Design Considerations

For many antenna designs, element tapers are simple and booms can be a single tube. As the frequency gets lower and the wind and ice survival requirements get higher everything gets more complex. Yagi Mechanical® includes capabilities to support this higher level of design and it is important not only to know how to use them but to understand the design methods where they are included.

The first important concept is how the stresses propagate along the different sections in your design. While the tip section only has to deal with the stress from its own weight and the wind acting directly on it, the next section has these direct forces plus those transferred from the tip section. Continuing in towards the Boom each section compounds those values.

You can easily see this in action by making a design with a single tube. A half inch tube by itself can be 6’ long before there is too much wind stress on it - but change the tube to be 1” diameter instead and you will find that you can get 9’. Add a 6’ half inch tube after the 1” and you will find that now the 1” section needs to be less than 6’ with the tip section included.

Another vital design factor is an extrapolation of that same idea - just as the stresses become more on each section as you move from the tip in towards the boom, the stress on any particular section increases in the same direction. What this ultimately means is that the stress values that YM calculates are not a level of force across the whole section but rather a maximum amount just prior to the next section.

While shortening the section can be an option for simpler designs, the main tool to shore up a weak point in a complex antenna design is to use multiple pieces of tubing that are sleeved together. This can be done in multiple ways and is covered more in detail below, but the important point is to understand that if this sleeving does not extend to the boom end of the section in question it will not improve the stress values. In fact, it is possible that by adding more weight without traversing the actual stress point, the stresses might be worse.

Methods and Examples

  1. Internal Double Wall - The simplest form of reinforcement is to add material internal to the existing design - this keeps the external dimensions the same, leaving the resonant frequency intact.

  2. External Double Wall - Stacking more tubing on the outside of your existing design will also improve your mechanical stability. As the eletrical characteristics are affected by the taper of the design, this method may require changing the length of other sections to maintain resonant frequency. If it is a significant change it should be re-run through your preferred electrical design program again too.