If you watch old science fiction or military movies — or if you were alive back in the 1960s — you probably know the cliche for a radar antenna is a spinning dish. Although the very first radar antennas were made from wire, as radar sets moved higher in frequency, antennas got smaller and rotating them meant you could “look” in different directions. When most people got their TV with an antenna, rotating those were pretty common, too. But these days you don’t see many moving antennas. Why? Because antennas these days move electrically rather than physically using multiple antennas in a phased array. These electronically scanned phased array antennas are the subject of Hunter Scott’s talk at 2018’s Supercon. Didn’t make it? No problem, you can watch the video below.
While this seems like new technology, it actually dates back to 1905. Karl Braun fed the output of a transmitter to three monopoles set up as a triangle. One antenna had a 90 degree phase shift. The two in-phase antennas caused a stronger signal in one direction, while the out-of-phase antenna canceled most of the signal and the resulting aggregate was a unidirectional beam. By changing the antenna fed with the delay, the beam could rotate in three 120 degree steps.
Today phased arrays are in all sorts of radio equipment from broadcast radio transmitters to WiFi routers and 5G phones. The technique even has uses in optics and acoustics.
There are two broad categories of phased arrays: passive, where one transmitter feeds a bunch of antennas and phase shift networks, or active where each antenna generates its own signal. The active antennas generally perform better but are much more expensive and Hunter focuses on the passive ones. But in both cases, the directionality depends on some signals canceling others out, and some signals reinforcing others.
One of the things we’ve always found interesting is that mathematically there is no difference between an antenna receiving and one that is transmitting. Hunter uses this to explain how the phased array receives a signal since that’s a little easier on the intuition.
The explanation covers some reasonably simple math with some helpful graphics. However, due to the complexity of math for practical examples, Hunter suggests using a Python tool called ArrayTool that does a nice job of handling the real math and shows you a nice graphical output.
Towards the end, Hunter points out a few ways to make your design cheaper or simpler, although — of course — not at the same time. That’s always the case though.
Hunter also had two interesting observations. First, if you are working on the usual frequencies, you can probably recycle an existing antenna design and save yourself some trouble. Second, the use of electrically-steerable antennas in 5G phones means there are now chips that will do most of the work for you. Today they are expensive, but as 5G phone production scales up, you can expect the price will plummet just like other mass-produced cell phone components. One other observation he had is that if you get into RF you’ll spend your days staring at test equipment wondering why nothing works. Sounds about right to us.
If you missed Supercon, you missed a lot. But you can catch up thanks to the magic of video. Why not find out how to turn a PCB into a motor? Or maybe you’d rather build your own vacuum tube.
Slides for Hunter’s talk are available on his website.
15 thoughts on “No Moving Parts: Phased Array Antennas Move While Standing Still”
Rotate all the beans!
“By changing the antenna fed with the delay, the bean could rotate in three 120 degree steps.”
Just in case it gets fixed :)
It is incorrect to say that ‘dish’ radars are rare : All of european civil air traffic control use them. Next generation civil are/will use a grid of groundlink stations with simple antennas in conjunction with GPS and Mode-S downlinked from each aircraft and will NOT use phased array radar. I only know of phased arrays being used by the military for primary surveillance (present or not), not secondary (elaborate protocols and data exchange).
Also : Phased arrays have not had any assurance or safety cases made for use by civil air traffic. Although cheaper in maintenance terms than rotating joint radar, next generation antennas are ‘simple wires’ and so very much cheaper than all other options.
Problem is, that all those “simple wire antenna” systems rely on the target aircraft cooperating.
Nobody seems to be the least bit concerned with what happens if whatever will send the GPS coordinates breaks / is turned off intentionally or even worse, spoofed. I hope I don’t need to remind any reader of this site that this would be fairly easy and cheap to do.
With an active radar running a phased array antenna, it get a lot harder and more expensive + it’s nearly impossible for a normal plane to disappear from said radar while in flight. ATC gets 3D position and vector regardles of the aircraft cooperating.
I worked as radar tech at this phased array radar years ago, interesting job, interesting equipment. I didn’t understand the math behind it, but it worked well.
Oh ya, PA running at 125%, rebootable after underground nuke test shot.
The good old days.
A simple test of rad hard circuits, and recover ability.
Rumor was, the PA was going to a DC museum.
I can neither confirm nor deny any of the above.
Interesting – same analogy was used by Goodyear for military tech’s intro theory for the APD-10 radar back in the 70s.
Synthetic aperture radars, with exception of the space-borne stuff, were considered the sine qua non for recon systems of that era. Carrier-deployable SLRs using beam-steerable antennae seem to date to the late 60s/early 70s. Anyone know the early history of the first carrier-based airborne systems, probably the APQ-102? Apparently the USMC or the USAF gave some these older systems to NASA for research; see
I remember working with these for Inmarsat in aviation – HGA-7000 in particular – that thing was really expensive. It was basically “satellite dish” which can be pointed any direction and it was completely flat.
The coolest thing about a phased array is that you can move the beam really fast. You can send out a pulse, move the beam, send out another pulse (and maybe do this several more times), and then go back and receive the return from the first pulse. In theory, as long as your phase shifters respond in a few microseconds.
If that’s too intense, then you can easily scan dozens of targets in a few miliseconds. With a moving antenna, it takes a good fraction of a second to move it very far.
State of the art T/R modules can change phase shift under 100 ns.
Also, not only can the beam move around quickly, by using more math you can play with the beam shape (pencil or fan) and/or supress side lobes.
You don’t have to “go back” on the receiving side. It is all software and processing the signal streams from all the elements of the array. Basically, you see all directions at once. And transmitting directions are nearly instant because it is just changes in the data streams to phase shifters or direct digital conversion devices. I would wager that the time limit is proportional to the bandwidth with higher bandwidth switching faster. There is a limit for how short a segment of transmitted signal can be without it’s frequency spreading out and becoming undetectable.
“if you get into RF you’ll spend your days staring at test equipment wondering why nothing works”
If you get into programming, you’ll spend some of your days staring into the screen wondering why nothing works,
and you’ll spend the other days staring into space and wondering why anything works.
Even AM MW (medium wave, not megawatt) broadcast towers typically use phased arrays. The towers are electrically short half dipoles (which have an omni radiation pattern in a flat plane) and the cheapest way to get any directivity out of them is to put up two or three towers with appropriate phasing.
In 1961 I worked as a student tech for Hughes Aircraft Ground Systems Group. One of my duties was testing phase shifters for S band radar destined for the USS Enterprise. As I remember, they were a plastic section of waveguide with a coil around the outside and a ferrite core in the middle. We used a small neon bulb on a stick to check for leaks.
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