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Correct Mast Choice
Mast Specifications: Same Size, Different Strength
Mast diameter and gauge are important when selecting the correct mast, but they do not tell the whole story. The aluminium grade, temper and tube manufacturing process can significantly affect the strength, stiffness and suitability of a mast for external aerial, satellite and antenna installations.
Key takeaway: Two masts may have the same diameter and wall thickness, but their strength can differ considerably depending on whether the tube is seam welded or extruded.
Why Mast Specification Matters
Recent years have seen more severe weather conditions affecting aerials, satellite dishes, brackets and reception equipment. As a result, installers and customers increasingly ask where product failure is most likely to occur and how to choose the correct mast for demanding installations.
Aluminium masts are commonly used because they are light, practical and corrosion resistant. However, the strength of an aluminium mast depends not only on its diameter and wall thickness, but also on the aluminium alloy, temper and manufacturing process.
Two Common Aluminium Mast Types
Two types of aluminium mast are typically available in the market: seam welded tubing and extruded tubing. Although they may look similar, the manufacturing processes are entirely different.
Seam Welded Tubing
Seam welded tubing starts as an aluminium ingot, usually a minimum of 5 tonnes. The aluminium is rolled and tempered into strips and coils. These strips are then rolled, pre-formed and formed into the mast shape before a high-inductance coil welding process creates the secure seam. The tubes are then deburred, cut, cleaned and packed.
Extruded Tubing
Extruded tubes also start as aluminium ingots. These are formed into blanks, pre-heated and forced through an extrusion die using a ram. This process plasticises the aluminium under pressure and forces it through the die to create the required tube shape. The lengths are then cut and packed.
Why the Manufacturing Process Affects Strength
Extrusion allows complex cross-sections to be formed and can be used with a wide range of materials. However, extruded aluminium can be less dense than seam welded aluminium and may contain air pockets which can weaken the wall.
Both processes use different aluminium grades, and these grades can vary considerably in mechanical strength. In addition, the rolling and forming process used for seam welded tube tempers the material and provides additional strength.
The alloy and temper details are therefore important when comparing mast types. Diameter alone is not enough to determine strength.
Mechanical Properties Comparison
The mechanical properties of extruded and seam welded aluminium masts can vary significantly, even when the masts appear similar in size.
| Mast Type | Alloy Type | Tensile Strength | Yield Strength | Elongation A50 |
|---|---|---|---|---|
| Aluminium extruded tube 2" masts | 6063 T6 | < 215 N/mm² | < 170 N/mm² | < 6% |
| Aluminium seam welded tube 2" masts | 5449 H195 | < 290 N/mm² | < 275 N/mm² | < 3% |
Compared with extruded tube, seam welded tube has approximately 34% higher tensile strength and approximately 60% higher yield strength.
Yield Strength and Permanent Deflection
Yield strength is especially important because it relates to permanent deflection. Assuming identical tube dimensions, an extruded tube in alloy 6063 T6 will bend and deform much sooner than a seam welded tube when exposed to storm conditions, wind loading or demanding installations.
Other important factors include the more homogeneous metal structure and tighter dimensional tolerances of seam welded tubing. For wall thickness, extruded tube typically has a tolerance of ±0.25 mm, whereas seam welded tube has a tolerance of ±0.08 mm. This accuracy can be important when fitting into castings, clamps or mounting hardware.
CAI Guidance
The CAI recognises these differences and recommends seam welded tubing in its Codes of Practice for the installation of aerials and antenna receiving equipment in single dwelling units, as well as for terrestrial and satellite TV reception systems in MDU and commercial installations.
Identifying the Difference
An extruded tube usually has a dull, matt finish and may be individually wrapped in plastic. A seam welded tube normally 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, ask your supplier before installation.
Extruded Tube |
Seam Welded Tube |
Conclusion
When using aluminium masts for aerial, satellite or antenna installations, seam welded construction should be used wherever strength and long-term reliability are required.
Please refer to the CAI Codes of Practice for further reading and confirmation of specifications.
All aluminium masts from Blake UK are seam welded to standard EN-AW-5449A, and all steel masts from Blake UK are seam welded to standard ERW EN 10305-3.
Bracket Selection
Correct mast selection should be considered alongside the appropriate bracket size and fixing method.