A common antenna tower configuration is a lattice tower structure with a tube mast out of the top of the tower to support antennas. The tube mast is usually rotatable, cantilevered and supported in bearings to resist the lateral and thrust forces. This article discusses briefly the design of that mast, and offers a spreadsheet design support tool. The article does not consider the implications for loading of the tower structure itself.
A range of materials are available for the purpose, but for most high performance applications, performance Aluminium or Steel alloys will be best suited. Not only are these tubes expensive, but transport costs can be even greater, especially in longer lengths.
The figure above show s a range of solutions for 50mm OD and 60mmOD tubes. The notation is the <product> <OD>x<WallThickness>.
The second bar is common 40mmNB water pipe, and shows it is a low strength option.
The VK2KRR option is his recommended sleeving of an aluminium tube with a high performance steel tube, being the first and third tubes shown above. It should be noted that unless the sleeve is effectively prevented from sliding at all under the highest safe bending moments, it should be treated as a sliding interface and the maximum bending moment of the combination is no greater than the sum of the maximum bending moment of each of the tubes. That is the main reason that the Steel 4130 50.8×4.77 option has much higher strength than the VK2KRR composite even the the latter is much thicker.
Attached is a spreadsheet design tool to assist with calculations.
Above is an upper section which describes one of the antennas installed on the mast.
Above is the summary section where all of the component bending moments are summed and the wind and tilt stresses calculated.
In this case, a Safety Factor of 1 is used for the wind calculations, so that the mast is likely to bend before tower failure, though that will not necessarily save the tower. The Safety Factor for tilt is 3 to make provision for, amongst other things, dynamic forces as the tower is raised or lowered.
Design wind speeds appropriate for location and environment need to be selected.
In this example, the required yield strength is within the strength of 6061 Aluminium alloy, and a relatively low cost tube that fits the standard 50mm opening in common towers will suffice. Grab the MastCalculations spreadsheet and try raising the antenna, or adding another antenna and explore the results.
The spreadsheet makes provision for up to three antennas, cable, mast, and rotator above the top of the mast. Obviously, if the rotator is inside the tower, it does not contribute to the bending moment of the mast tube.
No allowance is made for ice, wind forces are considered horizontal.
The spreadsheet can be downloaded, click on MastCalculations.
Warning, the spreadsheet does not replace the need for design competence. It is only a support tool, you MUST NOT depend on its results without independent verification. NO RESPONSIBILITY IS ACCEPTED NOR WARRANTY OF ANY KIND OFFERED.