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This article started me thinking.
Now remember, I'm an old man and years out of date about techie things, and worked in the world of spinning scanners, so treat me gently!

I realise that a modern 'scanned array' is an electronic way of scanning with a static aerial array of some sort.
I also have some experience of being in a CAT/PET scanning X-Ray machine.

Anyone know if the two technologies have been mated so that CAT/PET scanning can be done without spinning the mass of X-Ray emitters around the patient?

It works for me using Firefox under Win/10!
But try: https://aviationweek.com/defense-space/ ... 2a6d771d6d
or even: https://tinyurl.com/y2a5jjan

Not sure it would be an advantage. Radar sends a pulse off in a direction and looks for the return from the same direction. The antenna is (historically) turned round to scan the area of interest. Antennae can be steered by having multi-emitters and altering the phasing of the signal across them.

X-rays penetrate a body and the receiving sensor (historically film, nowadays semiconductor) records the radiation not stopped by the body. The spinning annulus on the CAT machine has an X-ray emitter on one side, and the receptor on the other side, i.e. 180 degrees away. This would be difficult to produce with a scanned array as both emitter and sensor would have to surround the body.

A phased array antenna relies on changing the electrical length to each element in the array so that when you sum the contribution in a given direction from each element, there's one particular direction where they all add up in phase. Think of all your trigonometry. That's way harder to do at X-ray wavelengths - easy enough (relatively) to design a fixed array to aim a beam in a particular direction, but making it variable is going to be problematic.

llondel wrote: ↑

Sun Dec 13, 2020 6:13 pm

A phased array antenna relies on changing the electrical length to each element in the array so that when you sum the contribution in a given direction from each element, there's one particular direction where they all add up in phase. Think of all your trigonometry. That's way harder to do at X-ray wavelengths - easy enough (relatively) to design a fixed array to aim a beam in a particular direction, but making it variable is going to be problematic.

I take it that by 'electrical length' you mean the 'wave length' at the given frequency.

Wavelength is a variable property. Light travels at about a foot per nanosecond in free space, but if you've got an electrical signal propagating on a printed circuit board then it goes much slower - on the normal fibreglass stuff it'll go about half the speed. so a wavelength is only half that of free space. That's why I call it an electrical length. Frequency is fixed, wavelength is not. Also see "velocity factor" and relative permittivity.

I thought it was called a phased array, because the phase of the signal was varied across the elements. At least that's how they told me it was done at HF when I was transmitting it 50 plus years ago. I assume the same applies at higher frequencies.

Yes, the phase varies, so if you do the maths, the signals are all in phase at a particular heading, or occasionally are intended to be out of phase and so cancel on a particular heading if you're trying to null something out.