Why do some radio towers blink?

https://www.jeffgeerling.com/blog/2025/why-do-some-radio-towers-blink

by warrenm • 9 months ago

179 118 9 months ago

scblock7• 9 months ago

FAA details the marking and lighting requirements here: https://www.faa.gov/regulations_policies/advisory_circulars/...

Johnny555• 9 months ago

One interesting fact I learned in a different discussion is that when LED lights are used for obstruction lighting, the FAA has standards that require infrared emitters to make them visible to night vision goggles, since unlike incandescent bulbs, some LED's can be invisible to NVG's.

https://www.faa.gov/sites/faa.gov/files/airports/engineering...

wolrah• 9 months ago

That is one of those wonderful facts that's both interesting and in hindsight completely obvious.

ranger207• 9 months ago

Speaking of, I was recently looking into what exactly makes lights NVG compatible, and found this informative PDF: https://www.consolite.co.uk/wp-content/uploads/2017/05/Basic...

tl;dr NVGs are sensitive to near IR so you want lights that are dim in those wavelengths (but not completely out since you still want to see them with NVGs) while still being bright in visible light. There's a neat picture of the flying controll tower on one of the UK's Queen Elizabeth-class aircraft carriers that shows two rooms with similar illumination levels in visible light, but under NVGs one room which isn't NVG compatible is massively brighter than the other. In the NVG-compatible room there were still a handful of panel indicator lights that were not compatible and lit up the whole space

pizzalife3• 9 months ago

This blog post has a really verbose format.

TLDR; White lights are used during the daytime, red lights at night (less annoying), towers under 200 feet don't need blinking lights.

jamesbelchamber• 9 months ago

It's a transcript of the video at the top.

skrebbel• 9 months ago

Wow that wasn't clear to me either, thanks for pointing it out

pizzalife3• 9 months ago

Yes, reading transcripts is a terrible way of ingesting information in my opinion.

cesarb• 9 months ago

It's a transcript of the video at the top.

Which is not obvious at all if you have JavaScript disabled by default, since it only shows up as a blank space, which could also be a blocked ad or an image which failed to load correctly.

The first few times I saw one of these transcripts with video at the top (IIRC, it was on Practical Engineering, not this site), I thought it sounded odd but didn't get that it was a transcript. Only later did I find out that there were videos (and they're great).

rollulus• 9 months ago

Slightly off topic: typically, lights of neighboring towers blink asynchronously. But sometimes they are synchronized. Very satisfying. Anyone knows how this works? My best guess is e.g. DCF77. Thoughts?

asdefghyk• 9 months ago

One option - Maybe the blinking time is set to a INDEPENDENT? accurate time piece - ie Blink on the change of a second

anyfoo6• 9 months ago

That's what DCF77 is. Or GPS.

winrid• 9 months ago

It's probably just a side effect of them being powered on at the same time or not?

anyfoo6• 9 months ago

If they don't sync to a common clock source, they won't stay in sync for long. Probably not even for a few minutes or so.

You'd get the same phenomenon that you see when operating turn signals in traffic. They seem to weave "in and out" of sync. The frequency at which that happens is the beat frequency, i.e. the difference between the two blinking frequencies.

scblock7• 9 months ago

When it's on purpose it's typically done through GPS driven clocks. This is how wind farms manage it, where all towers are required to blink together.

floatrock• 9 months ago

Yeah, I've seen it with windfarms. Always wondered why do they need to blink at the same time. The scale of the blink is pretty jarring at night (but also awe-some, in the same way any big enough infrastructure project inspires a kind of awe).

Wind farms have a certain amount of nimbyism because they "spoil the natural landscape." (So do regular farms -- nothing natural about grain silos or row crops, but that's a side topic...) Anyways, having that many towers blink in unison across that big a landscape is a weird effect when you first see it. I think there's an argument that if they blinked independently it would feel more natural in a way.

But since the blinking is all FAA requirements, I assume it's to help identify all the individual towers from the air. I suppose if they were all blinking independently, it would be a predator-trying-to-focus-on-a-single-zebra-in-the-herd problem, except in this case the predator is a pilot trying not to crash into a turbine.

Sure would emit more subtle 'part of the landscape' vibes though.

(Which I guess is exactly what you don't want when you're flying above them. Sigh.)

daemonologist• 9 months ago

I believe it's usually GPS/GNS (they all receive the time via GPS independently, and flash at predetermined times). The FAA requires synchronization for many classes of obstruction because it makes it clear that you're looking at obstruction lights rather than e.g. brake lights or traffic lights on the ground.

geerlingguy6• 9 months ago

Could they also use power grid sync? Not sure as I haven't talked to anyone in wind power, but grid sync would be pretty close to 1 Hz at least in the US.

Building a product that would sync at 1 Hz via GPS that works in the US and other countries with 50 Hz power would be a little easier than syncing to grid phase though.

askvictor• 9 months ago

My observation is not that they are sometimes synchronised, but some subset of the towers are synchronised (this was my observation in Melbourne AU). Upon asking reddit, it appears that it is the FAA-preferred option that all lights are synchronised: https://www.faa.gov/documentLibrary/media/Advisory_Circular/...

rts_cts• 9 months ago

Driving through a massive wind farm at night is a trip since they all blink in unison. Having them all independently would look interesting but could rapidly descend into madness:

https://archive.org/details/bigclive_20230516

SoftTalker2• 9 months ago

I’ve driven through wind farms where the blinking tower lights are synchronized. Highly distracting.

bluGill• 9 months ago

If you look close - almost impossible in the dark - you will see that most wind turbines don't have any lights at all. Too many lights is a distracion but all the synchronized lights are an easy to understand 'stay away from this whole area'.

There are FAA rules on this.

ck2• 9 months ago

muuuuch better article, lol

https://www.jeffgeerling.com/blog/2024/what-happens-when-you...

barbazoo2• 9 months ago

These beacons are also great for navigation. Aeronautical charts usually show the color/pattern of the light. You can use those as points to triangulate your position.

gorgoiler• 9 months ago

You have brought back to me memories from 30+ years ago, playing Microsoft flight simulator, trying to triangulate my position using VOR beacons as quickly as possible before my aircraft had moved so far on that I was no longer anywhere near my triangulated position, hah!

barbazoo2• 9 months ago

I never appreciated how realistic FS was until I started studying for the navigation part of my PPL. I flew a 172 too so the instruments felt very similar.

dylan6046• 9 months ago

IIRC, light houses are marked on charts similarly. The lights have different patterns, maybe color too?? Maybe not on aeronautical charts though.

clort• 9 months ago

Lighthouses are usually white. They used to rotate (sweeping the beam) but tend to flash now I think. Nautical charts show the elevation of the light, the visibility ranges and the pattern of on/off (eg Fl (2) 10s 41m 17NM). Some lights are sector lights, so red/white/green depending on your position, they indicate if you are in the correct approach channel.

It can be really confusing approaching a port at night. Never mind the city lights behind, and the cars driving about. 20+ lights flashing at different rates in the dark, some of them disappearing for seconds at a time. Oh, and if you turn your light on to refer to the chart, there goes your night vision!

Boat masts above 25m also have blinking red lights for the low flying aircraft (I guess its a bragging point)

skinwill2• 9 months ago

I worked at a television station years back that was designed in such a way that the lights going up the tower were powered by the separate phases of three phase AC with the one at the top powered from all three combined. This was pretty normal but what the engineer had done was rotate them at every level so that if a phase was dropped you could count the lights and quickly see from a distance that the power wasn't right. 4 lights was good, 3 meant you dropped a phase, and so on. I thought it was a pretty clever way of keeping light on all sides of the tower while being able to tell from a distance that a phase was out.

butlike• 9 months ago

Fascinating

dotancohen• 9 months ago

Phascinating

I couldn't help myself, downvote at will.

xenadu02• 9 months ago

This is best practice for anyone who uses three phase power.

A machine shop should connect 1/3 of their lights to each phase so it is immediately obvious if a phase gets dropped. Lots of equipment will suffer on two of three phases but with lower performance or even damage.

quickthrowman2• 9 months ago

This is best practice for anyone who uses three phase power.

No, it’s not. It’s a neat trick that visually reveals when the utility drops a phase, but there are better ways to handle avoiding equipment damage.

Best practice is to use phase monitoring relays that can de-energize a motor when a phase is dropped/reversed to prevent damage. The trip time is adjustable and it’s more reliable than manually hitting an e-stop. It also won’t let a motor with incorrect phasing start up either. You see phase loss relays on a lot of compressor motors and other large motors.

Here’s a flyer for an Eaton product: https://www.eaton.com/content/dam/eaton/products/industrialc...

genter• 9 months ago

I was told that you want lights on all 3 phases so that you can see spinning things spin. If the lights are on single phase, they will dim 120 times a second, and the strobe effect can cause spinning things to appear stationary. With 3 phase, at least 1/3 of the lights are lit all the time.

MisterTea2• 9 months ago

At work we run a lot of machinery with motors and its obvious when phase loss happens. From my office I can tell if the lights go out/dim and the usual shop "hum" becomes a buzzing grunt that is immediately identifiable. Older machines have to be manually powered down but the machines I rebuilt have phase loss protection in the PLC thanks to a power monitoring terminal in the IO (Beckhoff EL3453.) Since the PLC is on 24V DC I have a capacitor backup module fronted by a 24V PSU that takes the 480V three phase power which tolerates a phase loss. The machine safely stops the process and shuts down the pumps and any other AC loads, then waits to be manually powered off as the PSU and DC side doesn't care.

geerlingguy6• 9 months ago

Clever! I know I talked to the folks at Masterclock in St. Louis recently about one of their clock displays; they intentionally default the separators to flash if the clock is not synced to NTP, and then they go solid once the connection is established.

It's a quick way to know if something is down, using context clues that are already there to begin with!

skinwill2• 9 months ago

Jeff, it's very cool to hear from you as I have enjoyed your videos for some time. I would like to make one request though. Next time you do a project like "How I almost broke MrBeast's Ages 1-100 video" that involves setting up interactive electronics for an event, you should really call me. I managed interactive technology deployments for the automotive marketing industry for over a decade. It's a niche that has unique challenges which you fell face first into. I would be happy to help make things go much smoother should you ever do something similar. Proof: https://imgur.com/a/5z5JrwT I'm also on Reddit, same screen name if you want to dm me there.

ortusdux• 9 months ago

Blinking the lights also helps prevent bird deaths - https://en.wikipedia.org/wiki/Towerkill

Terr_• 9 months ago

Joe: [...] So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those?
Jeff: A full ghillie suit? Or I don't know what they're called.

If you see someone up in a tall tower wearing a ghillie suit [0]... that sounds like time to call emergency services while avoiding their line-of-sight. :p

(Perhaps they meant "Hazmat" [1])

[0] https://en.wikipedia.org/wiki/Ghillie_suit

[1] https://en.wikipedia.org/wiki/Hazmat_suit

mr_toad• 9 months ago

I can’t imagine why they’d need a hazmat suit either. It’s probably just protection from the cold.

fipar• 9 months ago

Seems to be needed only for old towers:

Tower painting has changed a lot over the years. The older towers have lead in them. So whenever there's a project on the tower, it's not unusual to see the guys in some kind of a, what do they call those?

geerlingguy6• 9 months ago

Haha, yes. For some reason, ghillie suit was the only thing I could think of at the time. I think they wear basic tyvek suits while doing paint work, not sure if they need full hazmat (maybe if the tower has lead paint?).

kcplate2• 9 months ago

If you see someone up in a tall tower wearing a ghillie suit

My first thought would be “that might be the dumbest sniper I have ever seen”…while I was taking cover, because even if they are dumb, they might still be a capable marksman.

MisterTea2• 9 months ago

I imagine that a properly constructed ghillie suit covered with chain link fencing, barbed wire, no trespassing signs and a blinking red light might go undetected...

hinkley• 9 months ago

Why do some radio towers NOT blink? Is the better question.

dylan6046• 9 months ago

Their height. Below a certain height, lights are not required

ofalkaed• 9 months ago

In the past year or two they have also added a quick periodic flash of white light for when visibility is low; like a camera flash that happens every few seconds. I think it was added this spring but don't quite remember.

loph• 9 months ago

It's not just radio towers. I've seen aircraft warning lights on tall buildings, particularly those near airports.

see https://www.ecfr.gov/current/title-14/chapter-I/subchapter-E...

emmelaich• 9 months ago

mentioned in the article

lxgr3• 9 months ago

Related: Some wind turbines apparently only turn on their position lights when there's any aviation traffic nearby (as detected by either local transponder interrogators (possibly ADS-B receivers?) or radar)!

scblock7• 9 months ago

Radar, called Aircraft Detection Lighting Systems (ADLS). The requirements are summarized in the FAA Advisory Circular covering aviation marking and lighting.

lxgr3• 9 months ago

As far as I understand, non-US systems don’t always use (primary) radar.

Razengan• 9 months ago

To clean their eyes?

punnerud• 9 months ago

Never seen a blinking light on a tower in Norway. Why the difference between countries, can’t be that huge difference risk if some don’t have them?

SoftTalker2• 9 months ago

Strictly speaking they should be unnecessary because there are published minimum safe altitudes for every air space over land. But some aircraft must be able to “See and avoid”

teslatree• 9 months ago

In Norway this is regulated by Luftfartstilsynets BSL E 2-1, and the blinking white lights on our towers are called "hinderlys", for example category "Høyintensitet, type B".

They are not uncommon in Norway.

If you go to one of our major airports you will see one on the tower. The blinking lights also sit on wind turbines and TV masts, and anything taller than 15 meters in rural areas or 30 meters in populated areas will have some kind of light on it, sometimes blinking, either red or white.

WalterBright3• 9 months ago

Blinking uses less power.

dylan6046• 9 months ago

Blinking attracts attention which is the real purpose. I'd assume based on how we detect motion more easily, these add a bit of "motion" to attract our attention

WalterBright3• 9 months ago

I know. But when I was programming LEDs for my single board computer, I found that blinking the LEDs used a lot less power. If the blink rate was fast enough, your eye did not distinguish that from the LED being on 100% of the time.

I don't know if battery powered devices generally use that trick or not.

ikkun• 9 months ago

the towers in my area all switched to LED recently. the slow, glowing blink of the incandescent ones probably isn't as visible as the modern ones, but I do dearly miss seeing it out my window.

spc476• 9 months ago

Changing a radio tower light bulb is not for the weak of heart: https://www.youtube.com/watch?v=9b9LahaBJIk

not4uffin• 9 months ago

Figured it was something to do with aircraft communications.

I live near an airfield and the runway has flashing white lights at night to help guide the aircraft's.

alex_young• 9 months ago

I love the narrative storytelling here, but the takeaways seem somewhat obvious.

Lights on towers mean stuff, especially to airplanes.

Lights are required for tall towers, and get this, towers next to airports.

You can guess how tall a tower is by looking at the lights.

themafia• 9 months ago

"My license to operate the tower is contingent upon my ability to appropriately light it."

gchokov• 9 months ago

that article is incredibly hard to read. Luckily, an AI can summarise it in seconds.

LargoLasskhyfv• 9 months ago

Flugwarnbefeuerung

wil421• 9 months ago

At my work we are working on automating NOTAMs when our tower lights go out.

Why do some radio towers blink?

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