Posted: 1/8/2015 4:21:28 PM EDT
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I'm getting closer to being able to put up my tower and wanted to get a little input from the hive.
I have around 150' of Rohn 25g to work with and would like to be able to set up a decent 160m vertical antenna as well as coverage for the other bands. I have a SteppIR 3 element 10m - 20m that needs repaired and a 3 ele. Cushcraft A3S 10m/15m/20m to choose from for the top and can can hang dipoles/OCFD/etc... off of the sides. Question 1: IIRC, Rohn recommends that the guy wire anchors are placed at 80% of the tower height from the tower base. How absolute is this? If I can safely bring the guy wire anchors closer to the tower by only setting them at say 60% of the height from the tower base, it would give the wires a steeper angle and take up less space in the yard to mow around or walk in to. Question B: I've seen egg shaped, ceramic, insulators on ebay that will insulate one section of wire from another. I would like to use these to insulate sections of the guy wires so that they can be used as a wire antenna. Will these things hold up to that kind of stress? If not, is there something that will? eBay Egg Insulator Link 
Question 3: Is there a reason to NOT go big? Since I already have the tower sections, the cost of putting up a 150' tower isn't going to be much more than putting up a 70' tower. I live in Central Florida (pretty much the center of the state) and am in the 110 mph wind zone. I don't have to worry about permitting or inspections, but I also don't my build to be unsafe. Thanks! |
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Quoted:
I'm getting closer to being able to put up my tower and wanted to get a little input from the hive. I have around 150' of Rohn 25g to work with and would like to be able to set up a decent 160m vertical antenna as well as coverage for the other bands. I have a SteppIR 3 element 10m - 20m that needs repaired and a 3 ele. Cushcraft A3S 10m/15m/20m to choose from for the top and can can hang dipoles/OCFD/etc... off of the sides. Question 1: IIRC, Rohn recommends that the guy wire anchors are placed at 80% of the tower height from the tower base. How absolute is this? If I can safely bring the guy wire anchors closer to the tower by only setting them at say 60% of the height from the tower base, it would give the wires a steeper angle and take up less space in the yard to mow around or walk in to. I know Rohn has an engineering division, but I don't know if they'll help you out with this. I STRONGLY recommend that you get a local structural engineer to help you. The engineer may even be able to get you a smaller footprint with multiple guys on each side. In addition, he can help you make good choices for anchoring your tower. Question B: I've seen egg shaped, ceramic, insulators on ebay that will insulate one section of wire from another. I would like to use these to insulate sections of the guy wires so that they can be used as a wire antenna. Will these things hold up to that kind of stress? If not, is there something that will? eBay Egg Insulator Link http://i.ebayimg.com/00/$(KGrHqEOKpQE23vwWljCBN1tSl6Zpw~~_12.JPG They only have 1/4" holes. The easy question is whether the cable you're using will fit. Also, I'm not sure I'd want a $3 link in the guy wires of my antenna when the wind picks up. Question 3: Is there a reason to NOT go big? Since I already have the tower sections, the cost of putting up a 150' tower isn't going to be much more than putting up a 70' tower. I live in Central Florida (pretty much the center of the state) and am in the 110 mph wind zone. I don't have to worry about permitting or inspections, but I also don't my build to be unsafe. Thanks! I don't mean to sound like a downer, but anything tall in Florida gives me the itchies because I am an engineer and I have to design some of them. |
| Why not get the answers directly from Rohn? They have detailed information here: http://www.rohnnet.com/rohn-25g-tower |
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1) If you go to 60%, thats basically 30ft closer to the tower and going to be quite a steep angle. I wouldn't suggest that without a PE signing off on that.
2) http://www.texastowers.com/towers/guyed/guyed-acc/ins.htm You'd have to insulate the guys from the tower as well if that's what you planning on doing. Much easier to just put a standoff and pulley on the tower and put your dipole on that. I used something similar to these. http://www.pfisterer.com/cocs-kategorien-produkt.php?id=73&cockat=17 3) Everything will be the same for a 70' or 150' tower except amount of guys and distance to guy anchors(56' vs 120'). 2 guys for 70', 4 for 150'. |
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I started with the county. Figured why design something unless I can have it. They said any tower over 30 feet tall needed permitting. I made sure they meant the tower "...not what's on it, right?" So I plan to have my A-4S at about 35 feet on the extension pole. I'm going to make a separate top-suspended pole at about 40-50 feet with my 2m beam on it. I may try to mount a 17m and 12m moxon on it as well. Then the electrically flipable 40m moxon above the house. Then the big vertical for 160. Then drop the G5RV down to 20 feet for NVIS. Then I will be happy...maybe. |
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Quoted:
I started with the county. Figured why design something unless I can have it. They said any tower over 30 feet tall needed permitting. I made sure they meant the tower "...not what's on it, right?" So I plan to have my A-4S at about 35 feet on the extension pole. I'm going to make a separate top-suspended pole at about 40-50 feet with my 2m beam on it. I may try to mount a 17m and 12m moxon on it as well. Then the electrically flipable 40m moxon above the house. Then the big vertical for 160. Then drop the G5RV down to 20 feet for NVIS. Then I will be happy...maybe. Exactly. They totally meant "permanent structure" at least that's what my county says. |
| Think about wind load of the antennas…Rohn designs the towers for a specific wind load. Double check the wind loading info on your antennas. Personally, I'd go 90-100 ft with that kind of antenna load. If you want to go taller, I'd suggest moving to 45 or 55. |
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Instead of trying to break up the guys with insulators why not use Phillystran. Then if you want to use your guys to support parallel running dipoles you can.
Moving from 80% to 60% you increase the vertical stress load on the the tower by a factor of X. You may find that with the increase vertical loading and additional wind load with that much antenna you will collapse the legs in the the lower sections. Once they collapse the rest of the tower will follow. If you are worried about mowing around the anchor points you can always elevate the anchor. Some elevate them about 6 feet to allow for mowing clearance. One thing you may want to think about is a simple 40m directional sloper design. At 70 feet you can hang the ends of 5 40m dipoles. Slope them 60 degrees back towards ground. Space them evenly around the tower. Feed them back to a relay switch mounted on the tower. You want to leave each unselected dipole open not grounded or shorted. The added length of feedline will make the unselected dipoles appear longer and act as reflectors. Honestly you can go too high for some things. Gain over ground is a losing battle unless you are running huge EXPENSIVE hardline. What you gain in ground gain is lost in feedline loses. Also think about take off angle. You can end up with placing some of the stations in the shadow of the tower. One main thing the county is going to look at first is SET BACK. While most tower fall within the guy radius, they will look at height vs set back from property live. Will any part of the tower fall on adjacent property. Just my ramblings for today sorry if too long just hate to see you go to a lot of work and find out you are not pleased with the results. |