Many beams with long booms use a truss cable for added support. While some manufacturers will include a spar mounted on the boom for this, many save a few dollars by having the spar attach to the mast.
The disadvantage to using the mast is that it requires more time on the tower for the climber to attach and adjust the turnbuckles — usually requiring a few steps up and down to get at eye level with the beam to check the adjustment.
Here is how Adam, N3TTT, and AA1K recently made up a spar for the truss on a Cushcraft XM240, a 2-element 40-meter yagi.
With this modification, the truss is installed and the turnbuckles adjusted while the beam is on the ground.
This also allows the beam to be placed higher on the mast , instead of 3 feet or more down from the top when the stock Cushcraft truss bracket is used.
A side benefit, if the beam is being trammed to the top as this one will be, a rope tag line may be attached to the top of the spar to tilt the boom to lift the elements above the guy wires.
The angle material is 6061-T6 aluminum obtained from a Delaware salvage yard, 3/16″ thick. We bolted two pieces together for extra strength. With 1/4″ thick a single piece would have worked. We used two 1/4″ stainless steel U-bolts with saddle clamps to secure the spar to the boom.
We used the original Cushcraft truss bracket and some additional hardware to add weight at the reflector end of the boom to physically balance the beam.
