Saturday, August 9, 2014

Site-B Antenna Mast 3.0

After the mast supporting my multi-band inverted vee collapsed in early winter I put up a newer, stronger mast that replaced the fibreglass parts with an 18 gauge steel fence rail. I called this antenna mast version 2.0. Both sat on a 19' long length of Schedule 40 steel pipe bracketed to the house. On my site plan this location was designated 'B'. Both the mast and the pipe were taken down in early June in preparation for a complete rebuild of my antennas and supports.

The Golden Nugget tower that had been at Site C (supporting a multi-band dipole and delta loop) has since taken the place of the steel pipe. That is the bottom support for what I am calling antenna mast version 3.0. It is stronger and simpler that its predecessors. And this time it's all steel construction.

At right is the newly completed antenna mast, complete with steel back stay and the pulley to pull up the antenna. The steel stay (or guy) and pulley are the sole survivors from the version 2.0 mast.

The new mast consists of two 10' (3 meters) sections of steel pipe. The bottom pipe is the 18 gauge mast that comes with the tower. It is swaged at one end to allow stacking.

Rather than purchase an overpriced mast from the tower manufacturer I found a suitable alternative at my local big box home improvement store. This is an EMT 1.25" (nominal) galvanized steel conduit. Its outer diameter is 1.51" and the steel gauge according to the EMT industry specification (and my measurement) is 16. It weighs 10 lb (1 lb per foot) and fits perfectly onto the swaged end of the lower pipe.

I drilled a hole just below the top for a bolt from which to hang the pulley and secure the steel cable. With all attachments in place it was quite easy to stand at the top of the tower in a stiff breeze and plant the pipe onto the lower mast.

The top of the completed mast is about 14.3 meters above grade: the tower is 8.8 meters high and the mast (minus overlaps) is 5.5 meters long. Wind load on the mast is 2.4 ft², or 22.5 kg (50 lb) at 135 kph (85 mph). If the load were close to the top of the tower this would be within the bracketed tower's capacity, but not when extended 18' above it. Hence the need for guying. The back stay is one guy, with the other two to be provided by the inverted vee legs.

The horizontal bar is the same rigid plastic pipe that was the back stay supporting the mast of the 40 meters delta loop. Its purpose is to allow the stay to be anchored to the tower rather than on the ground. As in the discussion of vector forces acting on a gin pole it is best that the pipe be midway between top and bottom cable anchors. Otherwise the wind would bend and possibly break the plastic pipe. The wind load carried by the steel stay is designed to result in a net horizontal load from the tower to the house bracket.

My plan for the next week is to modify the old multi-band inverted vee (30, 20, 17, 15 and 10) and mount it on the mast. I'll have more to say later about those changes and my motivation for making them. I am also making repairs to my old Ham-M/II rotator. It's amazing how things can deteriorate by simply being stored in the basement for 22 years.

For the present I am, again, QRT since I had to remove the dipole from the small tower to build the new mast.

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