Correct Mast Choice

Mast Specifications - Same Size, Different Strength!

Recent years have seen worsening weather conditions, partly due to global warming, significantly damaging aerials and reception equipment. Consequently, we have received numerous enquiries about the potential failure points of products (especially aluminium masts) and have provided advice on the correct product choice.

The mast diameter and gauge are key factors in 'selecting the correct mast', but the aluminium grade and type of tubing also significantly impact the mast's strength and suitability for the installation.

Two types of aluminium masts are typically available on the market: seam welded and extruded tubing. These manufacturing processes are entirely different.

1. Seam welded tubing (SWT) begins as an aluminium ingot (minimum 5 tonnes). It is rolled and tempered into strips and coils. These strips are then rolled, pre-formed, and formed into the mast shape. A high inductance coil welding process creates the secure seam. The tubes are deburred, cut, cleaned, and packed.
2. Extruded tubes also start as an aluminium ingot, which is formed into blanks, pre-heated, and forced (using a ram) into the extrusion process. This process plasticises the aluminium under pressure and forces it through a die to form the required shape. The lengths are then cut and packed.

Extrusion allows for creating very complex cross-sections and working with brittle materials. However, extruded aluminium is less dense than seam welded aluminium and may contain air pockets, which weaken the wall. Both processes use different aluminium grades, varying in strength. Additionally, the rolling and forming process for the seam welded tube tempers the material, providing extra strength.

Important factors to consider are the alloy and temper details, which can be compared (see below).

The mechanical properties of both types vary considerably:

Comparing both grades, the seam welded tube is significantly stronger:

• Tensile strength is about 34% higher.
• Yield strength is about 60% higher.

The yield property is crucial for permanent deflection. Assuming identical tube dimensions, in stormy weather, an extruded tube in alloy 6063 T6 will bend and deform much sooner than the SWT.

Other important points include the more homogeneous metal structure and tighter dimensional tolerances of the SWT. For wall thickness, the extruded tube has a +/- 0.25mm tolerance, whereas SWT has a +/- 0.08mm tolerance. This accuracy can be important for mounting and fitting into castings or clamping.

The CAI understands these differences and recommends using SWT in the Code of Practice for the "Installation of Aerials/Antennas Receiving Equipment in the Single Dwelling Unit" and "Installation of Terrestrial and Satellite TV Reception Systems (MDU and Commercial)".

Typically, an extruded tube (Fig. 1) has a dull, matt finish and may be individually wrapped in plastic. Conversely, the seam-welded tube (Fig. 2) has a brighter finish, with a visible seam weld running the length of the mast internally and externally. If you are unsure whether the masting you are fitting is seam welded or extruded, please ask your supplier.

In Conclusion:

When using aluminium masts, they should be of seam welded construction.

Note: Please refer to the CAI Codes of Practice for further reading and confirmation of specifications.

Note: All aluminium masts from Blake UK are seam welded to the standard: (EN-AW-5449A), and all steel masts from Blake UK are seam welded to the standard: ERW (EN10305-3).

Click here for guidelines on selecting the appropriate bracket size for the relevant mast.