How to visualize 24 GHz RF on a screen


Is there a (preferably inexpensive) way to visualize/image 24 GHz RF from a radar gun (of the type used to check baseball speeds) on a screen?

Here are some examples of what I mean, for other invisible EM radiation. We can use phosphorescent screens to get an idea of the relative intensity and distribution of x-rays from an x-ray emitting tube. We can use LIT pigment to "see" the pattern of standing waves in a (4 GHz) microwave oven, but this is apparently due to heating of the pigment, and probably would not work at a distance.

I have only one idea at the moment: place an array of NE-2 bulbs on a sheet, and power them to just below illumination threshold. Then perhaps the marginal added power of the incident radar beam would be enough to illuminate a subset of them, giving, for example, a rough idea of relative intensity at various distances.

Perhaps there some RF diode that will detect this frequency, in the way that an IR photosensor detects IR?

Best Answer

If you have a receiver for 24GHz signals as well as a directional antenna and RF level detector for that range, you might try something like this gadget I built a few years ago.

enter image description here

That's a parabolic reflector with a satellite receiver LNB attached to an RF signal level detector and an Arduino with a 24 bit ADC. The Arduino drives a couple of servos to aim the dish. Move the dish around, make measurements of the RF level at each point, assemble into a picture.

This is an image of a window made using my "microwave camera:"

enter image description here

This is an image of the television satellites visible in the sky south of my house:

enter image description here

The difficulty is, of course, that the image is a convolution of the receiving antenna pattern and the radiation pattern of the sources. That makes the generated images "fuzzier" than might be expected.

Another difficulty is that it takes about 15 minutes to make an image. The dish wobbles when you move it, so you have to move slowly else the image is nothing but wobbles. Even with a less wobbly setup, it would still take a while. It can make about 120 steps in each direction. With one pixel per point, that's 14400 pixels. It takes a while to mechanically scan around to make that many measurements.

I hadn't realy considered neon bulbs to be sensitive to RF, but it appears that they are. The difficulty with that is that the bulbs won't be characterized for that use - you'd have to test bulbs to see if it works and what kind of RF signal level you can detect - and if that trick will even work at 24GHz. Outside of that, the trigger level of each bulb may be different - you'd have to match the bulbs (or the untriggered voltage) to to get an evenly "illuminated" image.

You can use RF diodes to detect RF levels, but I don't think you can do much with them at 24GHz. I had plenty of trouble locating diodes that would work at 2.4GHz a couple of years ago when I made a simple RF level bargraph display. Discrete diodes don't work well at such frequencies - the inductance of the leads and the capacitance of the junction work against you. You need an integrated detector and someone capable of working with such high frequencies.

Related Topic