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[TowerTalk] Counter-balanced "Tower" Design

 To: towertalk@contesting.com [TowerTalk] Counter-balanced "Tower" Design Bob Bogash bobby1@rbogash.com Mon, 25 Feb 2008 10:42:45 -0800 mailto:towertalk@contesting.com>
 ```Counter-balanced "Tower" Design I have been looking at a number of options for a fairly light-weight, free-standing (unguyed) tower (mast) with a counter-balanced tilt-over. The antenna(s) I am considering are simple and light-weight, either 1) a 20 meter wire Moxon, or 2) a 20 meter - (or 5 band) Hex-beam. The mast height would be between 25 and 30 feet. My QTH is on a high bluff about salt-water. The mast base would be at about 100 ft elev. The ground then slopes very gently to the edge of the bluff, which then drops almost vertically about 80 ft to the beach. The distance from mast base to bluff is about 60-70 ft. The antenna would be facing salt-water from approximately 300 degrees True through North to about 120 degrees True. The shortest distance across salt-water to the nearest land (Whidbey Island) is about 18,500 ft; the longest in excess of 75,000 feet. Japan is about Heading 300; Europe 030; Caribbean/Brazil 120 degrees True. Modelling I have done using HFTA and my terrain profile indicate the following gains using a 2 element Yagi and an 8 degree take-off angle: My QTH - 25 ft antenna elev = gain 11 dbi Flat - 25 ft = 0.9 dbi Flat - 35 ft = 3.5 Flat - 45 ft = 5.8 Flat - 55 ft = 7.3 Flat - 65 ft = 8.3 Flat - 75 ft = 9.2 Flat - 85 ft = 10.3 Flat - 95 ft = 10.8 Flat - 105 ft = 11.0 "Flat" means over flat terrain. The higher antennas would only equal the 25 ft yagi height at much higher take-off angles - typ 15-25 degrees, or MUCH higher antenna heights. If I'm interpreting the data correctly, and if the model is accurate, I am reaping the benefit of the bluff height without needing a higher tower or mast. That's exclusive of any kick I get from the salt-water. It would take a 105 ft high yagi over flat terrain to equal my 25 ft yagi with an adjusted height of 125 ft (MSL.) (I am uncertain what the lower physical height would mean; that is the actual near-field height, which would be over average ground, versus the far-field height, which appears to the antenna to be much higher...) I am currently running a Steppir vertical on my metal roof house and it is performing very well, and is apparently loving that salt-water far field. For various reasons - esthetic and neighbors, and installation and maintenance, I am looking at a low elevation, low visual impact antenna with a tilt-over capability. Not a crank up. Not a "traditional" looking lattice tower. I don't want to climb. I want to lay the tower over for maintenance and wind (it can blow a lot here!) I have considered the MA-40. I think it is over-kill for my situation. I also don't like the tilt-over scheme - cable and winch. So, I considered a simple tilt-over. I have a DX Engineering vertical tilt-over. It tilts from the base. The combined antenna/mast/modest rotator would weigh about 25-30 lbs. Plus the weight of the mast. I could "walk" it up or down without too much trouble. But, it seems to me a counter-balanced scheme would be superior. I made up a simple drawing - you can find it here: http://www.rbogash.com/Tower%20design.jpg After I designed this, I noticed a similar installation in a near-by park. There, they have a number of wooden flagpoles. I've measured them as 5 inch butt diam, and 32.5 ft length. They are hinged at the 5 ft level. They don't use counter-balances. The supports are steel U-channels. I already have several big ones in my garage. All manner of sophisticated bearings, collars, bungees and shock absorber schemes have crossed my mind (being a dumb retired engineer), but for the purposes of this exercise, the above simple layout should be fine. For the mast, I have considered wood poles like at the park, aluminum section push-up masts, aluminum and fiberglass flagpoles, and steel parking lot light standards. Typical 25 - 30 ft flagpoles can be found here: http://flagpoleco.com/25taonepifl.html Light standards, 4 or 5 inches square, can be found here: http://www.affordablelighting.com/25ft_light_pole_5x511.html For weights, I would use a set of cement or cast-iron weights from a body-building free weight set. I could adjust the weights any way I wanted, right down to allowing the antenna to be walked up and down with one finger. A likely counter-balance weight would be 125-175 lbs. The mast would normally be locked in the vertical position and lowered for maintenance and wind events. However, I have also contemplated the option of leaving it unlocked and "floating." In the event of a wind storm, with say sustained winds of 50 and gusts to 75, the antenna/mast might float to an angle - say 20 degrees off vertical in the sustained wind, and, in effect, become "unloaded." When a 75 mph gust struck, the mast would then really be seeing only a 25 mph delta increment. Any and all comments and critiques of the above scheme are solicited. 73s Bob Bogash W7DDD _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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