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Antenna Attachment Methods

One of the most important considerations when designing and building a Yagi antenna is the method used to attach the elements to the boom. The reason for this is 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 or in direct contact with the boom. Consequently, the designer must change the physical length of each element. If, over the course of the life of the antenna, the quality of the contact between the elements and the boom change, then the performance of the system will demonstrably degrade.

The quality of the contact is most often lost due to the natural buildup of aluminum oxide (high dielectric) on the surfaces between the element and the boom due to exposure to the weather. Under stress from wind loads, ice buildup or extreme temperatures, elements that are not firmly bonded will tend to move, creating a progressively larger gap that 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. This can have very serious consequences for the viability of the system.

When considering the different antenna designs available for inclusion into your system, remember the following points:

  • The 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.

Consider the following attachment methods and the pros & cons of each:

DIRECT TO BOOM

Design: Element is attached directly to the boom surface by means of rivet or bolt.
Pro: least expensive process- antenna will be cheap & light.
Con: element may snap in center due to drilled hole. Contact between element & boom degrades quickly due to excessive movement between each. High maintenance.

CENTER SWAGED MOUNT

Design: element is attached to a machined saddle which is fixed to the boom by means of a bolt. May also include a ‘swaged’ sleeve over element.
Pro: more & better contact than direct-to-boom method. Will likely last longer in the field before obvious signal degeneration.
relatively inexpensive to buy.
Con: contact points still develop oxide (rust!) which acts as insulator. Also, swaged on sleeve may actually contribute to metal fatigue of elements, causing cracks and/or breaks at edge of swage. High maintenance.

THRU-BOOM MOUNT

Design: elements are installed in holes drilled thru boom. They may be press fit or held in place with a set screw, preferably stainless steel.
Pro: far stronger elements, not likely to break or crack.
Contact is against two points (each edge of the holes in boom), so element is much less likely to move back & forth. Antenna life is prolonged.
Con: screw tension reduces over time, permitting oxidation to occur between the two contact points. Ultimately this leads to de-tuning of antenna. Some maintenance.

SPOT WELDS

Design: elements are fixed to boom using a spot weld. Elements can be mounted on top or thru the boom.
Pro: a permanent attachment point ensures a certain minimum contact area. Typically more robust design than screws or rivets.
Con: the gap between element and boom still allows oxidation to occur, reducing the actual contact to the weld point over time. May be marginally more expensive due to increased manufacturing costs. Lo maintenance.

CONTINUOUS WELDS

Design: elements are fixed to the boom using a continuous weld. Elements can be mounted on top or thru the boom.
Pro: eliminates any potential for movement or oxidation. The antenna is electrically one piece; characteristics remain constant for the life of the installation. The single most robust design available.
Con: initial cost is more than other designs.
Comment: Of all the processes utilized for attachments, none is considered superior to full welds around the circumference of the elements. A full weld will eliminate the potential of the development of oxides at the contact point. Due to NO maintenance costs of a fully welded antenna, overall cost-of-ownership will be significantly LESS than ‘cheap’ models over time.

 

 

 

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