My new 140' (40 m) tower is primarily for my 15 and 20 meter stacked yagis. Recently I installed the mast for the upper yagis. The 15 meter yagi will be at the top of the mast and the 20 meter yagi at the bottom.
Last year I kept the top two sections of the new 140' tower on the ground so that the mast and rotation system could be fabricated to fit. The mast is supported by 3 bearings and bearing plates, a plate below them for the prop pitch motor and custom couplers between the mast and thrust (bottom) bearing and to the motor drive shaft.
I made sure to get it working before proceeding with installation of the tower sections and the mast. Correcting mechanical problems in the air can be extremely difficult. Time spent getting it right on the ground is well worth it. Avoid shortcuts and never imagine that a job of this magnitude will reward laziness.
There are two features of the rotation system for this tower that are different from that for the near-identical 150' tower. One is that the prop pitch motor is mounted upside down below the mast for direct drive instead of chain drive. The other is that the mast is aluminum rather than steel.
I was negligent about taking pictures of the construction and mast lifting. There are times I'm so focused on doing the work and keeping safe that the camera does not come to mind. Prose and graphics will have to suffice, complemented by pictures I took after the installation. The prop pitch motor, which isn't yet installed, will be left to a future article.
The mast is a surplus standard 20' length of 2-⅞" OD 6061-T6 aluminum alloy pipe. My original plan was to use this as half the boom of a 3-element 40 meter yagi. Then I realized the pipe was schedule 80 rather than 40 which is heavy for a boom. The wall thickness is 0.276" and it weighs 53 lb (24 kg). It has a few holes from its previous use for a commercial antenna. The cap is welded with a spike for draining atmospheric charge.
I plugged the yagi data and pipe data into a mast stress calculator. It should support my 5-element 15 meter yagi at the top and 5-element 20 meter yagi at the bottom for winds up to 180 kph. These are the upper yagis of my 15 and 20 meter stacks. Their electrically identical and lower twins are side mounted and fixed towards Europe.
The expected maximum wind for this region is 135 kph (85 mph). The mast has a good safety margin, including an allowance for the holes mentioned earlier. Since aluminum alloy has a difficult to determine cyclical stress (fatigue) capacity the large margin is more important than for a steel mast.
One benefit of aluminum is the low weight compared to steel. Two men were comfortably able to lift the mast to the top of the 140' tower. I rigged the rope with pulleys so that my helpers could work apart and respect the 2 meter separation to protect against COVID-19. The gross lifting weight was approximately 70 lb (32 kg) including the top plate and bearing and fasteners.
There are two reasons to lift the top plate with the mast instead of beforehand. First, it shifts the centre of gravity lower. My gin pole can lift a load a maximum of 7'-8" above the tower and that is below the midpoint of the 20' pipe. The plate moves the centre of gravity 2' lower, which is almost but not quite enough. More on that in a moment.
The second reason is to ease the drop into the top of the tower. The clearance through the 75 mm bearing is tight for a 73 mm diameter pipe. The pipe would have to be almost perfectly vertical to slip through. With the bearing plate attached to the mast it drop more easily into the tower. Once the mast slips into the middle bearing 5' below the top plate can be bolted to the tower girt.
The same technique was used for the mast for the other big tower. Except in that case the mast was shorter so the gin pole had adequate reach.
Some of the alternatives for overcoming this constraint were described in the article on topping the Trylon with a 19' mast. The technique used here is better and takes advantage of having two helpers on the rope who could quickly react to verbal instructions. I had to shouted so they could hear me 140' below; none of our radios has the hands-free feature.
Absent a picture I drew a diagram. The heavy bearing plate, held in position by a muffler clamp, lowers the centre-of-gravity (CoG) ~2' below the pipe centre (C). This is 6" short of the gin pole's reach. A muffler clamp higher on the mast snags the lift rope which effectively shifts the attachment far above the CoG. The clamp isn't too tight so it doesn't crush the rope. Use the U as the snag not the sharp saddle!
The mast hangs almost perfectly vertical. The only time it leans (as depicted) is when the bottom is still on the ground. I followed the mast upward in 20' steps to guide the mast and bearing plate past the tower, guys and side mount yagis. A tag line hung from the bottom for another steering alternative and as a safety line in case the mast swung out of control during the final stage of the lift.
When I'm as high as is safe at the top of the tower the snag bolt is loosened and slid down as my friends continue the lift. Eventually the clamp bottoms out on the rope coil that grips the pipe. That's when you need to hold the pipe vertical from the bottom so that it doesn't overturn. The tag line is trapped in my gear in case I need it in an emergency. The operation went smoothly without mishap.
As the mast descends into the tower the plate stops when it contacts the tower. The clamp doubles as a stop when it contacts the middle bearing. That's when the top plate is bolted to the tower. The rope coil is slid upward a few feet as the rope is slacked and then the weight is retaken by my friends. I descend to the middle bearing, move the clamp up a short distance and the mast is lowered until the clamp again holds the weight.
The previous day I lifted the mast coupler and associated hardware so they were already at hand. It's ~2.5' of 3.5" OD 6061-T6 schedule 40 pipe. The coupler is attached to mast and it is dropped until it is sitting on the lower bearing. That bearing takes the full thrust load of the mast and antennas. All the bearings are 75 mm deep groove double sealed industrial bearings (FAG) with a capacity far in excess of the axial and radial loads of this application. This is a good fit for 2-⅞" (73 mm) pipe. A shim can be used on the top bearing to reduce chatter if that's a concern.
Whether a shim is used or not check for free 360° rotation. The bearings will likely require some adjustment to centre the mast. I have a small misalignment that I will correct later.
The coupler I fabricated has a few functions:
- As a mast collar it transfers vertical load to the bottom (thrust) bearing
- Extends the mast to the design length, with 12' extending above the tower
- Attachment for the prop pitch motor drive shaft, keeping it centred and free from axial and radial loads
The coupler was partially described earlier. The pictures give a closer view (sorry, I couldn't keep my feet out of the frame). Shims are needed to centre the mast and lower coupler to the 2-⅞" mast and lower coupler. Unlike for the first big tower I machined the shims so that the bolts go through them and so cannot creep out. Jam nuts are better than lock washers to secure fasteners on a round pipe.
The shaft is 1-¼" steel schedule 40 pipe with a 1.66" OD. The coupler ID is 2.469". A grade 8 bolt transfers torque and two ⅜" bolts in tapped holes centre the shaft. The design is fully adjustable and permits easy removal of the drive shaft for maintenance of the system including motor removal.
The shaft came with the motor and I decided to try it out since it is well made, if a little rusty. Downward flowing water is shunted to the side and away from the motor. If the shaft is unequal to the job I will take it to a machine shop and make it stronger.
Although the bearing plates are ¼" steel they will deflect under load. To simulate the effect I used a winch to put several hundred pounds of load on the system when it was still on the ground. The deflection of the bottom thrust bearing was no more than ⅛" yet this is enough to be worth the effort to compensate for in the design.
Adapting antennas to the mast
The boom-to-mast clamps and boom truss are home brew and not adaptations of commercial clamps. The design takes account of the softness of aluminum alloy in comparison to steel.
On the downside all the weight bears on the boom and mast clamps. For a truss support on the mast -- the conventional method -- the weight on the boom-to-mast clamp is reduced.
There is provision for 4 saddle clamps to grip the mast. By distributing the load over more than 2 clamps they each don't need to "bite" as deeply into the aluminum alloy mast. Cold flow is reduced so that the grip force holds better over time. The pipe wall is so thick that marring by the clamps is not a structural risk. I have not yet decided how many clamps to use: I can start with two and add more later.
On the other hand this antenna is not terribly heavy. The 20 meter antenna that will be at the bottom of the mast is much heavier, but that force is reduced by the boom truss being mounted on the mast in the conventional manner.
Many of you will recognize the clamps from the DX Engineering catalogue. I have had good success with these "Cycle 24" galvanized saddle clamps in other projects. I prefer them to the stainless saddle clamps for their ease of use (special lubricants not required) and textured band for improved grip.
The mast clamps are offset 1" from centre to fit the truss support clamps beside it. The completed boom-to-mast clamp fits nicely on the yagi. If all goes well it will be raised before the hay grows too high. Otherwise it must wait until August. The larger 20 meter yagi is scheduled for lifting later this year.
Working on the mast isn't safe until the motor is installed since it is otherwise freewheeling. With the first yagi lift planned before motor installation a temporary solution is required.
The anti-spin grip is comprised of a muffler clamp and perforated angle stock. When the arm is tied to the tower the mast will not spin. The grip is easily removed or loosened when the motor is installed.
I hurried to install the mast so that I can experiment with the 15 meter stack. The phasing and switching work is yet to be started. With the hay now at knee height and the easing of the pandemic lock down I have a brief window before work must be delayed until August.
- Raise the 15 meter 5-element upper yagi and ensure that it works
- Build the coax phasing harness between the yagis and connect them to the main transmission line
- Design and test the stack switch
- Install the prop pitch motor and test that the mast and yagi rotate properly
In the adjacent photo some of the hardware for the 15 meter yagi tram line can be seen on the mast several feet above the tower. Once the yagi is tested and mechanically adjusted it will be raised to the top of the mast. This last step is not urgent since the stacking work can proceed regardless.
If all goes well, after the hay is harvested I will assemble the 20 meter 5-element upper yagi, tune it and raise it to the top of the tower. The phasing harness and stack switch can then be constructed and installed. With a little luck my 15 and 20 meter stacks will be ready for the fall contest season.