobander with a boom-length of 26 feet.
This antenna is arguably not one of the best monobanders made, but it is
what we had and it was about the same size as our tribander. On the other
tower was a trapped 6-element Hy-Gain TH-6 DXX tribander having a 24-foot
boom. The entire tribander boom-length was used on 20 meters, so both
boom-lengths were comparable.
The transmitted signal strength of the two antennas was compared on
20-meters. This test involved many DX stations and one local amateur 5 miles
away. With both antennas pointing toward the receiving station, a carrier
power of 10 watts was fed from the transmitter, and held constant while the
antennas were "hot" switched several times. (The power level was unimportant
as long as it was held constant on both antennas). None of the many DX
stations involved in this test could see any difference
in either antenna, and, yes, their analog meters could discern a difference
of one dB. These tests by themselves were not conclusive because of the
possibility of fading signals (QSB). A second series of tests was performed
with a local ham when 20 meters was dead. Testing with him was done to
eliminate QSB from spoiling the results. He could also measure no difference
on his S-meter. He could also see a one-dB
difference on his analog S-meter. As a third series of tests, the antennas
were switched while we looked at the signals on the S-meter from distant
stations and the local station. No differences in received signals were
noted. Maybe the difference was a monobander has only a few tenths of a dB
less loss, such a small amount of difference no one was able to see it on
receiver S-meters. Certainly, the difference in the two antennas was less
than one dB.
Conclusion: The Hy Gain TH6DXX and the 204-BA antennas perform equally well
on 20 meters at a height of 56 feet.
The SteppIR Antenna
The latest developments in yagi designs are found in the ones being sold by
SteppIR Antennas. There are two, three, and four element versions. All these
versions are frequency agile and cover continuously from 13.5
to 54 MHz. The MonstIR adds three very long elements for 6.9 to 13.5 MHz.
The elements are made of fiberglass tubes with beryllium-copper ribbons
inside.
Each element has stepping motors to wind and unwind the copper ribbons to
change their lengths inside the tubes. A multi-wire control cable connecting
the control box to the stepping motors accomplishes this. The proper element
lengths for all frequencies in its range have been calculated by a computer
and stored in the control boxs computer. As you move from frequency to
frequency, the control box in the shack readjusts each element length. Thus,
the antenna is configured into a properly tuned monobander for any frequency
in its range. These antennas are expensive, but the hams who own them say
they are worth the money.
The Log-Periodic Array
Another beam antenna that looks like a yagi is the log-periodic antenna. It
is configured using many elements with each element being shorter than the
one behind it. This means the longest element is at the rear of the array
and the shortest element is at the front. All elements are divided in the
center and insulated from the boom, and all elements are driven. On both
sides of the insulator at the center of each element, wires run from the
front element of the array to the rear element. Each wire criss-crosses the
other ones but they do not touch. That makes a 180-degree phase reversal
from one element to the next one behind it. The feed-point is across the
insulator at the shortest element. The feed-point impedance is about 200
ohms and a 4:1 balun is used to feed it.
The advantage of the log-periodic antenna is that it is very broad banded
and it can cover all frequencies with an SWR below 2:1in its design
frequency range. The disadvantage is the gain of a log-periodic antenna is
lower than a yagi with an equal boom length. There are designs being sold
today that cover continuously from 14 to 30 MHz. In Fort Gordon, Georgia,
there used to be a monster log-periodic at the MARS station that covered
from 2 to 30 MHz. The boom length was 120 feet and the antenna was
rotatable.
***
Edited from:
Understanding Antennas For The Non-Technical Ham
http://www.hamuniverse.com/n4jaantennabook.html
A Book By Jim Abercrombie, N4JA
Dr. Ronald E. Milliman
Retired Professor of Marketing
President: Millitronics, Inc. (millitronics.biz)
President: A3 Business Solutions (a3businesssolutions.com)
President: M&M Properties
President: South Central Kentucky Council of the Blind (SCKCB.ORG)
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