[TowerTalk] Rohn RSL tower

[Byron Tatum]

Hello-    I was at a friend's QTH recently viewing his new Rohn RSL100L10, a 100' self supporter put together with 10' sections R-10H through R-1. It is rated for 25 ft/2 wind load at 90 MPH, dropping back to 11 ft2 at 100 MPH. We were discussing his planned installation of a KT-36 and M2 3 ele 40 meter yagi. He wishes to have the KT-36 about 15' above the tower top plate with the 40 meter yagi approx. 1' or 2' above tower top. Both antennas are fairly close in weight (approx. 95#), boom length (36') and wind load (10 ft2).   I wanted to get opinions of folks experienced with large antennas and self supporting towers if this planned installation is OK for this tower. Per Rohn "generalized rules of thumb" when extending the effective projected area up above the tower top, to get an approximation of the wind load capacity you look at the ratings of the tower in increments as it gets taller. In other words the 90' tall version is rated for 39 ft2 at 90 MPH so you lose 14 ft2 capacity in going from a 90' version to the 100 foot version. I checked the differences between 70' going to 80' (24 ft2), 80' going to 90' (17 ft2) and 90' going to 100' (14 ft2). The increments of wind load reduction get smaller ( 24 to 17 to 14 ft2 when going from 70' to 100') so take the figure of a 10 ft2 reduction, as an approximation, in wind load capacity when going from 100' to 110'.    It appears to me there are two main concerns regarding using this tower for the planned installation - avoiding exceeding the wind load capacity of tower and choosing the correct mast. Regarding the tower, if placing the KT-36 15' above tower top, it is my feeling that at a minimum the top R-1 section should be removed and a new top plate and rotor shelf obtained to use the R-2 section as new tower top. Possibly even remove the R-1 and R-2 sections and use the R-3 section as new tower top.  Regarding the mast would it be a good idea to move up to the 3" high strength thick wall steel mast, or would a smaller mast suffice?  I am just trying to get some recommendations, and the thoughts of others, since I have never owned one of these model towers myself.Byron W5FH