Date: Tue, 12 Mar 1996 15:35:59 -0500 From: KE6BER1@aol.com Reply-To: KE6BER1@aol.com To: cq-contest@tgv.com Subject: Rohn 25G Summary Thanks to everyone who responded. There are lots of responces here, in the order I recieved them. Hope the information is helpful. > Ok, here goes. I got the Rhon catalog over the weekend, and I"ve been > pouring over it, trying to decide what tower to consider. Now I had my heart > set on putting up 25G, but as I looked through the catalog I was throughly > surprised. Here is what I saw that most caught my attention, the windloading > numbers. It seems to me that many hams are using 25G that has much more > windload on it than is shown in the Rhon catalog. Are Rhon's numbers that > concervative? > > Second question. I'm looking at putting up 80ft of 25G with something like a > Force 12 C3XL (less than 10sq ft) and at a later date a Force 12 Magnum 2/2 > (about 10sq ft). Now, this 20 sq feet of antenna would excede the > windloading of 80 ft of 25G at 70mph by 5 feet. If the winds were at 90mph, > this would be TWICE the recommended windloading. Any answers? > > If there is interest, I will summarize responces and post them. Thanks. > > Al, KE6BER/1, KE6BER1@aol.com > > -- > > You can't learn if you don't ask. --- Most Hams ridiculously overload their towers; this is a dumb thing to do if you really want to keep everything up! You are wise to be doing the research! Follow Rohn's guidlines! Rohn makes the "mistake" of conservatively rating their towers, but, if you follow their guidlines, you will never loose a minute's sleep worrying about the tower coming down! Look for Rohn 45 or 55. 73, Steve KC2X kc2x@nebula.ispace.com Narcoosee, Florida --- Hi Al, I don't think the answer is very complex: hams tend to overload thier towers. I think this is particularly true of 25G. Unless a ham lives in a particularly windy area, he doesn't worry a whole lot about the actual designed wind load -- just assumes there must be some conservatism in the stated wind loads and puts the antennas up. Most of the time you can get away with this. Personally, I'd be a little more concerned about the wind load I would be placing on a guyed tower because it is always up. I have a crank-up tower that is probably at or in excess of its designed wind load, but I rationalize it with the fact that the tower is usually in the down position and if it's up and windy, I crank it down. I guess the bottom line is how windy is your area, what will the tower fall on if it falls, and how much risk are you willing to take? GL. 73 de Bruce, WA7BNM (bhorn@netcom.com) --- Al The answer is Rohn 45. Rohn 25 is good tower but many people overload it (as you have noted). Look at the numbers for Rohn 45. It should be o.k. for what you want. 73 Gene W3ZZ --- Al, Many hams, including me, tend to overload Rohn 25 tower! Mine is 70ft, with 3 sets of guys with tork bars and guy equalization plates. I have the following antennas on my tower: 4 elm 10M cushcraft 4 elm 15M Force 12 4 elm 20M Force 12 80 & 40 Sloper 160 Sloper 4bay 2M phased array and: G-1000 SDX Rotor Tic-ring Rotor for 20M beam 20ft 1/2" wall aluminum mast Rohn thrust bearing Rohn work platform Top Guys - Phillystran lower two set of guys 3/16 Rohn with insulators House bracket 4ft bottom/base section in 1 yd of concrete This tower, even with all of the guys, TWISTS in the wind! Most Rohn 25G towers TWIST DOWN! It is a real pain to climb -- When it does twist down I will probably replace it with Rohn 55G -- which is a MUCH better choice ---- Yep - lots more bucks, but you won't have to worry about it twisting down --- I recommend NOT using Rohn 25G -- 73 - Bill AB5YG (ex K2MHJ) --- well, you have seen my rohn 25g towers. the 15m one has that 8 ele 15m which runs over 15 sq ft. plus 5 4-ele smaller ones. plus feed lines and the 4-square ropes from the top. i would hate to think how much over the rohn specs it probably is. 73, dave --- Hiya, Al -- Yes, Rohn's (correct spelling) specs are conservatively engineered and YES the number one tower failure problem is overloading. As far as windloading goes, give me the county that you are building in and I'll give you the TIA-222 wind speed. That is THE number you use for the minimum figure for your system construction and, if in doubt, over-engineer everything. The Rohn 45G offers a little bit more wind loading but the siginificant improvement is with 55G. My advice is to study the Rohn book religiously and then BUILD EVERYTHING TO THE MANUFACTURER'S SPECS. You'll be in a potentially dangerous position if you don't. This is the short answer; give me a call at 206-485-7913 if you want to talk more about this. 73, Steve K7LXC "Up The Tower" now appears in CQ Contest magazine --- My experience is that Rohn specs everything conservatively. Rohn 25 will take a lot of antennas if you guy it properly. However, there are other reasons to use 45G instead. One problem with 25 is that you have to either put the rotator in the space provided very close to the top of the top section, or do what many people do and cut the cross bracing about 7' down from the top and put it there. I'm sure cutting the cross bracing weakens the top section, and once you cut it you can't move the rotator up or down another (cross bracing) notch. With 45 or 55, you can put the rotator anywhere you want to without cutting bracing. Also with 45 or 55, you need fewer sets of guy wires. Guy wires are a nuissance to install, they're unsightly (to neighbors), and they limit the places that you can side-mount other antennas. Unless you use expensive Phillystran, you have to break it up at short intervals to prevent resonances. When I put up another tower it will be 45 or 55, not 25. The cost difference is small compared to other considerations. 73, Ed Gilbert, WA2SRQ --- I ama pretty big guy, with big feet....we have two towers up, 25 & 55....the Rohn 25 is a real pain to climb after you have just been on the '55 (which has the same face size as '45) and I kick myself for investing in it years ago (and not just selling it off to finance some 45 (or 55)...if you plan to do any tower work, which as a contester you will always be doing, bite the bullet and go 45 (or 55 if you can find a good deal!). Go to 90 feet (not 80) if you can, the 90 to 100 foot range is great for 20 meters, you will probably hear this from a bunch of guys....if you read Lawson's (W2PV) yagi antenna book you will see him essentially postulate that 1 1/2 wavelengths is a real good height for a single yagi. Jim K1ZX --- Hi: Saw your message regarding the Rohn 25G tower. I had the good fortune to grow abt 30 Miles from the Rohn factory in Peoria, Il and have installed over 200 of them. An outstanding tower and company. They are very conservative on their specs... and they have to be with all the sue-happy lawyers out there. I have put up, in my back yard, a 100' rohn 25g with the spacing of the guy wires only 50' from the tower, not the required 80'. The antennas were a 40', 5 el 20m beam with a 5 el 15m beam on top. This lasted successfully for over 6 years in the ice storms of central Illinois. Now, understand that I built this tower way over specs... I guyed it every 25', not the expected 32'. I used 1/4" guy wire, not the 3/16" recommeded. I used more concrete in each guy anchor than required. The Force 12 antennas are light weight, you should have no problems if the tower is installed correctly. Not one tower I installed has fallen down in the last 20 years... and I did it without being burdened with an engineering degree! Write if you have any questions... respectfully, Michael Bill AA7NO --- My 70's vintage Rohn Catalog (Rohn Dwg Nr B-690712 R2 dated 1-7-75) shows the "proper" way to construct a clipped guy cable connection with thimble and clamps. The 14" "dead" end of the cable has the strands cut off to different lengths and "served" (each strand seaparately wrapped around the live cable). Now c'mon, Steve. Nobody in the WHOLE WORLD really does that, do they? They just cut off the dead end some polite distance beyond the last clamp and call it good... Big Don --- Al, Obviously many people have gotten away with loading more on these towers than the manufacturer's rating. It would be interesting to know whether anyone who ever put up Rohn 25G with guy wires spaced every 25 Ft actually loaded it with so much that the tower itself failed. It is likely that the antennas will destruct first and the tower will survive unless we are talking about hurricanes or tornados. In my experience Rohn 25G has been good for an installation up to about the size of a TH6 and 2 element shortened 40M yagi with 10 Ft spacing. More than that is pushing it a bit much. Regarding rotators, I would say that if you can get by with a HAM M you can get by with 25G. If it is absolutely necessary for you to use a Tasiltwister for the antennas you have selected, you probably need 45G. If you have to have a prop pitch or equivalent (if there is an equivalent), it could take anything from 45G to a microwave tower. In 1981 I put up 192 ft of 25G. Never put a yagi on it. Figured when I was on top it was probably overloaded - Hi. Sure worked FB with 9 radials (three on each leg) elevated at 60 ft level and the top 132 ft shunt fed on 160M! Back then elevated radials were a "secret weapon". Stan, K5GO --- The increase in load handling with the Rohn 55 is due to the increased size and the heavier gauge steel used in the legs. Review the engineering data book. The improvements from Rohn 25 to type 45 are minor. This is because they use the same gauge steel. The 55 is always best if you can afford it - and look closely at the upcost for it over 45 - not that much when you consider guys, misc hardware, etc. Genuinely, Patrick WB9IQI --- I looked it up in my Rohn catalog, why buy 45G comes to question on this one. What a joke, looks like someone at Rohn missed it? Let me know what you hear! -- 73 Robert WB5CRG w5robert@blkbox.com --- If I had to guess, I would say the reduced load capability of the 45G in the above comparison is due to the fact that you have 15 feet of unguyed tower above the top set of guys. You have to understand that the guying diagrams given in the Rohn Catalog you are looking at are only a few typical examples of HUNDREDS of different possible guying configurations. I am sure that if you added a THIRD set of guys at the top of the 80 foot 45G configuration above, you would raise its load capability considerably. How much you would raise it is a problem for a qualified structural engineer to solve. If you look at the very back of that Rohn Catalog, you will see a green page marked "Rohn Services Available". About 4 inches from the top of the page, Rohn lists the following service: "Product Review of Non-Catalog Towers" and "Simultaneous Product Review for Optional Loads". You will also see that Rohn wants $3,000.00 (Minimum) Per Tower plus $300.00 Per Hour to provide this service. This is the service you are asking for but you are asking mostly unqualified people to give you this service for free. The advice you get here will be worth about what you pay for it . . . Actually, some of the advice you get will be very good, but it is an extremely tough job for you to decide which advice is good and which advice is bad. To make make this decision, you really have to already be an expert on the subject yourself, in which case you don't need any advice. This is why you might want to seriously consider hiring an expert to give you advice on this matter. I suspect you doubt that many towers have ever had the kind of engineering consideration I am suggesting here. You are right. Most hams put up what they think will stay up. Most are smart about it (or lucky) and some are not smart or lucky. I am a Rohn Dealer and it would be sheer foolishness of me to suggest anything but the most conservative and safe approach to your tower installation. My suggestion remains for you to consult a qualified expert. Obviously, there ARE tower configurations that can handle 32 square feet of windload. They just don't happen to be the standard tower configurations shown in the Rohn Catalog you have. I really feel empathy for what you are going through with this situation. Hundreds of us have gone through it and we each come to our own conclusions in different ways. One possible approach is to find an actual installation very similar to what you want to put up in an area that has winds similar to what you expect at your location. If it has been up 10 years or so and has survived several strong windstorms, you might want to consider duplicating it. Nothing builds my confidence in a antenna/tower design as well as an established successful track record. Another useful exercise is to find out as much as possible about what went WRONG with towers that have come down and learn by the sad experiences of others. Good luck, Al. Stan w7ni@teleport.com PS My three towers have been up since the early '70s with no failures. One of them is 102 feet of Rohn 25. --- Al You are not making an equal comparison. For the 25 you have 5 feet of tower above the last guy. For the 45 you have 15 feet above the last guy. I think that is why the strange numbers and the much stronger tower appears no stronger. I am at a loss to explain why this example is given. I think 45 is usually guyed every 40 feet. 55 is bigger 18 inch tower than 45. I would use it if I could find it. I will have to ask the local commercial tower guru why the strange guy distances the next time I have dinner with him. The tower I have, AB105 (a military tower equivalent to Wind Turbine 2400) is 24 inches on a face, is put together like an erector set (nuts and bolts) and is supposed to handle about 30 sq feet at 240 feet maximum, guyed every 50 feet. At 80 feet it would probably handle better than 50 sq feet guyed at 40 and 80 feet. 73 Gene W3ZZ --- It is conventional wisdom among contesters that Rohn is excessively conservative in its ratings. What you've suggested putting up at the height you've mentioned should work fine, in my opinion. K3LYW, local bigtime commercial tower guru, did a presentation to PVRC a few years back about the Rohn specs and he pointed out some things -- we commonly misinterpret Rohn's specs and they really allow more than it seems -- but I can't remember the details. You may want to contact him. I had a TH6 at 100' on a Rohn 25 in a very windy place (open farmland in a Maryland valley) and never had a bit of problem for the five years I lived there. Took it all down and moved it. 73 - Rich, KE3Q --- Al, You cannot learn mechanical engineering from the Rohn catalog. My advice would be to get yourself a copy of ANSI/EIA/TIA-222-E, dated 1991, read it, and then contract with a professional tower company and/or a licensed PE, in your state, to specify/design/construct your system. Bill, N3RR --- >Ok, here's the STRAGEST thing I have ever seen in my life. These are numbers >directly out of the Rhon Catalog for 25G and 45G at 80 feet in 70MPH winds, >round element antennas, no ice. > >25G - 15.7 Sq. Feet, 3 guy wires (25', 50', 75'), 3/16" EHS, 400#. >45G - 15.3 Sq. Feet, 2 guy wires (35', 65'), 3/16" EHS, 400#. > >Is this reduced rating becuase of the number of guy wires, or guy wire >spacing? The catalog does not list Sq. Feet numbers for 55G at 80 feet, but >here is the closest thing in the catalog on 55G: My guess is spacing and especially number of guys > >100' of 55G, 70mph, round elements, no ice. > >55G - 26.6 Sq. Feet, 2 guy wires (42', 82') top wire is 1/4" EHS, 660#, >bottom wire > 3/16" EHS, 400#. > >Now they way the numbers reduce for every 10 feet between 150 and 100 feet of >55G, I would say that 80 feet would probably handle about 28-29 Sq. Feet of >antenna. THIS IS THE ONLY TOWER, ACCORDING TO THE RHON CATALOG, THAT WOULD >BE ACCEPTABLE FOR MY PURPOSES. > >I do understand from some responces that if I increase the guy wire size, or >the tension, I will increase the windloading capability. to a point, but at some stage the limiting factor is the compression strength of the tower. Wind forces are converted into pull on the guys and compression on the tower. If you put a set of unbreakable guys on un-pull-out-able anchors on a tower, sooner or later it will buckle between the guy attachments. Are the windload >numbers on the 55G due to the increase guy size and tension? I'm assuming >for the most part they are. Does this mean that if I used a similar guying >strategy on 25G, I would get it to handle the windloading of the antennas I >proposed before (about 20 Sq. Feet total). You'd need to figure all the forces to know what would give first. > >Just as an aside. After looking into towers some, I have discovered that I >have not seen ANY that will handle say 32 sq feet of antennas (except VERY >low towers). How come all these big rotors out there can handle that, if >there isin't even a tower to hold that much? > >Al, KE6BER/1, KE6BER1@aol.com > >-- > >You can't learn if you don't ask. > > 73, Pete Smith N4ZR n4zr@ix.netcom.com --- Al, Put 45 or 55 up. Believe me you'll be sorry later if you don't. Not so much because your current proposed windload, but what you'll want to do a couple years from now. 73, Steve --- Hiya, Al -- All of the Rohn towers are engineered independently based on their individual design, materials and capabilities. Believe the Rohn figures and follow their specs when building any Rohn system. >I do understand from some responces that if I increase the guy wire size, or >the tension, I will increase the windloading capability. Are the windload >numbers on the 55G due to the increase guy size and tension? I'm assuming >for the most part they are. Does this mean that if I used a similar guying >strategy on 25G, I would get it to handle the windloading of the antennas I >proposed before (about 20 Sq. Feet total). If you increase the guy wire size AND the guy wire tension, you will be exerting more compression on the tower legs which they WEREN'T designed to handle. Unless you have a Professional Engineer design your installation, DO NOT mess with the Rohn specs. And, yes, if you want to put 32 sq. ft. of antennas on top of a tower, the 55G is about the only way you can go. BTW, you haven't let me know which county you're in so that I can tell you what the TIA-222 wind speed spec is. You should probably be looking at the 90 MPH Rohn charts or higher depending where you are in Mass. >. How come all these big rotors out there can handle that, if there isin't even a tower to hold that much? Don't worry, Al, those rotators won't really hold 32 sq. ft. either. 73, Steve K7LXC --- Al: your ideas of the C3 on the Rohn are just fine, but the idea of stacking a Magnun 2/2 with the C3 on Rohn 25 is just dreaming, or certainly, asking for serious problems. Rohn 25 is not stout enough to handle that much antenna. I would not even think of putting the 2/2 on the Rohn 25. If you are seriously looking at that big an antenna, you should be looking at nothing less than Rohn 45 or Rohn 55, and guying those solidly... with torque bars and guy brackets, the works.. I've run a TH6 and a 2-element short 40 and that is Ok, but near the top end of rohn 25 load capabilities. 73 Gary W5VSZ --- You might want to contact N3RR (CBA or Contest Reflector); he hired an engineer, I think, to review his tower project and draw it up, etc. I think it was relatively reasonable and he was happy with the results. 73 - Rich, KE3Q --- Al, Just a thought....drop by your local utility and see if they've got a structural/civil type that might do it for you as a good public servant. Doesn't mean free but cheaper. If not that then they might be able to point you towrads their consultant de jour. Go for the 45G unless you wanna real big tower. 55G is cool if you can afford it but 45G is just as good for our line of work. Gary K7FR --- Hello again, Al -- By using a real engineer to help you design your tower installation, you will be doing yourself a big favor. Ask around locally on 2M or your packet cluster to see if anyone can give you the name of some PE that's familiar with Rohn/tower specs and will do it for a reasonable price. Try to find a sole practicioner because they will generally charge less than an engineering firm. You can probably figure 4 hours @ $100/hour so I'd budget around $400 for this service. If you provide him or her with the Rohn calculations, it'll obviously save you money since they won't have to recalculate everything. You'll probably need to have stamped drawings to get your building permit. 73 and good luck, Steve K7LXC --- I do agree that studying the Rohn Catalog will not teach you Mechanical Engineering. But it does give some interesting insight if you look for strange things. Then the conservative application of shade tree knowledge about leverage and mechanics and you get some ideas of where to look. I think if you look at the Rohn literature you will see the table that has the foundation reactions. I am working from memory here so please double check the numbers. But I think an 80 foot Rohn 25 G in 90 MPH wind district has a compressive base reaction of 6000 pounds. But a 190 foot tower has a compressive base reaction of 18,000 pounds. It is the same tower section with almost the same guy spacing so I think it is safe to say that increasing the top guy size to 1/4" will not fail the tower. In fact, here in ice country, I recommend and use 1/4" for all guys. I do however strongly recommend that you use the Rohn guy bracket if you increase the guy size. This will prevent having the guy attachment causing local stress problems. Most of the problems that I see in amatuer installations is the guying system, especially the guy anchor points - ie. not big enough, or deep enough. Towers fail in three primary ways. The wind loading exceeds the available guy system strength. The wind, guy tension and weight exceed the compressive strength of the tower section, or more likely, the strength of one leg. The shear strength of the tower, as provided by the latice, is exceeded by torque applied to the tower under high compressive loads, and/or the bending shear load on the tower by having lateral loads applied between guy clusters. Torque is caused by side mounts or aerodynamically unbalanced antennas, as well as long booms in severe gust conditions. When I started looking at towers, I reverse engineered the published Rohn literature against the the EIA-222 standard. My analysis found that for towers of less than the maximum height listed, the top guy is the first limiting factor. How much you can gain before you hit the second limiting factor requires some pretty thorough analysis. One thing my analysis indicated is that proper guy tension is critical to the life of a tower at maximum load. Another thing that I found is that side mounts and ring rotors really load the latices in the tower in certain wind conditions. As was also stated somewhere in this thread, there a a million ways to load and guy a given tower installation. Rohn has just started with one set of assumptions, based, I think, on the maximum height possible, and then scaled downward to give everyone a safe starting point. I have been giving a presentation to the local clubs and the MARS guys on "Tower Engineering 101". One of these days I am going to take my presentation and use it to write a short book. de n0yvy Steven H. Sawyers PE --- Hi Al... I can't argue with the experts on here... They are way too sure of themselves for me to debate... I can only relate my personal experience... Let me try to give you a point of view, i.e. a thought analysis of the problem, as opposed to computer analysis... note that Rohn lists the guy patterns for use of 25G to 190' feet... at max load this equates to an 18,000# compressive load on the tower.... I have a 150 foot of 25G... It is guyed every 30 feet with 4000# Phillystran.... the 30/60 guys are to an 8" screw in anchor (clay hard pan) and this guy point is 60' out ( note the 45 deg angle on the upper guy and the flatter angle on the lower guy)... The 90/120 guys are to an 8" screw in anchor at 120' out, same flat angle on the lower guy .. The 150' guy anchor is 150' out from the tower, to another 8" screw anchor, but this one is buried in a 3 foot square of poured concrete with wire rebar, sunk 4 feet into the hard pan... All the anchors came from the local electric/gas power company and are heavy, galvanized steel used for anchoring electric poles... There is a 4 element 40 meter beam on top - KT34XA at 120' and 90' - and another 4 el 40 at 70'... This far, far exceeds Rohn's catalog.... Orion 2800P and Ring rotors... Our highest winds are approx 73 mph, unless we get a tornado... Now, start analyzing at the top... Let us make a stipulation/assumption that the lower guy will not fail... So, if you assume that the 120' guy will not fail for the moment, then what the top guy is holding in that wind, is a 30 foot tower wind load, plus the beam on top... In my case, think of this is as a 40m-yagi on a 30 foot tower sitting on top of a 120' hill, guyed with 4000# philly at a 45 deg angle... Do you feel that this will fail?... It seems unlikely... Rohn's catalog shows this to be a freight train solid installation for a 30' tower... Now analyze the load on the 120' guy... assume that the 90' foot guy will not fail for the moment... same line of reasoning concerning the wind load of the KT34XA plus the 30' of tower it is holding up, plus in this case, about 1/3 of the projected area of the 30' tower section above it... continue down the tower sequentially, assuming for each section that the next lower guy will not fail, then examine that 30' tower section as if it were ground mounted, add the wind load of the beam mounted there, plus the wind load of 30' of tower, plus 1/3 the wind load of the projected area of the tower above that guy, and see how you feel about that guy failing on a 30' tower installation.... If you are concerned about any given guy set or section, then beef it up...once you have assured yourself that each guy is unlikely to fail, then we start thought analyzing the next topic... Torque and twisting... An aside: In my case thoughtful analysis shows that I would be better off with separate anchors for each 30 section, instead of doubling the guys on the anchors at 60 and 120 foot out from the tower... In defense of this doubling, note that the lower guy line of each of these pairs is very flat, coming off the tower at roughly a 33 deg angle, which is a high strength, low stress situation... the weak point here is the single anchor for a 60' tower section... In fact, I am going to add the 'between' anchors this summer and relocate the ground end of the 30' and 90' guys... Interestingly, _none_ of the prepackaged (and guaranteed) tower sets the Rohn sells include torque bars... When I pointed this out, they just shrugged and said that if I wanted torque bars they would be happy to sell me some... The hidden message here is that these short torque arms add little, if anything, to ultimate survival... The statement made was that they had never seen a failure in a tower that was properly guyed and TENSIONED... they said that most of the amateur installations they see are not tensioned adequately... for 3/16 ehs, you are supposed to have 400# of preload... this is very stiff... they said that on most amateur towers they can wiggle the guys with one finger... if the tower is tensioned to specs it cannot twist\rotate far enough to buckle a tube or a diagonal, and should not fail unless a guy lets go... okay, nuff of this drivel.... sorry to bore you... Cheers ... Denny --- Ok guys, that was it. Once again I want to thank everyone who responded. I now know I'm going to get in touch with a PE and find out what I really need to do. All the information in this document came from cq-contest reflector readers and should be taken with a grain of salt. The best policy is to ALWAYS check with a professional who has an education in the field in question, an don't to just rely on people who sort-of know. Al, KE6BER/1, KE6BER1@aol.com