Eli5: Why do radar antenna still need to spin?

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Eli5: Radar are built to spin around, send out, and capture a signal to create a 360 degree image of the surrounding area that regularly updates.

One would think that you could build a stationary antenna that electronically pulses and limits the area it is searching to do the same thing, removing the complication of the moving parts.

Why isn’t this the norm? And is it even possible?

In: Engineering

23 Answers

Anonymous 0 Comments

It’s a matter of timing. When the radar bounces off an object you want to know how far it is.

The direction it comes back, plus how long it’s been since it was pointed in that direction let you.know how long it took.

Additionally it allows you to pump a very strong signal down a corridor so you’re able to detect better.

With an omnidirectional you’ll have lower energy pulses and it’ll be harder to get distance as you couldn’t be positive which pulse is causing the return signal.

Anonymous 0 Comments

Yes, aesa radar will use phase interference to direct the beam and it’s used in many applications, including Starlink.

A nice and funny video:


More info:


It’s done a bit differently by generating pulses with delay in a way that waves cancel each other unless they’re in a specific angle.

Anonymous 0 Comments

> Why isn’t this the norm? And is it even possible?

Yes, there is Active Electronically Scanned Array (AESA) radar which uses a computer-controlled antenna array to aim radio waves without moving the antenna. They have been in use since the mid-60’s.

However there are limits to how much their signals can be steered. It can’t steer them around to go completely backwards for example. So either you have multiple antenna arrays to get a 360 degree view, or you just spin the same antenna array around.

Duplicating equipment is expensive, spinning something in a circle is really easy.

Anonymous 0 Comments

We have 360 degree radar. It’s not as precise as narrow radar. The antenna can get better resolution if it’s a little “zoomed in”, or focused on a narrow area of the sky, but obviously it can’t see the whole sky that way. It’s also a little easier for the computer to tell exactly what direction something is in if you are spinning.

Not all radar that spins actually spins. The f35 has a radar under it’s nose that scans back and forth without moving. Its made out of a bunch of little antennnae arranged in a pattern. It’s possible to adjust the timings of the different antennae so that the beam comes out at an angle. This is called a “phased array”, and it’s convenient because it has all the benefits of a scanning radar with no moving parts.

Anonymous 0 Comments

moving parts dont always equal complexity; we’ve had rotating devices for centuries. parts are plentiful and easy to repair or replace.

Rotating the radar device is independent of the radar technology.

now build a new radar that’s capable of blasting 360; but limit it to a few degrees at a time. you now have a more complex radar.

since the radar is more complex it has more points of failure and requires more technical knowhow to fix. if it only blasts 180 degrees you can still make it work by…….physically rotating it

Anonymous 0 Comments

They don’t.

You’re right. It is entirely possible to build radar systems that don’t require moving parts.

Why don’t they? Well, the general consensus was “ThEY’vE AlWaYS spUn LIkE ThAT!”

I never understood why. From a practical point of view, the fewer moving parts, the less maintenance it needs. Also, being able to wrap it around the mast would eliminate blind sectors.

But noo…still spinny-winny because…

Anonymous 0 Comments

>One would think that you could build a stationary antenna that electronically pulses and limits the area it is searching to do the same thing, removing the complication of the moving parts.

A radar is basically an antenna that produce a radar wave and then received it. The antenna can only send the signal in one direction so you need to point the radar toward the object for it to work, which is why you rotate the whole radar to look around.

We do have stationary radar, the first kind is PESA (Passive Electronically Scanned Array). You still have one transmitter/receiver, but instead of having one big antenna, you have a bunch of really small antenna on a flat surface. If you send a signal from all those antenna at he same time, you basically get a normal antenna sending a signal straight ahead. But if you delay some antenna, the interference between all those different signal will shift the whole signal toward a direction. This way, even if all those antenna are fix you can send a signal in a large angle ”cone of vision”. Since all those antenna are small, it mean that the whole radar look like a plate and you can put those plates around a fix platform to get a 360 degrees vision. This is what we often see in warship. PESA are really good at looking at a specific area really fast and get precise information, which is pretty good for weapon targeting systems. That said, if you are trying to search a wide area to find targets, then a big rotating antenna is still better. You can make the antenna very powerful, put it at the very top of a pole and have it rotate. An other disadvantage of the PESA is that you are limited in the number of target you can track at the same time.

Another type of stationary radar is AESA (Active Electronically Scanned Array). Improvement in technology meant that we could not make the transmitter/receiver small so each small antenna can now have their own transmitter/receiver. The main difference now is that you can have a bunch of antenna send a signal to the left, while the bunch of other antenna send a signal to the right. Now instead of having to scan one region at a time in your cone of vision, you can scan all your cone of vision, and then when you find a target have more antenna send a signal toward that target if you need more data. And you can now tracks a lot of targets at the same time.

With all that said, even with a PESA or AESA radar, you still only get a cone of vision. Even if those radar are small enough to be put on four surface to have 360 degrees, it’s still frequent that the best solution is to have one larger plate of radar on a rotating base. Typically the radar that rotate are for searching, while the stationary radar are for targeting (general rule of thumb).

Anonymous 0 Comments

I don’t know how to explain this like a person is five but newer military radars do not need to spin because the array can be aimed.

Anonymous 0 Comments

They don’t. Not if they’re phased array radars. As an example, look at the superstructure of an Arleigh Burke destroyer. You’ll notice four rectangles with clipped corners on it at the 1:30, 4:30, 7:30, and 10:30 positions. These four panels are the AN/SPY-1 radar, capable of drawing a hemisphere of coverage around the carrying ship from sea to zenith.

Some PARs need to spin due to limited slew angles off the array face to obtain 360° coverage.

Phased array radars are becoming the norm, but replace mechanical complexity with electronic and computing complexity. Benefits include being able to scan a volume nearly instantly, identify anything in it, and maintain radar tracks on all objects deemed worthy of further attention, while simultaneously guiding rear-illuminated ordnance.

Anonymous 0 Comments

On top of what everyone already said, the moving part isn’t that complicated. Say you have a motor burnout or a gear seize up (both of which are rare), that’s a lot cheaper and easier to replace than replacing the many extra electronic components that a stationary 360 antenna needs. If there’s one thing salt water loves its destroying nautical electronics, so usually the equipment on shios is built as “dumb” and robust as possible