Comment by d4ng

Imagine a power splitter + phase shifter that produces a correct phase shift for each element in a phased array to produce a directional beam from one radio transceiver.

Now this clever arrangement, instead of having only one radio transceiver port, has multiple. And each of those ports corresponds to a different set of phase shifts, producing a directional beam at a different azimuth angle.

And because this is an entirely passive device, it's linear, and all ports can be active at the same time (principle of superposition essentially). So you can use a single phased antenna array to serve multiple directional beams at the same time.

It's a way to create constructive interference in an RF signal in a certain direction (the signal gets stronger in one direction and weaker in others without changing the hardware). It's commonly used in LTE and Wi-Fi as a way to increase SNR directionally for clients.

https://en.wikipedia.org/wiki/Beamforming

Not just switching, you can use all of the beams simultaneously.

If you monitored all the inputs simultaneously instead of switching, you could make a low-tech radio-wavelength camera. Presumably with less SNR per "pixel" than you'd get from monitoring just one input though.

It creates pancake beams so you would usually use a separate Rx and Tx orthogonal to each-other to do imaging.

No, this is to concentrate the radio waves into a desired shape. Stronger in this direction, weaker than that.

Think of it very vaguely like a parabolic mirror on a flashlight directing the light vs a naked light bulb putting light out in all directions. (this is a bad metaphor for what's going on but it's the basic idea of the goal)

To change the direction you have to physically move the antenna OR have an active phased array antenna with an electronic component which has a variable phase change to be able to move the beam around while leaving the antenna fixed.

That's kind of it, yes.

You can aim an aerial by physically rotating it. You've probably done this, gone up on the roof to adjust the aim of your TV aerial or satellite dish. It makes sense, right? A Yagi aerial - a reflector, a driven element, and a bunch of directors - focuses the beam in a kind of aubergine-shaped blob in the direction of the pointy end.

But you can also aim aerials by having two of them, and varying the phase that you send a signal into them. This sounds a bit mental but consider how direction finding equipment like LoJack works - you have a transmitter in an unknown location and you have a cluster of aerials connected to one receiver. By comparing the phase of the incoming signal between two aerials you can work out which one it's nearer to! This trick works well enough if you make two dipoles spaced a half wavelength apart that you can easily homebrew something where by switching in a 180° phase shift at an audio rate, the difference in phase can be heard as a tone.

In this case you've got a bunch of aerials attached to the ports along the bottom and the phase of the signal reaching them depends on how long it's taken to cross the microstrip. If you fire it in at the top in the centre it'll be equal (notice the middle "legs" have kinks in them to keep the path length the same?), if you fire it in at the side then one of the ports at the side will get the signal sooner and its phase will appear advanced compared to the other one - and the beam will bend that way.

Basically, it lets you steer a beam very precisely in an arbitrary direction, as if you were physically rotating the transmitting antenna.

Usually you'd need to rotate the antenna or else use a large number of controllable phase-shifting elements to send an output beam in the desired direction. But a Rotman lens lets you use a small number of phase shifting elements to merge a small number of source beams (still > 1, though) into a single precisely-steered output beam.