A gin pole for the lower sections of a Trylon or similar self-supporting tower must be very robust. The heaviest section I am raising is the #11 section which lists as 168 lb (80 kg). This is substantially more than the 50 lb sections of DMX for my earlier gin pole. The structural requirements are challenging. Not only that, lifting that amount of weight safely and with enough maneuverability (to position the section for splicing) is also a challenge.
Pole and tower "hooks"
I have many lengths of 1.5" schedule 40 water pipe lying around that look suitably attractive for this purpose. It isn't especially light -- many prefer aluminum alloy pipe or tube -- but it's up to the task and I have it on hand. Water pipe is regularly dismissed as unsuitable for mechanical applications, such as masts for yagis, yet it perfectly usable when its limitations are kept in consideration. It also did well when bending large diameter rebar.
Since the forces are primarily axial rather than radial they can be used for gin poles. My only concern was getting the height right and being cognizant of the difficulty of handling its weight (2.7 lb/foot) from the bottom end when attaching and detaching it from the tower. The gin pole I constructed comes it at about 35 lb (16 kg) and 10' tall. It is cumbersome enough to easily get away from you when trying to hold it vertical from the bottom. You must be strong enough to handle it on your own or invite a friend up the tower with you.
I was offered the free use of a gin pole of my choice from the stable of tools kept by a local tower service company. However all of them were quite long and heavy and would require two people to manipulate. They, of course, make tools for large commercial towers which must necessarily be heftier. The gesture was appreciated nonetheless.
One nice thing about much of the scrap pipe I have is that it comes pre-drilled with ⅜" holes at top and bottom. (The pipe had previously served its previous owner as a long mast for a ground-mounted rotator, hence the holes for angle stock pipe splices.) There is therefore less machining required than with new pipe. Only one more hole needed to be drilled for the second attachment point to the tower.
To avoid the need for welding or metal bending I designed the gin pole to require little metal work. The detail of a tower "hook" is visible in the above photograph (close up adjacent), while the #11 section is being attached to #12 base section.
A 3" long ⅜" grade 5 bolt holds a sandwich of several parts. In order, from the inner side of the pipe:
- 1"x⅛" steel stock 3" long with a ⅜" hole in the centre. This is a "keeper plate" to prevent clockwise rotation (as viewed from above).
- Flat washer that butts against the tower leg.
- Shaped 1"x1"x ⅛" angle stock 2.5" long that provides a seat for resting on a downward diagonal cross-member, and as a keeper plate to prevent counterclockwise rotation. The shaping allows further insertion into the leg interior before bottoming out on the smaller (upper) tower sections.
- Flat washer. A lock washer is a poor choice since the angle stock must rotate slightly when attached to the tower because the diagonal orientations (angles) are all different.
- Grade 5 Nut.
Pole length and splice procedure
The next picture shows the 150 lb #11 section lifted and ready for splicing to the #12 section.
With this 10' pole the maximum lift is 6.5' (2 meters) above the lower section. This is enough for an 8' tower section. Since lower sections are so wide (3'+) any attachment to the perimeter of the section will result in the section hanging diagonally. As you go up the tower the lean will decrease as the ratio of height to width increases.
It is not difficult for even one person to grab and swing the section into vertical orientation. The bigger problem is judging whether the lift is just right for the section to be fitted. Due to the heavy weight there is only a small margin unless there are two strong hams up the tower.
To ease the process with just me up the tower, with or without someone working the winch, I first tie a rope around the gin pole and leg attached to the cable. This keeps the section near enough to vertical to allow fine adjustment of height for the remainder of the splicing operation. Despite appearances it is the other two legs that first contact the lower section when it is rotated to the vertical orientation.
When the outer legs are hanging inside the lower section legs I inserted bolts into the lower legs to prevent the upper section from sliding down. Thus when the section is lowered further to slip the inner leg into the lower section it is in near vertical orientation and is unlikely to jam due to excess lean. The bolts can then be removed and splicing can proceed to completion.
It's important to attach all 12 bolt loosely at first. If a bolt is tightened too soon it may be impossible to insert the bolts on the other face of the leg due to misalignment. Gradually tighten all 4 bolts per leg splice in a rotation to achieve maximum alignment.
There are several possible ways of putting muscle into a gin pole to lift heavy sections:
- Lots of friends, or at least two strong ones
- Winch, motorized or manual
- Tractor or similar motorized vehicle
I purchased a winch capable of 1,400 lb working load. A smaller capacity winch would suffice but the ones I investigated didn't meet my expectations for spool capacity, ratchet quality or other criteria.
I am using 3/16" aircraft cable with a 6,600 lb breaking strength. Smaller cable is, again, sufficient but a poor choice due to the risk of pulley cage pinching, abrasion or kink failure and tangling. I happened to enough left over from a previous project. One end went onto the winch spool and the other was terminated in a forged steel hook with a thimble and cable clamps.
I constructed a platform and tower clamp for the winch out of surplus lumber and some bolts. The outside half holds the winch. The inside half is cut to fit underneath the upper diagonal and atop the one below. These two diagonal converge away from the tower leg so that when the two halves are bolted together the platform cannot move outward. Both halves sandwich the tower leg to secure it against slipping inward or outward. The inner half is 2x6 lumber and the outer is 2x10. There are 4 bolts tying them together. The outer half is wider so that the winch handle clears the tower leg.
On the right you can see the winch in action, fully supporting the weight of the #11 section. By running the cable onto the outside of the spool there is increased outward force on the platform. With a platform made of lumber there is some risk to doing this. However the bigger risks are abrasion of cable and tower and cable snagging were the cable run to the inside of the spool.
As this is written I have only 3 sections raised, sections #12 through #10. Weather and other tasks have delayed further progress until next week. It gets easier as you go up since the sections rapidly decrease in weight. The #9 section (see picture) is the last that is over 100 lb. Eventually I'll dispense with the winch and use a rope and muscle. I've already fitted a rope cleat to the tower, as I have done for all of my towers.
I have some work yet to finish on the rotator and bearing plates (#5 and #4 sections, respectively) so I needn't rush. There is also ongoing work on the LR20 150' guyed tower which I'll write about when it's ready to go. The gin pole for the Trylon requires only a few modifications to be used on the LR20 since both towers use a similar leg design.
If all goes well the Trylon should be fully built within two weeks. I can hardly wait to have a real antenna again. A simple, low antenna doesn't appeal to me the way it did when I returned to the hobby in 2013.