Re-aligning the 40 Meter 3-element Yagi

Here's a retelling of an old joke:

Riddle: Where does a 300 lb antenna sit?

Answer: Anywhere it wants to.

When I raised the 3-element 40 meter yagi last December I omitted a step. The plate for the boom-to-mast clamp was firmly tightened on the mast but not too tight. Getting the boom to sit flat on the plate when the tram line was slacked might require twisting it, and I wanted to be able to respond quickly. 

Our team did such a great job that it was positioned perfectly. All I had to do was slip on the u-bolts and thread the nuts. I was so happy that I skipped a critical step. My mistake became apparent a few months later in the aftermath of a wind storm with gusts of 90 kph. That's when I remembered.

The antenna was no longer aligned with the 10 meter yagi above since it had turned on the mast. The difference was 30°. I confirmed that the 40 meter yagi was responsible when I climbed the tower and saw the marks on the mast and the skew of the boom truss cables. I thought back to December and could not recall tightening the several clamps holding the mast to the clamp plate. Sure enough, that was the mistake I made.

The antenna wasn't going anywhere so I put it on my to-do list and mentally adjusted the true bearing of each yagi when rotating them. During late summer I stepped the mast in preparation to hook the top of the mast -- it rises 10' (3 m) above the top plate -- and attach it to a tensioning device and a harness for the boom. I wasn't satisfied with my first mechanical design so I delayed the job. 

It is no trivial matter to take the 300 lb weight of the antenna and being able to freely rotate it on the mast without also having it slip down the mast. I kept putting off the job since it wasn't urgent. My preference was to come up with an alternative design that would be easy and foolproof.

This is the contraption I discarded. The weather is cooling and we've had high winds, neither of which is conducive to mast climbing. That was one point against it. Another was the difficulty of holding the hook firm on the rim of the mast pipe until there was tension on the cable, especially since it would have to lever out over the mast clamp and boom of the 10 meter yagi located less than 1' below the top of the mast. 

The final concern was the turnbuckle. I liked it because it's lighter than alternative mechanisms. However, it is difficult to adjust when it's on the mast since the upper and lower eyes must both be fixed in place and there is no good way to do that. When using them on tower guys the lower screw is bolted onto the anchor rod plate (or equalizer plate) so you only need to hold a rod or big screwdriver in the eye of the upper screw while turning the turnbuckle.

Shackles on the boom cradle would help to secure the lower screw. I tried that with several sizes of shackle and none of them fit in the small triangular gap between the boom pipe, u-bolt and mast clamp plate (there are no saddles under the u-bolts). In light of these deficits I decided to try something different. 

We had several days of unseasonably warm November weather and I wanted to take advantage. The morning of the arrival of the rainstorm remnant of hurricane Nicole I draped 30 lb of steel equipment on and over my safety gear and climbed 150'. I had about a 2 hour window for the job before it began to rain. I was probably lifting 40 lb in addition to my own weight! That isn't fun but I can do it. It was another win for brute force over ingenuity.

On the left, ¼" aircraft cable is threaded through the narrow gap under the boom clamp u-bolts. The ½" u-bolts are a strong anchor point. A chain or belt around the bottom of the boom clamp is easier to attach but it would be heavier and might slip out while awkwardly setting up the lift.

Rather than climb the mast with the aforementioned hook, a scavenged mast clamp that came with the LR20 tower is attached to the mast below the boom truss clamp (middle panel). To make room for it I cut the ties binding the LMR400 coax to the mast. The ½" u-bolt and heavy gauge steel angle will withstand more than the required vertical force required to support the yagi. A 5/16" chain is looped around the mast above the clamp with the ends connected by a shackle. 

The movable hook of my 40 year old 2-ton come-along (hand winch) slips onto the shackle and the fixed hook snaps onto the aircraft cable. This arrangement is easiest for operating the winch without interfering with my fall arrest gear and permitting clear access to the mast clamp. In seconds the winch had the weight of the yagi and I loosened the 8 nuts on the ½" bolts of the mast clamps.

With the yagi free all I had to do was rotate the yagi. That's easier said than done!

Developing sufficient leverage at the centre of this behemoth isn't easy. It's particularly difficult at the top of the tower where you have limited options for where to brace your feet and how far out your hands will reach and be able to push. Think about it for a moment and you can perhaps understand. My solution was to loosen my positioning belts, lean out from the tower, crouch and plant my feet on opposite sides of the tower. I could then pull the boom and push the driven element, or vice versa.

Once the antenna starts moving its momentum keeps it moving. When the boom was approximately aligned I tightened a few of the mast clamp bolts. I rechecked the alignment and found that the yagi had rotated back several degrees. Only after the second attempt did I look around to find the reason for the backlash. 

It was the boom truss mast clamp. It is fixed to the mast so that rotation past its neutral point increases tension on the truss cables. The tension gradient in torque turns the boom truss into a spring.

This is more difficult to fix and I didn't have the time with Hurricane Nicole literally on the horizon. I gently nudged the antenna to minimize backlash and gradually tightened the bolts. As you can see the alignment is out by a few degrees. For a 3-element yagi, and a 5-element for that matter, there is little practical impact. The yagis' beam widths are pretty broad.

As long as they align I can be flexible on where the yagis point because the prop pitch motor direction indicator permits setting of the zero point. A tweak of a pot in the controller and it's all done.

Next year I'll redo the alignment. I'll let the 40 meter yagi come to rest with the boom truss cables orthogonal to the mast clamp, which is exactly where it wants to sit. I'll then step the mast, climb up and align the 10 meter yagi with the 40 meter yagi. An inspection is in any case timely and I may take the opportunity to lift it the final foot to the top of the mast.