Excerpts from the YS Manual
It is important to note that the ideal element, from a structural standpoint, is one that is constantly changing over its entire length. If the wall thickness were constant, the diameter of the element would be a linear taper, getting larger at the inboard end. If the diameter was held constant, the wall thickness would be a linear taper thickening towards the center. And if we were really "smart", and knew what the optimum tradeoffs were for size, weight, and stiffness, the thing would utilize both.
The "perfect" element, being accurately sized to withstand the bending loads at all locations along its length, would use a section diameter or wall taper designed
to obtain the same stress across the whole element. In the real world, this element would be too costly for most of us. Its advantage would be to provide a minimum weight and area element.
The best we can do to approximate the "perfect element" with constant wall cylindrical tubing, is to build an element that looks like the "perfect" element,
changing diameter and/or wall thickness frequently. This is done by using many short sections instead of a few long ones. Another taper approximation
technique is to thicken the section walls as one moves inward. This can be done by sliding a slip fit piece of tubing (called a doubler) inside a section at the
inboard end and/or sliding one over the outside.
Sliding a doubler inside is best because it does not increase the element surface area and adds less weight. The outside doubler is more efficient at lowering the
stress but it will usually result in overloading the inner single wall sections due to its increased wind loading. This little tidbit is offered as an endorsement for
more efficient element design.
DESIGNING NEW ELEMENTS:
Before you run off and spend a bunch of time with your electrical design software, mechanically design something that will survive the anticipated wind speeds.
(1) Create a 1 element antenna with any boom length (I usually make it very short). Specify the proper design frequency and take a stab at how many sections
to use and what their size would be. This doesn't need to be very accurate. Set all section lengths short at 6". This will prevent them from becoming overloaded
as you are adjusting the outer sections.
(2) When YS loads it, and go to the Element Stress screen. Set the wind speed to the desired value. This will lock the wind speed at that value. F1 will overide
(3) Start at the element tip, and adjust the section length until the stress is just below the allowable stress. Then move in to sections 2, 3, 4, ...etc, adjusting their
lengths up to near maximum stress. While you are doing this, keep an eye on the Current Resonant Frequency (the original value is useless for this exercise).
You want to end up with a reflector (the longest, most highly stressed element).
The Resonant Frequency should be somewhere around 500 khz below the design frequency. It is better to be low than high, as this will result in a longer,
stronger element than required. If you will be adding a dummy section to your electrical model to approximate the attachment, use a resonant frequency
100-200 khz below design. If your resonance is too high, you need to lengthen the element. A low value requires a shorter element. If you guessed right when you made the model, you will be able to get to the target frequency with all sections near maximum stress. If not, you will need to add or delete sections.
If you run out of length (frequency too high, all sections at max stress), add another section after the inner one, and lengthen it up to the limit. Keep doing so
until you reach the target frequency. If you get to the target frequency before all sections are at max stress, delete the innermost one and lengthen the remaining
sections that have low stress.
(4) Once you get to the target resonance, print the screen and save the file.
(5) Now go build your electrical model with this element. Make all of the elements the same and space them equally on the boom. YO will move things around
as required. It will only tune the element by adjusting the tips, so you will end up with the elements stronger than required, the reflector being the weakest
(6) You may then make a YS model of the YO design and go back and adjust the sections to reduce surface area and more evenly distribute the stress across
There are many variations on this process. You may find it better to change the diameters or wall thicknesses, or use slip fit internal doublers at the inboard end.
Professor Murphy dictates that your scrap pile will not render the sizes you need to build your dream element, but you will be suitably entertained for hours
figuring out how to get the most out of what you have. The number of iterations required to reach a final design will decline as your experience increases.
MODIFYING EXISTING ELEMENTS:
Strengthening an existing element presents a more challenging task than building from scratch.
The first order of business is usually to redistribute the stress across the element by adjusting the section lengths to obtain equal stress levels in all sections. I
recommend trying to maintain resonance by equalizing the resonant frequencies as you go. This will keep you from getting too far away from your electrical
design and having to undo much of what you have accomplished structurally. Quite often, it is not possible to equalize stress, due to limitations on the available
lengths. Do the best you can and see what gain is possible. It is usually not enough to produce a significant increase in wind speed capability.
The next step is to insert doublers inside the inner end of the sections. This is done by selecting the < A >dd Section option from the menu bar. When
prompted "After which Section", input the section number of the section you want to strengthen. Then, using the on-line editor, fill in the dimensions for the new
inner portion of the section. The diameter of the section will remain the same but the wall thickness will be thicker. It should be the old section thickness + the
doubler thickness. Input a length that seems appropriate. The total length of the doubler section will be longer than the exposed length by the joint overlap.
Then reduce the old section length by the amount that was added in the doubler (exposed length). The exposed & total length of the old portion of the section
will be the same.
EXAMPLE: Old Section
Section Diameter Wall Exposed Total
2 .750 .058 57.000 60.000
3 .875 .058 68.000 72.000
4 1.000 .058 33.000 36.000
Press < A >dd Section then input < 3 >. Display will show section #4 with blue highlite. Press < E > to edit, use arrow keys to select the dimensions to enter
the new dimensions.
Section Diameter Wall Exposed Total
2 .750 .058 57.000 60.000
3 .875 .058 44.000 44.000
4 Dblr .875 .116 24.000 28.000
5 1.000 .058 33.000 36.000
The doubler section #4 was added to section #3. The doubler was a piece .750 dia x .058 wall x 24.0 long This doubled the wall thickness to .116 thick. The
original 4" joint overlap appears in the new section #4. Section #3 was reduced in length by 24.00" to retain resonance with both lengths being the same
because the lap joint is now in section #4.
Now that the doubler is in place, adjust its length to raise the safe wind speed. Make corrections for resonance to the original portion of the section. Continue
adding doublers to the other element sections and adjust their lengths to increase the safe wind speed and maintaining uniform stress distribution and resonance.
It is possible for single doubler additions to be inadequate for achieving the wind survival speed you desire. In this case, you will need to add secondary
doublers inside of the first ones. The procedure is identical to the one described above. The secondary doublers will always be shorter than the first ones.
Remember, it is imperative for you to adjust the element to maintain resonance before you return to the main menu or you will lose track of the element length
required to maintain your electrical design.
The procedure just described assumes that you must maintain the same element diameter at the boom to facilitate the connection. Hence, all improvements are
via internal doublers. If you are one of those free thinkers, and your element is too whimpy, then free think your element attachment into one that will accept a
larger element. This will relieve the frustration you will experience when the existing element has not been designed with enough taper.
ELEMENT TAPERS :
As previously discussed in the "Ideal Elements" section, it is best to design an element with a tapering diameter. This constitutes the most efficient use of the
material. There is another compelling reason for tapering the elements, and its name is "Vortex Shedding".
Vortex shedding will result in element oscillations at certain wind speeds. The elements seek their natural mode which usually resembles a sine wave. This motion, over extended periods, causes the material to either become work hardened and brittle, or if the stress levels are high enough the element is subject to fatigue failure. Eventually, the XYL notices some new growth in the rose garden.
Plainly stated, vortex shedding occurs as a result of disturbed air flow over the surface of the section. The size and shape of the section determine the range of
wind speeds where this occurs.
If we build an element with the same diameter over its entire length, the whole element experiences the oscillation phenomena at the same time, producing the
greatest vibration amplitudes.
If we break up the element into many sections with different diameters, the sections will want to oscillate at different wind speeds. Thus, it is rarely possible to
generate high amplitude vibrations across the entire structure, and the life of the elements will be extended.