The Bizarre Bases of Antenna Towers

[00:01] In 1974, a new world record was set for  the tallest structure on Earth. Soaring   to 646 meters or 2,120 feet, the Warsaw Radio  Mast was built to broadcast radio programs to  

[00:16] Polish-speaking audiences across Europe. If  the atmospheric conditions were just right,   those signals could be picked up from  nearly anywhere in the world. But   like all big infrastructure projects,  building it was only half the battle.  

[00:30] Maintaining a structure that tall—and that  slender—was incredibly expensive. Over time,   the guy wires that held the tower upright  began to wear out. By 1991, many of them  

[00:42] were frayed and overdue for replacement, a job  that wasn’t just costly, but also fairly complex. To replace a guy wire, two temporary guys needed  to be attached to the mast first. Then the old guy  

[00:55] could be removed and swapped out for a new one.  But on August 8, 1991, the sequence got mixed up.   Reports vary, but it seems that one of the main  cables was disconnected before the temporary ones  

[01:09] were fully installed. A gust of wind twisted the  tower, pulling the temporary cables away, and the   unsupported mast collapsed. Incredibly, no one was  injured in the failure, but it was a catastrophic  

[01:23] loss nonetheless. Usually, the tallest structures  in the world lose their position because something   else is built taller. In this case, a tower  in North Dakota regained the lead by default.

[01:35] It’s actually not an unusual story.  This particular type of structure,   called a guyed mast, has some seemingly bizarre  structural characteristics that make it possible,   including the sometimes unusual bases that  seem to defy logic. But they come with risks,  

[01:52] too. At least nine guyed masts taller than  600 meters have collapsed, mostly in the US,   and hundreds of similar shorter structures around  the world as well. They’re pretty interesting  

[02:05] structures: cool to look at, incredibly tall,  just rare enough that seeing one is kind of   special. So this video is an ode to guyed  masts, and of course, I built a little demo  

[02:17] in the garage to help explain how they work.  I’m Grady, and this is Practical Engineering.

[02:33] Radio communication is a remarkable technology  that enables a huge variety of wireless devices,   from garage door openers to cell phones. electromagnetic radiation, even just the  human-made stuff, we would be completely  

[02:52] overwhelmed by the volume and variety of  information moving through the airwaves.   Many of the frequencies used for communication,  especially those broadcast by radio and television   stations, require a clear line of sight; the  path between the transmitter and receiver  

[03:08] has to be relatively unobstructed, at least  by objects that are opaque to radio waves,   like the earth. That’s why many antennas  are mounted at the tops of hills, mountains,   or (lacking those) gigantic towers. The higher  they are, the further their signals can extend.

[03:26] Antenna towers are some of the tallest  human-made structures in the world,   with many topping out above 600 meters  (roughly 2,000 feet). At that height,   the distance to the horizon is more than 50  miles (or 80 kilometers). To achieve that  

[03:42] has required some very clever structural  engineering. Let me show you what I mean. This is my model antenna tower. Pretty basic;  just a steel welding rod stuck in a plate.   This isn’t going to match the structural  behavior of an actual mast, but it’s close  

[04:00] enough for a garage demo. The main load on  a tower like this, besides its own weight,   is wind. So let’s apply some wind and see what  happens.

[04:18] The tower’s still standing - it   didn’t collapse. But structural engineering  isn’t all about strength. A structure can “not   fall down” but still fail. We also have to address  the concept of serviceability: does the structure  

[04:34] actually do what it’s meant to? And in this case,  hopefully it’s clear that the answer is no. Many   antennas are designed to be directional. It takes  a lot of power to radiate signals, so you don’t  

[04:46] want to waste it sending them where they’re not  needed. This varies a lot depending on the end   use. Radio and TV broadcasts are less sensitive  to movement than microwave communications, but in  

[04:58] general, we can’t have antenna towers wobbling  around like floppy wet noodles in the sky. You can imagine that to adequately stiffen this  tower, it would have to be a lot wider at the   base. And that’s just what we do with so-called  self-supporting towers. They’re designed to be  

[05:14] freestanding and stable against the wind entirely  on their own. Self-supporting towers don't take up   much space, so they are ideal in urban areas where  land comes at a premium. But, they are expensive  

[05:26] to build because of all the extra material  required for stiffness and stability against   lateral wind loads. In fact, their cost goes up  roughly proportional to the height squared. For  

[05:39] guyed masts, it's roughly height to the power of  1.5. You need more land for a guyed tower since   the guys extend so far out, so there is more  cost there, but above a certain height (that  

[05:51] depends on those land costs), it becomes the most  economical option. And for really tall towers,   it’s really the only technically feasible one.  They are just so structurally efficient, it's  

[06:03] almost unbelievable. To give you an example, at  324 meters tall (or 1,060 feet) the Eiffel Tower   weighs around 7000 tons. A guyed tower of the same  height would weigh roughly five percent of that.

[06:20] So let me add some guys to my tower  and we’ll see how it works. Of course,   you can’t add just one. Wind can come from any  direction, and don’t forget one of the most   important adages of civil engineering: you can’t  push a rope. So it takes at least three guys to  

[06:37] get some tension in every direction. Some towers  use four lanes, but most stick with three. This   seems like a more stable situation, but now we’ve  got a new problem. Watch what happens when I apply  

[06:49] a lateral load. It's still just not that stiff,  and actually, the tower buckles. And here’s why: The guys can’t pull horizontally on the  tower to resist lateral loads directly.   They have to be anchored to the ground,  which means they meet the tower at an  

[07:04] angle. Any tension in the cable is going to  necessarily put the tower in compression as   well. And what happens with skinny  compression members? They buckle. Steel can take a lot of compression.  Theoretically, this rod is strong  

[07:19] enough to hold my entire weight without  a material failure. If it were short,   it’d be more than capable of bearing a full  Grady, but when it’s tall and skinny like this,   it can barely hold its own weight. When the tower  takes a lateral load, the guy wires transfer that  

[07:34] into compressive force. And unless the structure  is stiff enough, it buckles. you can see it takes a lot more wind load to buckle the  

[07:46] structure. Less cable tension is needed for an  equivalent horizontal force. And this is one of   the many structural tradeoffs with guyed towers.  You have to balance the land cost of extending  

[07:58] anchors outward against the cost of a stiffer  tower that can withstand steeply angled guys. But you can see we’re not quite out of the  woods here. Some shorter guyed towers can  

[08:10] get away with one level of supports, but  mine is still pretty flimsy in the middle.   and it’s still prone to buckling under  compressive loads, like, for example,  

[08:23] the weight of an antenna mounted to the top. We’ve got supports on both ends and loads trying  to bend the structure in the center. So we can  

[08:37] do what the bridge engineers do: either stiffen  the structure or add more intermediate supports.   It’s a little more complicated than that though,  since every guy adds additional compressive load   on the tower, in addition to providing lateral  support to reduce the unbraced height. You’re  

[08:53] kind of adding to both sides of the equation.  Luckily, the lower you go on the tower, the   shallower the angle of the cable. Just as a little  demonstration of this, let’s compare the loads   my little tower can support as we add more guys. 

[09:10] This can barely support its own weight, let alone  any extra on top. With a second level halfway up,   it’s quite a bit stiffer. I could get 100 grams  on top with no failure. Adding two more levels,  

[09:28] now this thing feels rock solid. I’m  not sure if it comes across on camera,   but the change in stiffness is dramatic.  It passes the wind test with flying colors.  

[09:41] It couldn’t quite hold a kilogram,  but Brady could sit on it just fine,   even if it made him a bit uneasy (since his  hard hat is still damaged from the last demo). One of the other tradeoffs with this is  the pre-tension of the cables. These guys  

[09:57] sag along their length; they’re not perfectly  straight. Under high wind, they tighten up and add   stiffness. But in calm conditions, that slack can  cause the tower to wobble. The obvious solution   is to pre-tension the guys to take the sag out,  but again, that pretension puts extra compression  

[10:15] on the tower, requiring stronger members or  more guys. So this is a balancing act as well. And then there’s the base. You have essentially  two choices here. We’re used to seeing large   columns with a rigid attachment to the foundation.  I did a whole video on base plates diving into  

[10:32] this topic deeper if you want to learn more. You  can see in my model that, with a fixed connection,   my tower holds itself up just fine without  loading. Obviously, this rod is solid steel - not   a thin latticework of individual members -  so the behavior is a little different. But  

[10:48] remember that buckling is a function of the end  connections of the column. With the bottom fixed,   it takes about 140 grams to buckle the rod.  When it’s free to rotate at the bottom,   it buckles at around half that. The  problem in this case is that fixing  

[11:04] such a tall tower rigidly to the foundation  makes the design a lot more complicated. If you want rigid restraint, you have to have  a way to transfer the loads into the ground. So  

[11:16] the foundation has to be designed to resist  rotation and pullout forces, and for not a   lot of structural benefit. So the other option  is to use a spherical bearing or pin support.   And if you keep your eye out, you’ll see that a  lot of these masts have these sorts of unusual  

[11:33] bases where they taper down to a narrow  point. In this way, you can just rely on   the guys to handle almost all the restraint. The  foundation only has to resist the vertical force,   and maybe a touch of shear. This allows some  movement or settlement of the foundation  

[11:49] without inducing stress into the structure.  And it just makes the design process easier.   Removing the restraint simplifies the structural  response and makes the tower more predictable,  

[12:01] so you don’t have to be super conservative  or spend tons of engineering effort and   use sophisticated modeling software in the  design. Finally, some towers aren’t used to   mount antennas; they are the antennas themselves.  For lower frequency transmissions like AM radio,  

[12:18] you need a big antenna, so the tower  itself is energized. In those cases,   the base needs to be electrically insulated from  the ground, which is much easier to do at a single   point. If you look closely at some towers, you’ll  see they’re actually standing on a ceramic disc.

[12:35] Beyond structural design, these masts come with  a lot of other engineering challenges. Of course,   there’s the hazard to aircraft.   painted in alternating orange and white  bands and equipped with warning lights,  

[12:53] whose color and flash rate  are carefully prescribed,   and can even be synchronized with nearby  towers to avoid dazzling pilots at night. Ice is another big one. These towers stretch  into colder, wetter layers of air where ice  

[13:08] can build up on the mast and guys. That  adds weight, but it also adds surface area,   sometimes dramatically increasing wind loads. When  it melts, it can fall and damage anything below,  

[13:20] so often you’ll see protective structures  over the radio transmission lines. Lightning is another threat. For most towers,  it’s not a question of IF, but rather HOW OFTEN   they’ll be struck. Towers are often equipped  with lightning rods or other protection devices  

[13:36] and robust grounding systems to keep stray  voltage out of the transmission lines and   sensitive equipment on the ground. Obviously,  those mast radiators I mentioned earlier,   where the entire tower services as the antenna,  can’t be grounded for lightning protection.  

[13:53] So most use some type of spark gap to keep  the tower insulated. If lightning strikes,   the air in the gap ionizes, allowing  the surge to safely reach the ground. Like all infrastructure, antenna towers need  maintenance - painting, changing light bulbs,  

[14:09] and servicing antenna equipment. Technicians with  specialized training for heights and electrical   hazards have to do the work. Some tall towers are  even equipped with elevators to provide access,   but most require some manual climbing.  Although the frequencies used for radio  

[14:25] communication are non-ionizing (meaning  the waves can’t break apart molecules),   that doesn’t mean they aren’t dangerous.  Electromagnetic radiation can generate heat;   it’s the fundamental principle of a microwave  oven. And if the tower itself is energized,  

[14:41] With so much of our telecommunication happening  through the internet these days, it’s easy to   forget the importance of large-scale radio  broadcasting and communications. The cells for  

[14:56] cellular communications are small, so we’re used  to seeing those antennas relatively close to the   ground. But you have to look way up to remember  how critical the other wireless systems are,  

[15:08] especially in emergency situations where radio  and television signals can be an essential link   to information. So next time you pass one  of these towers by, take a closer look,   and I hope you’ll appreciate some of the  thoughtful engineering that goes into them.

[15:25] I plan these  videos out in advance, and I actually bought   a telephoto lens for my camera about a year ago so  I could get some of the shots in this video. So,   I went outside to test it out, and the first  thing I took a picture of was a bird. Of course,  

[15:41] then I wanted to learn what kind of bird  it was. And that basically snowballed   into a full-on new hobby of birding. I’ve got  feeders in the backyard, fancy new binoculars,   and the Merlin app on my phone. I try to get  out at least once a week, and so far I’ve seen  

[15:56] about 160 species. But once you start paying  attention and learning more about birds, it   can be a little disheartening. For example, I see  Loggerhead Shrikes pretty regularly here in Texas,  

[16:08] but if you live in the northeastern US, they’ve  pretty much disappeared. The species has lost   about three-quarters of its population in North  America since 1966, and that’s just one example. Another is Little Owls - cute little guys who  have seen major population declines in some  

[16:25] parts of Europe. My friends at Planet Wild have  been working with conservationists in Germany to   help re-establish the population there. Planet  Wild is a community-based organization dedicated   to protecting our natural world, including  wildlife. It’s basically crowdfunding for nature.  

[16:42] Every month, all the members fund a new  project related to endangered species,   oceans, or forests. And then they produce  a video documenting the project so you   can see for yourself where your money  is going and the impact it’s having.

[16:55] I love the idea, which is why I’m a member.  It makes me feel more connected to the causes   I care about and part of a community who are  working together to accomplish something bigger   than any individual could do on their own. If  you’ve been looking for a neat way to give back,  

[17:10] I think Planet Wild is a great way to do it. And  to prove it, for the first 100 people who sign up,   I’ll cover your first month. Just scan the QR  code or click the link in the description and   use my code PRACTICAL10 to get your first  month free. No catches - you can cancel  

[17:26] anytime. If you’re not sure yet, go check  out their project protecting the Little   Owl in Germany. You can give whatever amount  - big or small - that feels right to you. Your   money will go towards really cool conservation  projects that you can watch happen here on  

[17:40] YouTube. I hope you’ll consider joining. Thank  you for watching, and let me know what you think.