Starlink: Tower

Published Wednesday, May 19, 2021 by Bryan

One reason I haven't worked on fixing my obstructions before now is that winter makes the ground impenetrable, and the roofs treacherous. The second reason is that I didn't want to attempt any solutions without having data to guide me.

"What data do you need? Just get the dish up as high as you can!" is the sentiment I've gotten from the Starlink subreddit. Forty-foot Rohn towers are the "put a bird on it" of that community. But the idea of just ordering $1000 or more of tower, and pouring a large cement pad for it, just to see if that fixes things, doesn't sit right with me. What if all I needed was 20ft of tower at a different location? What if I actually needed something taller than 40ft?

Don't even get me started on the, "Just cut down the trees," crowd.

So once it got warm enough to be outside without thick mittens for more than a minute at a time, I did what any engineer would do: I hacked together a sextant, and mapped the positions and heights of my obstructions - trees, mostly.

Surveying with what you have: tripod + walking stick = reference, protractor + straw + string + weight = sextant.

Then, because my particular engineering specialty is computers, I created a 3D model of the property, and put Dishy in it.

Dishy's original location, about 4ft off the ground. North is along the positive Y-axis, which runs to the right (view is toward WNW).

A cone with a 100º peak, rotated northward until its edge is 25º of of horizontal represents Dishy's field-of-view. I was lazy about tree modeling - they're just cylinders rising to the measured height, at the correct location. I assume that if I can get the cone above the peaks, it will also be outside of the rest of the tree shape. Comparing this model to the obstruction view in the Starlink app, I think I got close. Each tree I see inside the app's window, I see inside the model's cone.

Dishy 10ft above my roof. The blue cylinder near it represents my chimney. The tree in the cone to the north is far more in Dishy's field-of-view than this model shows, because many tall branches extend southward.

With hope that the model is correct, I started moving the Dishy cone around. My first question was whether I should plan to put Dishy on my roof. Unfortunately, there is a tree very close to the north side of my house, not to mention the east and west sides. The cone wasn't clear until Dishy was ten feet above my roof. To confirm, when the ice had cleared, I climbed on the roof and pointed the app around. The view from the app made me think this might even be optimistic, as some branches of the tree reach over the roof, in a way that makes the simple tree-cylinder model too simplistic.

Dishy a few feet southwest, and 16ft higher than its original location.

If the roof mount was going to require a tower anyway, how much tower would I need from the ground? I began moving Dishy's cone up from its original position. When I reached twenty feet, moving it a little south and a little west seemed to almost entirely clear the cone. If I raise the aim or narrow the field at all, as is expected to happen when more satellites come online later this year, the cone is completely clear.

So I should have just ordered a 20ft Rohn and put it there, right? Maybe, but, I'm not quite that confident in my mapping. My hacked sextant gave very coarse readings, made worse by the fact that many readings were over 45º, where sine grows faster than cosine. My compass liked to swing 5-10º between eye level and ground level. I think the model is a decent start, but I'm not willing to risk the permanent installation of a steel tower on it yet.

But I do like building things, and even though lumber prices are higher than usual, it's manageable for small projects. How close to 20ft can I get?

3D model of a 18.5ft wood-and-pipe tower. Adding Dishy's 16in stem places the center of the dish 20ft off the ground.

Pretty close, it turns out! A tripod made of 12ft 4x4s, with 7ft of a 10ft pipe sticking out the top, plus Dishy's 16in stem comes to almost exactly 20ft. If something like this will work, it offers a few nice features:

  • The pole can be raised and lowered to make installing Dishy easier.
  • The pole or the legs can be extended, if just a little more height is required.
  • If ground anchors are enough to keep it from tipping, it's possible to reposition the tower.

That last point is half of an important question: is this design strong enough? Luckily, radio operators have been mounting antennas on pipes for decades, so the engineering isn't hard to find.

Total Wind Torque < S40 Bending Moment. Hooray!

Dishy's stem has a 1.5in. outer diameter. Schedule 40 1.5in. steel pipe has a 1.5in. inner diameter, so mounting Dishy on such a pipe wouldn't even require the backordered adapter. Is it strong enough? I chose what seemed like the worst case scenario: Dishy's flat face pointed straight into an 80mph wind. Our winds generally come from the west (parallel-ish to Dishy's face), and average high gusts are 40mph, so this should give us a good margin of error. Luckily, even in the worst case I've specified, math says that 8ft. of 1.5in. S40 shouldn't bend. Hooray!

Total Torque << Ground Anchor Torque. Hooray!

What about those ground anchors? If I use the wind force already calculated, an calculate the leverage at the pivot point at ground level, I get almost 25,000in.lbs. I found ground anchors that say they provide 2250lbs. of holding power "in normal soil conditions." Given that they will be 4.5ft from the pivot point, that comes out to over 120,000in.lbs. of leverage. That's so much more than the wind leverage that even if my soil conditions are abnormal, I think I'm safe. (Yes, I have ignored wind torque on the wood tower as well. Given that it's much lower to the ground, and the anchor torque is so much higher, I'm not concerned.)

Dishy's mounting point: a 1/2in. bolt fits the squeeze-button holes perfectly.
Center braces assembled: opposite chiralities for some sense of stability.
Pipe clamps: bolts will screw into the t-nuts from the outside.
Treated pine cut to shape. Ignore the extra three 4x4's - they're for other projects.

If my well pump hadn't failed just a couple of hours into construction, I might have had the tower up in two days. It took three. The tower was stable enough for me to lean my ladder against while mounting Dishy. We've only had light breezes so far, but Dishy doesn't seem to wiggle too much.

Dishy powering up in its new home. View is to the southwest - these trees were outside even the original field of view.

So, the did-I-save-over-a-Rohn question: did I fix my obstructions? Almost. We had rainy days following installation, so obstructions have bounced around a bit. In the days before relocation, I was seeing over 35 minutes of obstruction per 12 hr period. Since relocation, I've seen as low as 2 minutes of obstruction per 12 hr period, and no higher than 12 minutes (during the thickest cloud cover). So it seems like I fixed at least 60%, and maybe almost 95%. Our leaves are finally starting to come in, so I'll probably let this setup gather data for a bit before deciding how much farther to push it. Come back in a couple of weeks for my May outage data analysis to find out what effect this tower had on my connection statistics.

If you're interested in building a similar tower, I've published my plans here:

Dishy looking (mostly) over the northward obstructions.