Tower Layout And Mechanical Analysis

Tower Layout (Not to scale)

Tower Layout (Not to scale)

Once we determined the height of our tower and the antennas on it, the next step was to determine the location of the tower and the associated guy anchors on our property. Rohn provides specs for guy anchor heights, layout angles and distances for a given tower type and height. Using Rohn’s specifications for a 100 ft 55G Tower, we selected the location for our tower in such a way that the guy anchors were about 100 ft or so from the tower on the edge of our backyard in the woods. These locations allowed the guy lines to rise above the yard enough to walk under them before reaching the edge of our lawn. This layout placed two of the guy wires below the ground level that the base of the tower sits on. To ensure that our guy angle’s would be as expected, we used a surveyor’s transit to measure the elevation of the guy anchors relative to the tower and to locate them at the required angular separation. A little bit of trigonometry and a spreadsheet allowed us to determine the angle of each guy wire relative to the ground using the following formula:

Guy Angle = arctan( Elevation / Distance ) where:

Guy Angle = Guy angle relative to the ground
Elevation = Total rise of guy wire
(height of guy on tower + distance of guy anchor below tower base)
Distance = Distance from tower to guy anchor

These calculations should be done to determine angles for the upper and lower guys wires at each of the three guy anchor points. As long as the distances and angles are within Rohn’s specifications, the tower layout is ok. We also had a special requirement here because we are planning to rotate on of our SteppIR DB36 beams around the tower using a KoXG rotating ring.

K0XG Ring Rotor

K0XG Ring Rotor

The concern here was that the lower SteppIR DB36 antenna might interfere with the upper set of guy wires as it rotates.

SteppIR Antenna on K0XG Ring

SteppIR Antenna on a K0XG Ring

SteppIR’s specifications for the DB36 indicate that this antenna has a 26 ft turning radius. To allow for the size of the K0XG ring and some margin for error, we settled on a minimum clearance of 30 ft between the lower rotating SteppIR DB36 Antenna and the upper guy lines. Again, some trigonometry and a spreadsheet help us determine if our planned design will work.

Clearance = ( Upper Guy HeightAnt Height ) / tan( Guy Angle )

The Guy Angles are those computed in the previous step. Note that the Upper Guy Height includes both the height of the guy on the tower plus the distance of the guy anchor’s elevation below the base of the tower if any.

We created a simple spreadsheet to calculate the clearance between all three of the upper guy lines and our planned SteppIR DB36 antenna at 65 ft and determined that we had more than the required clearance for all three guy lines.

– Fred (AB1OC)

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