Sunday, July 21, 2013

Windy Test

On Friday July 19 I finally got what I've been waiting for: high winds. Hams with towers and HF antennas tend to fear potentially-destructive winds, but not me. Or at least, not right now.

My tower, though only 9 meters tall, is unconventionally guyed. Although I designed for tree motion in high winds I could not be certain how it would perform in practice. The trunk of the smaller tree -- a 40"-circumference (100 cm) tamarack -- was observed to move several centimeters in heavy gusts, though before the tower was erected. This motion was at a height of several meters; it is, of course, zero at ground level.

It isn't possible to say with certainty how strong Friday's winds topped out at. I happened to be here at the time and carefully observed how trees in the area responded. I would estimate that the strongest gusts peaked at 90 to 100 kph. Funnel clouds were spotted as near as 10 km south of here but did not touch the ground. Several large tree limbs came down in my own neighbourhood.

The tower did fine. As the large trees all around, including the two used as guy anchors, bent sharply and shed branches at the peak of the storm  the tower barely moved at all.

The only lasting impact was that the dipole (my so-called TH1vn) rotated so that it pointed into the wind. This is typical "weather-vaning" that is more typical with HF yagis, where the forces on the antenna try to turn it to the direction where it has the least surface area. This is not unexpected since I did not tightly bind the antenna mount to the mast. This is not a tower that can withstand significant torque.

The plus side of the weather-vaning is that the antenna favours Asia and South America. I've decided to leave it as is for a while so that I can try for some over-the-pole contacts. The proof of which is that I've now worked my first Japanese station (15 meters) since getting back on the air earlier this year.

Let me return to the tower test. There are two items in particular that I wanted to test:
  • Tree trunk motion would not compromise the tower.
  • Wind load well-above the top of the tower proper would not compromise the top section.
I took several steps in the design to protect against the first threat:
  • Lowering the anchor position on the trunk - Motion decreases as one gets closer to the roots. There is compromise in that I want to avoid having the anchor within hand reach to reduce opportunities for tampering. For the more bendable tamarack I placed it at 2 meters height, so it is reachable.
  • Moderate pre-load tension - The tension must be high enough to keep the tower vertical and rigid, but not so high that lateral tree motion pulls apart the tower!
  • Flexible tree trunk cradle - The anchors on both trees are constructed from ¼" aircraft cable. This is large enough that it doesn't cut the tree bark or fully deflect under the moderate pre-load tension.
In combination these steps appear to work well. A point to keep in mind is that lateral tree trunk displacement does not equal the distance between the tree and the guy point at the top of the tower. Trigonometry explains this. If the angle between tower and guy is 30° and the trunk moves 1 cm away from the tower the change in distance between the anchor point (back of tree) and tower guy point is 0.5 cm.

When this occurs the guy tension rises and the trunk cradle elongates and pivots upward, roughly compensating for the increased distance. Steel guy wire (aircraft cable in this instance) has almost no stretch, so a small change in length can cause a large increase in guy tension. This is why it is important to ensure that the effective guy length increases in proportion to tree trunk displacement. The lower rope guys stretch, so they require less design effort but at the price of lower reliability and endurance.

The second test item is placing load above the tower. This is one reason why I placed the TH1vn so far above the tower top, with the mast very extended. There is almost no performance reason to do so, other than somewhat to reduce high-angle radiation by getting the antenna exactly λ/2 above ground on 20 meters. It is generally considered a good practice to extend a mast past the tower top no further than it extends downward inside the tower to the lower attachment (usually a rotator). Even then it is critical to use a mast that can withstand the leverage of an extended load point, and a tower that can handle more than the wind load of the antenna plus mast. In other words, what I did is not a good idea for a permanent installation.

The test is a (very) rough proxy for a mast that extend 6 meters above the tower. That mast will support a 40 meters delta loop (apex at 15 meters) and a 30 meters delta loop. There may also be antennas lower down. Golden Nugget tower is nominally "rated" at 3 square feet of load at the tower top when extended one section (10') higher than the guy or house anchor. I quoted "rated" since engineering details are not in the spec sheet I have.

The mast plus antenna for the test add up to approximately a little over 2 square feet. The planned 6 meters of mast will have a similar wind profile. There is no easy way to translate these complex loads to an equivalent load placed directly at the top of the tower, but it will certainly be well above 2 square feet. This is due to the multiplied leverage of the higher load: the mast is a lever pushed by the wind, with the lever fulcrum where the mast meets the tower.

On the other hand the top guy is at the tower top, not one section lower. This should increase the tower's capacity. Then again little of the mast will extend into the tower, ~0.5 meters versus 6 meters mast above the tower. I plan to guy the 6 meters of mast to the existing guys so that should greatly reduce its impact on the tower, by limiting the bending moment at the anchor point within the top section.

The lever action, if the force produced by the wind load is high enough, can push the mast past its yield strength where it meets the tower. Worse, the lever multiplies the wind force at the mast clamp. This bending moment can push the horizontal or vertical structural elements or welds past their yield strength. We want none of these things to occur.

Complicated and confusing? Yes. There is no simple way through this. However, just because it is difficult does not mean one should give up and do nothing. While exact quantification isn't possible it is feasible to mitigate the risks if one is aware of the forces acting on the tower, mast and antenna.

It is always best to strictly follow the manufacturer's engineering specifications for tower installations. When that isn't possible, such as in my particular situation, it is vital that every detail is thought through, paying close attention to potential failure modes, and then calculate and test. Even then things can go very wrong, so think about where the tower will fall if disaster strikes. If this tower falls it will not hit the house and it is highly unlikely that it could touch a neighbour's property. Ensuring that this is so provides me some measure of comfort.

Detail of the final design and construction will come later, though probably not until early fall when the delta loops are scheduled to be put up.

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