By Joseph H. Reisert
One of the most important considerations when designing
and building a Yagi antenna is the method used to attach the
elements to a boom. This is true because the boom influences
the electrical length of the elements. In effect, a boom shields
or shorts out that portion of the element where it comes in close
proximity with or in direct contact with the boom. In response,
the designer must change the physical length of each element.
Should the quality of the contact between the elements and the
boom change during the life of the antenna, the performance of
the system will demonstrably degrade.
The quality of the contact is most often lost
due to the buildup of aluminum oxide (an excellent
insulator) on the surface where the element and
the boom form the contact point. Under stress from
wind loads, ice buildup or temperature extremes,
elements that are not firmly bonded to the boom
will move, creating a gap which will collect moisture
and induce the development of aluminum oxide. The
loss of contact becomes permanent and the performance
of the antenna can be degraded by several dB.
Therefore, in reviewing methods
of attachment of elements to the boom, the following
points should be remembered:
amount of contact area between the element and
the boom must be compensated for in the design process
• The mechanical attachment must assure that the contact area remains constant
over the life of the antenna, especially in the presence of corrosive environments
and mechanical stresses.
• The mechanical attachment process itself should not induce stresses,
which will contribute to the failure of the antenna.
The following is an overview of
some of the more common attachment
methods and the demonstrated performance
CENTER HOLE MOUNTING
One of the earliest element attachment
methods was to drill a hole through
the center of the element and mount
the element directly to the boom, sometimes
referred to as the Ohmic contact method.
This approach often fails when the
element snaps in the center, weakened
by the presence of the hole. Contact
between the element and the boom is
also easily lost as the element bends
away from the boom under windy and
CENTER SWAGED MOUNTING
This element mounting method is not
reliable because the element
breaks in half or loses contact with
Some manufacturers add a
swaged on sleeve over the center of
the element to strengthen it. However,
this approach also has reliability
problems. After the element has been
subjected to wind and vibration for
a period of time (as little
as 6 months
to a year), the element often breaks
off at the edge of the swage point
due to metal fatigue caused by micro-cracks
developed during the swaging process.
CENTER SCREW MOUNTING
One seemingly simple solution for element
mounting is to use a boom made from
an insulating material. However, most
acceptable boom materials will still
have some affect on the electrical
performance of the elements. At the
same time, the mechanical strength
as well as the cost of using a boom
material other than aluminum comes
Another element mounting method
is to drill lateral holes in the
boom. The elements are then inserted
and centered on the boom. The element
is held in place by a hole drilled
or tapped through the boom above
the element. Then a screw is inserted
and tightened until it presses the
element in place. However, contact
is limited to the two points in the
side holes against which the elements
are pressed. If the screw tension
changes (as it often does), the element
is jarred or the boom distorts, oxide
will build up in the hole around
the element, creating an insulation.
This results in a change in the tuning
of the element and hence a deterioration
in antenna performance.
A viable alternative to assuring contact
is to eliminate it entirely! To achieve
this, a-larger diameter hole is drilled
laterally in the boom and an insulating
sleeve is inserted and secured. The
element is centered in the insulator
and fixed in place with small press-on
lock rings or "keepers".
The use of inexpensive insulator materials
such as nylon can cause failures as
the insulator breaks down after exposure
to ultra violet radiation, weather
extremes, or as it absorbs water, creating
a contact where none is desired. High
quality insulating materials will drive
up the unit cost. The method for fixing
the elements in the insulators often
fails allowing the elements to physically
move and thus impacts performance.
Furthermore, the larger holes in the
boom can also contribute to a reduction
of the mechanical integrity of the
Spot welding is similar to center screw mounting in that the process
establishes limited contact between the element and the boom. The
open gap between the element and the holes still creates an environment
for the buildup of aluminum oxides. A major manufacturing concern
is the tendency of the boom to distort
during the welding process.
The designer must compensate for the impact of the distortion by
assuring that the design of the antenna is sufficiently robust.
A continuous weld around the full circumference of the element
will minimize the potential of the development of oxides at the
contact point. This process is more involved and will increase
the unit cost of the antenna. A major concern with welding is the
cleaning process, usually caustic, to remove the splashing from
the welds. This may cause the boom to etch at a later time.
Astron Wireless Technologies, Inc. has developed a unique process
to provide an expanded bonding between the element and the boom
thus increasing element to boom conductivity while minimizing mechanical
stress during assembly and in the field. The center of the element
is knurled to increase its surface area. Then a fitted mounting
bracket is placed over the knurled area and attached to the boom
using stainless steel hardware. Applying the proper torque to the
hardware compresses the knurl into the surface of both the boom
and the element mounting bracket and creates an extensive joining
surface with excellent conductivity. In essence, a metallurgical
bond similar to a cold weld is created. This secures the element
in place and minimizes the potential of oxide buildup. Mechanical
stress on the element is evenly distributed by the bracket. In
turn, the bracket also distributes any stresses across the boom.
Furthermore, there are no holes through the element or large boom
holes to weaken the structure.
The bracket mounting method developed and used by Astron Wireless
Technologies, Inc. meets the requirements of quality and durability
for mating antenna elements to the boom.
The electrical impact of the contact area is fully compensated
for in the design of the antenna.
• The clamping action
of the bracket, combined with the cold weld effects of the
knurl, assures constant maintenance of the contact
area, especially during peak load situations such as ice and
• The mechanical attachment process itself does not impart
stresses on the element or seriously degrade the integrity of
• Antennas produced and field tested using this element
mounting process demonstrated excellent electrical performance
reported failures, even after being subjected to a five (5) year
accelerated salt bath.
Astron Wireless Technologies, Inc. and the author retain the rights
to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.
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