On Wed, 17 Jan 1996, Steve Nace wrote: As I type this we are experiencing 80+ mph winds in southern New Mexico and far West Texas. I recently read that one should point his/her yagis 45 degrees off of the wind. I thought the prefered method is to point directly into the wind. 73 de Hose KN5H ------------------------------------------------------------------------ Date: Fri, 19 Jan 1996 16:11:55 -0500 From: David & Barbara Leeson <0005543629@mcimail.com> The choice of how to aim a Yagi in a windstorm depends on which of these is the weakest point of the antenna system: Elements Boom Mast and tower Rotator and coax If the elements are the weak spot, you don't want to have them broadside to the wind. If the boom is the weak spot, you don't want it broadside to the wind. As Stan, W7NI, points out, it's better to lose an element than to lose the boom plus the elements, so boom with the wind is a conservative choice. The only time the antenna area comes into play is if the mast and/or tower are the weakest link. The antenna area depends on the areas both of the elements and of the boom, and is calculated under older standards to be maximum, but under newer standards minimum, with the boom around 45 degrees to the wind. Of course the mast area is the same for all wind directions. Yagis tend to weathervane along the boom, even if they are symmetrical, and the response to gusting in different directions tends to break the mast loose (goodbye coax) or destroy rotators. There ae still a lot of unknowns in this department. Using the model that was current at the time I wrote Physical Design of Yagi Antennas, Electronic Industry Association's EIA RS-222-C, you calculate the area to be the square root of the sum of the squares of boom and element areas (i.e., bigger than either one by itself), and the angle to the wind for maximum side force is somewhere around 45 degrees. NI6W's excellent YS software is also based on RS-222-C. Since that time, models that give a better picture of forces on cylinders at an angle to wind have been published by EIA (EIA-222-E) and by the American Society of Civil Engineers (ASCE 74). In these standards, the wind force on a cylinder in yaw is modelled on the idea that the force is all due to variations in surface pressure around the cylinder, with no viscous force along the axis. Since pressure acts perpendicular to the surface, the force on a cylinder in yaw is also perpendicular to the axis. If the wind were purely horizontal (which it isn't) you could say that all the element force is directed along the boom. Using the newer model gives the same limits for element strength. Because the element force is perpendicular to element axes, it predicts you could make booms slightly less beefy, but not by much because you can't ignore the effect of the vertical gust component on boom requirements. A typical failure of big Yagis is the boom breaking upward; I use wind models with gusting at 15 degrees from horizontal over flat terrain and 30 degrees in hilly terrain. The biggest difference between the old and new models, and one that has caused confusion, is the prediction of the side force on the mast and tower. Modelled under the newer standards the maximum area is simply the larger of the boom or element areas, and the effective area is less than either one at angles near 45 degrees. So the predicted side force under the newer models (remember, the real world hasn't changed, only the model) is less, the minimum rather than the maximum is around 45 degrees and maybe you can get by with a little less mast or tower if you want to take the gamble. I experimented with scale-model Yagis in a poor-man's wind tunnel (mounted on a bearing out the sunroof of a car) and satisfied myself that the more recent standards describe the physical situation better, but I also found that there were stable points both with the boom aligned with the wind and with elements aligned with wind (0,90,180,270 degrees). This weathervaning may be caused by shadowing of the boom by the elements and vice versa. It isn't predicted by the simple force models, but you can sure see it on your own antennas in a wind. Symmetry, especially boom balance, reduces the rotating torque needed to rotate the antennas in the wind. But even with static balance, weathervaning with different gust and vortex directions results in your mast coming loose or rotator breakage no matter how carefully you align it with the mean wind. The required rotator braking torque depends on the rotational resonant frequency (it goes way up if there's any bang-bang mechanical backlash) and on the mass and area polar moments of the antennas on the mast, which scale with element size and the square of the boom length. The bottom line for me is that I don't let myself overload the mast or tower at the local 50-year maximum windspeed (defined as a medium-term average called the "fastest mile of wind" that doesn't include the shortest gusts), I balance the booms and the elements and I aim the antennas with the boom along the predominant storm wind direction. I still lose rotators from gust weathervaning (the prop pitches aren't ready yet) and I use a short coax jumper at the mast so I only have a small piece to replace if things get loose. This year's storms measured 134 mi/h plus gusts on the ridge here, so theory really got well tested. 73 de Dave, W6QHS --------------------------------------------- From: Dave K8CC, DAP14@infoctr.chrysler.com re: Pointing Yagis into Wind de K8CC KN5H asked the question about what is the optimum direction to aim yagis under conditions of high wind. Here is my opinion (if the opinion of a electronics/software engineer can count on a mechanical issue...). >From messing around with NI6W's Yagi-Stress program, it can be seen that the maximum wind load for a given antenna occurs not parallel to the wind direction, nor perpendicular, but rather at a angle that combines the wind areas of the boom and the sum of the elements. Steve is looking for the opposite case - minimum load. In this case, the correct direction depends on whether the boom or the elements present the worst wind load area. For example, a 204BA has four 35' elements a 26' boom - although the boom has larger diameter, it likely has less area so a 204BA probably should be aimed perpendicular to the wind. On the other hand, a 7L10M beam (which might have a large diameter boom and small diameter elements) might exhibit the least wind load aimed into the wind. Another factor to consider is whether you want to purely minimize wind load or rather present a balanced surface to the wind, which would reduce torque on the mast/tower/rotator. In this case, aimed into the wind might be preferable, since element spacings are rarely equal anymore in the post-YO era, while I have never seen a yagi with elements mounted off-center. 73, Dave/K8CC --------------------------------------------- Date: Fri, 19 Jan 1996 16:12:06 -0500 From: M Glenn Vinson Jr This subject was addressed by Dick Weber, PE, K5IU, at the New Orleans DXC last year in a presentation entitled, "Important Mechanical Issues for Yagi Antenna Installations." It was printed as an article in one of the journals. Pls refer to the actual article for the details, but what he says generally is that the direction of maximum wind load is either boom or elements broadside to the wind, and not in between. For minimum wind load into your tower during storm conditions, he presents some graphs based on boom area and element area, saying "when in doubt point your boom 45 degrees to the wind". Tom Schiller, N6BT, of Force 12 Antennas says that, "most beams have more element than boom wind load." Based on this proposition, I point my Force 12 using the 45 degree rule, but as if the elements were the boom, i.e., I point the elements 45 degrees to the wind (yes, I should be more precise and use Dick's charts). In the big storm we had here last month (85 mph winds here), nothing moved on my tower while a friend's Force 12 free-wheeled into exactly the direction that I had my beam headed. Just some anecdotal information. 73, Glenn. W6OTC --------------------------------------------- Date: Fri, 19 Jan 1996 16:12:12 -0500 From: Steven Affens I'm more concerned about rotor destruction or having the mast torqued off true direction within the rotor than antenna damage. So what I have tried on numerous occasions is release the brake during heavy winds and let the array (TH6 and 2el 40 both pointed in the same direction) seek its own direction just as a wind vane would. EVERY time, the array ends up "beaming" right into the wind and settles down. Virtually NO strain on the rotor. (assuming a fairly constant wind direction) So there's by 2 cents... find your array's "happy" point re the wind and leave it there. Good Luck. Steve K3SA --------------------------------------------- Date: Fri, 19 Jan 1996 16:12:17 -0500 From: "Charles H. Harpole" I think someone wrote in QST (?) about aiming the ends of the elements into the wind, but I prefer to point the beam directly into the wind with the front end of the boom looking directly into the wind, I think that you have to caculate where the greatest wind resistance is, but also take into account flex-- that is, the elements will bend back into the wind and take some of the strain that way, whereas the boom will not bend nearly so much, taking little strain relief (the bough that bends usually does not break). I got the chance to watch my old four element Swan (that used to be Hornet, remember them?) tribander looking directly into 70 mph (I was standing in the lee of my auto, but outside, and reporting into an ht about cloud rotation just above my head in Illinois), and saw what I think was the best beam heading: directly into the wind. Good luck. 73, K4VUD --------------------------------------------- Date: Fri, 19 Jan 1996 13:16:25 -0400 (EDT) From: btippett@ctc.net (Bill Tippett) Here is my experience from Colorado where we routinely had several 100+ MPH Chinook winds per season and two 140+ in 1982. I had a 5 el KLM 20 and 3 el KLM 40 on 148 feet of Rohn 45 for over 10 years with no rotator or antenna damage other than slight bending of the elements. For high winds (50+) ONLY, point the array 180 degrees from the wind direction. This is the most stable situation since the KLM 20 is forward offset. If you have a balanced antenna boom, into the wind is probably the same. When I put my KLM 20 back up this spring, I have added a wind balancing plate to the shorter end of the boom. For wind AND ice loading conditions, point the array with element tips into the wind. This minimizes ice loading on the elements which are the weakest part of the antennas. Unfortunately I learned this lesson the hard way by losing 10 and 15 meter elements from monobanders on another tower. Our power failed and I could not rotate that antenna after I realized we were having icing conditions (rare in Colorado). 73, Bill W0ZV --------------------------------------------- Date: Fri, 19 Jan 1996 12:19:36 -0500 From: K8DO@aol.com I have calculated which surface provides the least wind load for each of the multiple beams on my tower... But, I do not expect the tower to blow over or to fail in any manner, this side of a tornado, in which case it will not matter in the slightest where the beams were pointed .... so, knowing the surface area is interesting, but not particularily useful in less than ultimate storm conditions... Those who expect a hurricane will point the beam based on their calculations of least surface area and best mechanical position... Keeping the beam pointed during the storm passage is another story.... I am more interested in what angle to the wind provides the least torquing and shaking.... I found this by observing my beams on windy days, and then turning them a bit and watching some more until I found the angle to the prevailing wind that provided the steadiest ride for each beam.... For my installation it appears that the mounting point on the tower and how far each is above the buildings/trees determines the wind response more than the projected area of the elements/boom...They seem to to do best with combined element and boom sideloading... This probably maximizes the load on the tower, but as I noted above, this is not going to result in failure so it is of no concern.... The yagi on top of the tower at 150' is steadiest at 30 to 40 deg off the wind... The yagi at 120' on a ring mount, is best at 10 to 20 deg off the wind... The yagi side mounted at 90' seems best directly into the wind... This is only part of the story... Wind seldom blows from the same direction all day in michigan ( the coasts may be a different story) with a 90 degree change from the time I leave in the morning until I get home 14 hours later, being the norm.... Also, windy days in michigan are gusty, with the instantaneous wind varying 30 or 40 degs, as often as they are steady from one direction.... So, in rough weather, I point the beams based on the average expected wind direction for the day... I recommend ON4UN's book on antennas... He discusses this topic and offers methods to calculate ultimate wind survival for your antennas... Cheers ... Denny k8do@aol.com --------------------------------------------- From: Cleve D Leclair Yes - point the boom into the wind. Or in my case my case CDE-45 rotor with KLM 40M-1 @ 55' Telrex @ 50'... The big winds that we had one day here in western Oregon keep swinging my disc-brake rotor to 45 degs and I wud turn it to 15 degs...Then after a few min it was back to 45 or so degs...This leads me to the conclusion that pointing a boom into the wind is the point of least wind resistance... --------------------------------------------- To: cq-contest@tgv.com From: w7ni@teleport.com (Stan Griffiths) So after reading a lot of words about this subject, I am still left with some questions: 1. Does an antenna, if left to rotate to any position it wants to, rotate to the position of least wind resistance? Be careful before you say the answer is an intuitively obvious "yes". Kites don't seem to. They seem to seek a position that catches maximum wind . . . or at least "balances" the load about a pivot point. 2. Does maximum antenna windloading occur broadside to the elements or boom, whichever is greater, or does it occur at some angle in between where the wind catches some of both areas? As Dave, W6QHS, pointed out, EIA RS-222C said maximum antenna area was the square root of the sum of the squares of the elements and the boom and was somewhere around 45 degrees across the array. But now we also have EIA-222-E and ASCE 74 which are newer models and tend to indicate the 45 degree angle of the array to the wind actually catches LESS wind. Dave also points out that the laws of physics have not changed, only the models have changed. So do the new models actually come closer to the truth than the old ones? Dave, W6QHS, do you have any more comments on these two questions? Even if the calculated answer is different, depending upon the assumptions of your model, there must be a good way to measure it empirically, like using a scale miniture Yagi in a wind tunnel and taking actual measurements. But really, how important is the direction you point your Yagi in a strong wind? If you are really concerned about your tower or mast failing and you think the direction the antenna is pointing can make the difference between tower or mast failing or not failing, your tower or mast are just too light. If you are trying to protect your rotator, I think Dave, W6QHS, has done an outstanding job of telling us how to minimize that risk by balancing the torque. I think of my elements like I think of a fuse in my amplifier. That is the place I want a failure to occur if something has to fail. I would not consider wiring across my amplifier fuse if it blew once in awhile. Nor would I consider beefing up my elements (or orienting them in the wind) so that the tower, mast, or boom would fail first! ANYBODY can build a tower or antenna that won't fall down. It takes a REAL ENGINEER to build one that just barely won't fall down . . . Stan W7NI@teleport.com ---------------------------------------------