In the fall of 2018 I decided to experiment with constructing an antenna mast with ABS pipe. The saying is that “the only experiment that is a failure, is one where you don’t learn anything”. So my experiment was a complete success when the mast catastrophically collapsed.
Masts and Towers
First off, being a geek, I had to look up the difference between a mast and a tower. Basically, a tower is a structure that is way taller than it is wide. It is not intended to be used as a residence or an office. It is totally self supported. A mast is a tall narrow object that is guyed to keep it from swaying on its own. Like a ship mast. So I’m building a mast.
Whenever you look for an example of how to string a dipole antenna in a tree, they have these amazing fantasy trees that don’t exist. The trunk goes straight up. The first branch doesn’t come out until about 70 feet up, and is about 4 inches in diameter and sticks out to 10 or 15 feet away from the trunk and all you have to do is throw a line over that amazingly big branch and you’re all set to go.
The photo of actual spruce trees is actually larger than it should be to be in scale with the drawing. When they show off dipoles and masts, it’s kind of like at the RSOC. Relatively flat, with mowed grass as high as 2 or three inches.
I’m located in Bear Valley, in the southeast corner of Anchorage, up the hill from Rabbit Creek Road. Bear Valley is in the “except” part of the weather forecast. When you listen, they will say something like “the winds will be light at 15 miles an hour from the East except 25 gusting to 45 along Turnagain Arm and the upper hillside”.
I have scraggly spruce trees on my lot. Some of them are about 40 feet tall. The trees whip around in the wind. I used to have a wind sock, but the galvanized steel pipe mast broke along the threads (but it didn’t have guy lines, so I guess it was really a tower.)
The terrain up here is steep and incredibly wet. Looking at a topographic map of the area, the contour lines are 80 feet apart. Just to the East of my house is a swamp symbol. In the immediate area where the swamp symbol is, I measured the distance between the contour lines. When I calculated the grade, it was between 13% and 18%.
The maximum grade for Federally funded highways is up to 6% generally allowed in mountainous areas and hilly urban areas. In my neighborhood, the feds put up a swamp symbol over an area that is twice as steep as they would allow to build a highway.
The other fun thing for putting up masts and dipoles is the ten foot high alders that shelter the five foot tall Devil’s Club infestations. When you’re dealing with Devil’s Club, regular 3 for $10 work gloves don’t hack it. Neither do heavy leather work gloves. They just trap the stinging barbs in the leather, so that when you put them on again months later, you get stung. The only thing that I found that works for handling Devil’s Club is full blown heavy leather welding gloves.
ABS pipe is black, comes in ten foot sections, and is only about $15 a pop. I had done some projects with ½” and ¾” PVC pipe, so I figured that a larger diameter ABS pipe might work for a mast. The larger diameter makes the pipe stiffer.
I started with some 2 inch diameter ABS pipe. A ten foot section drooped a little bit when I held it in the middle. Attaching two of them together, the 20 foot assembly did droop, but didn’t seem too noodly. I figured that when I guyed the middle it would be OK. For the base of the mast I used some 3 inch diameter pipe. The 3 inch pipe by itself didn’t seem to bend on its own at all.
The next trick was to glue the pipe together. I had done lots of connections with ½” and ¾” PVC pipe. With ½” pipe, there is an area about inch and a half around by half an inch deep for the glue joint. You just push and twist the two parts together. With 3 inch pipe, there is about 9 inches around by an inch deep of glue area in the joint, significantly more glue and friction than the ½” pipe connections. Plus, the pipe is quite a bit bigger in diameter.
I can’t palm a basketball, and I can’t wrap one hand around a 3 inch diameter pipe. Plus you have to shove the parts together pretty hard to make sure that they bottom out fully in the connector. Did I also mention that the sections are ten feet long? Kind of hard to do solo. I finally figured out a system with some straps, 2x4s, a strap wrench, and something solid to brace the pipe against. With that, I could put a block of wood against the end of the pipe and beat on it with a dead blow hammer to seat the connection fully.
I wanted the mast to start out straight before I put it in the air, so I had to search for a while, looking for a straight, long place to put the mast while the cement cured. Someplace not in the driveway.
After a while I had a 30 foot long mast, two 2” sections connected to a 3” base pipe, I picked it up. Unbelievably noodly, magnitudes more noodly than a twenty foot section. Like the whole thing was made out of flexible plastic! It weighed less than 30 pounds, so with the exception of the Three Stooges effect of swinging it around anything breakable, it was easy to move around. In some cases it was easier to poke it though the alders, walk around the long way, and pull it through from the far side, rather than alder bash with a 30 foot mast.
I had cleared part of the area where I wanted to set up the mast. Now I had to figure out exactly where I was going to put it. In the beginning I thought about putting one of the guy ropes on a cardinal heading, or maybe putting a guy line going straight up the hill, or straight into the big winds, but nothing was ideal. I had one end of a dipole high up in a tree, but there wasn’t a straight shot to anywhere good that I could use. The area that was open and far enough away for half a dipole was limited. I have some downed spruce tree trunks that I can use for anchors.
The 120 degree spacing was driving me nuts. I’d aim a guy line at a good anchor, and the other two lines would conflict with something. I’d move the base 5 feet in another direction, and aim a guy line at an open area, and nothing else would line up. After much trial and error, I found a spot for the mast. I’d only have to clear out a twenty foot long by eight foot wide section for the guy lines.
I had a 35 foot tall ABS mast, with a 15 foot base of 3 inch diameter pipe with 20 feet of 2 inch pipe on top.
I figured out the guy line lengths for 15, 25, and 35 foot high attach points, and a 70 foot long halyard to hoist the middle of the dipole up to the top.
Did I mention that this was on a hill? I measured out from the mast about 25 feet for the anchor for the guy lines. On the West, downhill side, the anchor point was about 5 feet lower than the bottom of the mast. Of course, all three guy lines on that side were too short. I was able to loop a galvanized steel cable around a downed spruce tree and attach it to a thick steel perforated strap that had large smooth holes in it for the guy lines. Unfortunately it was almost out of reach. I’d have to pull all three guy lines down, and bend the mast a bit to adjust the guy lines, release them, and see if the adjustment was good. What fun.
I tried, and tried to tip the mast up by myself. I could pick it up with one arm, but I couldn’t tip it up at all. I was able to brace the bottom against a tree stump, and push up against it. I just managed to create a 35 foot long arch that went up maybe 12 feet high in the middle. Guy lines tied to the top of the mast wrapped themselves around alder and Devils’s Club stumps. As the mast was waving around in the air, guy lines got tangled up in a spruce tree. I tried tightening some guy lines, hoping that it would get the top into the air, but they all drooped lower than the mast and ended up guying the top of the mast down to the ground.
The next day I got some help from my friend Lenny. With two of us pushing as hard as we could, braced against the stump, we managed to get the end of the mast almost 3 feet off the ground. Kind of like those fly fishing ads where the pole is arched way over with the tip almost touching the water, except we were not smiling, and we didn’t have any fresh fish for dinner. That afternoon was a lot of work. Period. Sweat, mosquitos, and frustration. We didn’t get the mast up.
The next day I got the big step ladder and the slingshot. I put a new guy line up into the tree, using it as a high line to get the top of the mast up first. Then we could walk the rest of the mast under it. After an hour or so, and many tries, the mast was up.
Once the top of the mast was in the air, I could pick up the mast and move the base where I wanted. I just needed some slack in the guy lines. I could move it if it was horizontal, or vertical. I just couldn’t move it from horizontal to vertical.
The mast was not really straight, but at least it was mostly vertical. Actually it was a strange 3-D corkscrew kind of shape. This is when some of the optical illusions kicked in. The scraggly spruce trees. Black mast with diagonal black guy lines. Stand near the mast and look up. The mast is wiggly, with 9 guy lines and the two lines for the halyard. Nothing straight and vertical anywhere near it to even tell if we’re within 15 degrees of vertical. Shift positions to one of the guy line anchors. Looks funky. Go to another anchor. Now it looks like it’s mostly leaning in a different direction than it looked at the first anchor. Go to the third anchor, and now I have even less of an idea of which way it’s leaning than when I started.
I finally decide to loosen the guy line at the 25 foot level on the east anchor. It’s obviously too tight, and loosening it will definitely help. I go that guy line, and it’s slack. Only the weight of the guy line pulling on the mast. What the heck?
After futzing with the guy lines for a long time, using a level on the mast, and letting the halyard hang free as a plumb line, I got the mast looking OK. I hoisted my dipole up, and everything was working.
A week or so later, it got a little windy. Only gusting into the upper 30s. From my office window things looked OK. I can’t see the mast through the trees from my office, but the horizontal dipole looks like it’s heading in the right direction. Later on I walked out in the rain to take a closer look. Something is not right. The antenna coax and ladder line are wrapped up in two different trees and some alders. Almost every guy line seem to be loose. How could all the guy lines be loose? Hmmmm. Why is the cleat for the halyard 18” off the ground? It was at least waist high two days ago. Hmmm. Just for the heck of it, let me lift up the mast. What the heck? The weight of the mast, the guy lines and wind jiggling made the mast sink into the peaty soil close to 18” deep.
During the setup and adjusting the guy lines, it hadn’t sunk in more than two inches. I’m not about to dig a refrigerator sized hole full of rebar and concrete for a base, so I’ll come up with something else. I ended up with a length of 2×8 treated lumber, cut an arc out of the middle, and hose clamped an “L” bracket to the mast. I put the board parallel to the slope and tried to straighten the mast with the guy lines again. After too much fighting with the 120 degree angles, I decided to add some more guy lines to the lower sections and make them 90 degrees apart.
At the 25 foot level I was able to create a big loop above all of the 25 foot guy lines. I created a noose, and with some tugging and poking with some 10 foot sticks, was able to tighten the noose at the 25 foot level. With 4 sets of guy lines at 90 degree angles it was a lot simpler to get the mast adjusted, but it still had a genetic disposition to curve. I did the best I could to get it to an acceptable state. After all, this is an experiment.
In November, we got some medium wind. It was steady in the 40s with gusts into the 50s, and it was below freezing. As I was heading down the driveway the next day, I looked over at the mast, and it wasn’t there. I walked down into the clearing, and it was obvious that the mast had catastrophically broken, into three pieces. Three pieces!
PHYSICS: Basile Audoly and Sebastien Neukirch of the Université Pierre et Marie Curie, in Paris, for their insights into why, when you bend dry spaghetti, it often breaks into more than two pieces.
B. Audoly, S. Neukirch
Bent dry spaghetti do not break in half but instead in three or more pieces. With the aim to explain this surprising phenomenon, we studied a related problem, namely the dynamics of an elastic rod that is bent quasi-statically and then suddenly set free. Counter-intuitively, we find that the mere release of the rod induces a stress increase. The multiple breaking of bent rods, like dry spaghetti pasta, can then be understood as a cascade of releases (loss of cohesion upon breakings) followed by stress increases leading to new cracks.
Maybe that’s why my mast broke into three pieces. That’s my story, and I’m sticking to it.
Walter Yankauskas – KL7WY