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.
Slides for Hunter’s talk are available on his website.