VHF/UHF Yagi Antenna Design Help
by Martin E. Meserve
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Antennas | VHF/UHF Yagi Help
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This is intended to go into a little more
detail on the VHF/UHF Yagi Design program. The design program is long
enough without being choked with detailed explanations.
The design program is best suited for antennas in the VHF,
UHF, and Microwave frequencies because lower frequency antennas,
that have gains between 11.8 to 21.6 dBd, tend to be physically
impractical. For example, a yagi antenna for 50 MHz would have elements
that are around 8 - 10 feet (2.5 - 3 M) long. For a 3 or 4 element antenna
the overall size would not be excessive and would be practical. But, this
would only get you limited gain.
If you wanted, say, 12 dBd of Gain, it would require
10 elements. This would make the boom 2.3 wavelengths (44.9 feet, 13.695 M)
long. That may not be very practical.
Even though the program specifies a gain between 11.8 to 21.6 dBd,
you can still specify a smaller gain, like 6 dBd. You may get a
warning when the dimensions are displayed, but, the design should still
be accurate. However, in order to prevent the program from performing
needless calculations the upper limits are protected. You can not calculate
an antenna with a specified Gain in excess of 21.6 dBd or a specified
Boom Length in excess of 39 Wavelengths.
A bandwidth of 7% of the center frequency,
at the -1 dB forward-gain points, is typical for yagis designed here. The
Design Frequency should be this center. Also, the Design
Frequency should be limited to the VHF, UHF, and Microwave frequencies.
Use the Select box, next to the the Design Frequency entry box,
to specify the frequency dimensions in Mega Hertz (MHz) or
Giga Hertz (GHz).
All internal calculations are performed in Wavelengths and converted
to the various dimensions for output, as necessary. Some of the equations
and conversion that I use are listed to the right.
You can define you antenna with a required Gain or
Boom Length. If you enter a required Gain, the program
will calculate the necesary Boom Length. Conversly, if you
enter a required Boom Length, the program will calculate
the possible Gain. Specifying a Boom Length is useful
when you have stock lengths of boom material or length restrictions.
The Boom Length can be specified in inches, millimeters, or Wavelenghts.
The Gain of a Yagi style antenna is directly proportional to
Boom Length. This is because longer booms allow for more Director
elements, and hence, higher Gain. Current indications are that
doubling a Yagi's boom length will result in a maximum theoretical
gain increase of about 2.6 dB. In practice, the increase will be
a little less due to construction errors.
Gains are specified in dBd (dB Gain over a Dipole).
This is different than dBi (dB Gain over Isotropic). An Isotropic
radiator is considered to be omni directional (radiates equally in
all directions) and has 0 dB gain ,whereas a Dipole is considered
to have 2.1 dBi. So when you specify an antenna with 12 dBd, for
example, you are also specifying an antenna with 14.1 dBi.
Boom Length is calculated from Gain using:
Conversly, Gain is calculated from Boom Length using:
Reflector and Director Spacing
The default Reflector spacing is 0.2 Wavelengths
behind the Driven element. Spacings between 0.15 and 0.2 Wavelengths
are valid and have little effect on the foward Gain. However, the
Reflector spacing can affect the feed impedance. It may be useful to
make the Reflector element spacing adjustable in order to obtain a
better match on the Driven element.
DL6WU designs generally use the 0.2 Wavelength Reflector spacing
and the ARRL designs use the 0.15 Wavelength Reflector spacing.
You can select either spacing or any other spacing between them,
in 0.01 Wavelength increments.
One place where adjusting the Reflector spacing may be useful is, if you have
a boom of fixed length. By reducing the Reflector spacing you may be
able to fit another Director on the Boom.
An example of this is a antenna
designed for 440 MHz on a Boom with a fixed length of 1000 mm (1 Meter).
If a 0.2 Wavelength spacing is used only 5 Director elements will fit
on the boom for an effective Gain of 9.732 dBd. However, should you reduce
the Reflector spacing to 0.16 Wavelengths, 6 Director
elements will fit on the boom for an effective Gain of 10.483 dBd.
This is not a very big Gain increase (0.75 dBd) and will be much less
for longer boom lengths, but it may be useful.
The default Director element spacing is calculated from an
array that gradually increases from about 0.65 wavelengths, for the
first driven element, to 0.4 wavelengths for element 14 and beyond.
A user can select the spacing defined by DL6WU or the ARRL and they
only differ for the first 3 driven elements.
Boom Correction, Boom Type, Mounting, and Dimensions
The type of Boom, metalic or non-metalic, and the method
of mounting the elements effects the length of the antenna array elements.
Close proximity of a metalic boom to the antenna array elements
causes a shortening effect which raises it's operating frequency.
Lowering the operating frequency back to the design frequency would
require the elements to be lengthened. If the elements are actually bonded
to the metalic boom, as opposed to being insulated from the boom
by grommets, even more of an effect is realized.
The precise amount, to lengthen the elements by, is based on the
design frequency and the relationship of the Boom Diameter to the
design frequency, in Wavelength.
The following equation describes this relationship for Boom Diameters
less than, or equal to, 0.055 × Wavelength. This yields the
percentage of Boom Diameter which must be added to element lengths.
BC equals the Boom Correction and BD_WL is the boom diameter, in
BC = 733 × BD_WL × (0.055 - BD_WL) - 504 × BD_WL × (0.03 - BD_WL)
Selecting Metal Boom (Bonded) means that a Metal Boom
is used and the elements pass through and are physically Bonded to it.
This is not a good choice unless the elements are welded to the boom.
Otherwise, air currents can cause vibration of the antenna elements
which in turn will cause the mounting holes to widen and the elements
will get loose. Loose vibrating elements can introduce noise into the
Selecting Metal Boom (Insulated) - means that a Metal Boom
is used and the elements pass through it, however, they are
insulated from the boom. This is usually done with rubber grommets.
When this is the case, the calculated Boom Correction is divided
Selecting Non-Metallic Boom means that a materal other than Metal is used
for the boom, i.e. wood, plastic, etc., or a Metal Boom is used but
the elements are mounted on insulators, above or below the boom,
such that the metal-to-metal spacing is greater than the boom radius.
NOTE - The last option means that there will be no correction
made for the influence of the boom and that any Boom Diameter that
is entered, will not be used in the calculation
of the element lengths. If in doubt, select the first or second
option and you can review your decision later.
You can choose to accept the Boom Correction listed in the
select boxes or set your own value. You can return to the program
calculated value at any time by clicking on the
Init BC button.
For these designs, the diameter of the Driven and Parasitic
elements should be limited to between 0.001 and 0.02 wavelengths.
The spacing of the Reflector element is fixed at 0.2 wavelengths,
for all designs. However, this value is not carved in stone and can
be anything from 0.15 to 0.2 wavelengths. In the ARRL Examples, listed
at the bottom of the main page, a 0.15 wavelength Reflector spacing
The spacing of the Director elements along the boom is calculated
from a Exponential curve for the first 13 elements and then remains
constant at 0.4 wavelengths for elements 14 through N.
As with the Reflector spacing, the ARRL Examples use a slightly
modified equation for calculating the spacing, from one element to the
Element Spacing from Previous Element (wavelengths)
Viewing the Design and Sample Designs
On the Yagi Design web page, this section provides you with a way of
collecting all of the design information into one single page. This gives
you a better overview and is easier to read than skipping back and forth
on the design page.
You can start up the design viewer at the start
of your work and leave it up throughout. It will not update on it's own, but,
all you need to do is press the view button and all the latest information is
These are antenna designs from The ARRL Antenna Book, 18th
Edition. They were designed using a method similar to the one in this
web page, however, the element spacing and length curves are slightly
different. That being said, don't expect to design an antenna, with this
program, that exactly matches the ARRL designs.
The director spacings gradually increases until a constant spacing of
about 0.4 Wavelengths is reached. The director lengths start out
longest with the first director and then decrease in length. This method
of construction results in a wide gain bandwidth. A bandwidth of 7%
of the center frequency at the -1 dB forward-gain points is typical for