"Metallic Delay Lenses" paper (1948)

[cdev]

From the venerable Bell System Technical Journal, no less.

https://ia801604.us.archive.org/18/items/bstj27-1-58/bstj27-1-58.pdf

[Alex Eisenhut]

Wow, that's amazing!

[cdev]

This is an ancestor in spirit of "metamaterials"

also something Copper Cone has discussed here, the use of RF absorbent materials in gradients.

Also called "stealth" materials.

Which can be surprisingly easy to make if you like me, use filters for your drinking water, they come with quite a bit of activated carbon which can be used in projects..  You can use the spray foam you buy in cans to make layered foams with different densities of carbon. It sort of works.

I was trying to make a better GPS antenna.

[CopperCone]

that stuff looked like it was really fun to build.

[cdev]

that stuff looked like it was really fun to build.

It must have been, totally..

Have you ever heard of a Luneburg lens?

Very similar concept.

[Richard Crowley]

There were several commercial implementations.  For example the JBL "venetian blind" acoustic lens:



And the "potato masher" lens:

[KE5FX]

What will really bake your noodle is how much this hardware resembles the polarizers and collimators used by Bose around fifty years earlier.

[LaserSteve]

Delay lenses were used for the first transcontinental microwave links in the US.. They were later replaced by the  famous KS-15676 Horn..

Dr. Winston  Kock worked in sonar, radar, microwave communications and holography. His lenses and textbooks are amazing.

He created a method of visualizing sound waves with a neon lamp, a signal generator, and a scanning microphone that makes amazing pictures using a camera with an open shutter..



Steve

[cdev]

That sounds interesting!  Do you have any links? is it anything like
https://en.wikipedia.org/wiki/Schlieren_photography  ?

Dr. Winston  Kock worked in sonar, radar, microwave communications and holography. His lenses and textbooks are amazing.

He created a method of visualizing sound waves with a neon lamp, a signal generator, and a scanning microphone that makes amazing pictures using a camera with an open shutter..

[LaserSteve]

Easy, make a pivoted arm that scans vertically through the near  field signal to be mapped from your speaker or microwave horn/lens.

. Place microphone or detector diode on the tip of the free end of the arm.  Koch used a neon lamp, but these days we have LEds.
Place the light just behind the detector on the wooden pivoting arm.

The above is your Y scanning axis, and drive the arm with a motor or a pully and string.

The X axis is the pivoted arm described above, mounted on a cart on a track.

The lamp is driven by a mixer, amplitude detector, or phase detector , depending on what you seek.

The signal source is a reasonably stable, coherent,  oscillator  split between the sending transducer and providing a phase reference to whatever drives the lamp.

Place a camera at right angles to the track in a darkened room.

Turn everything on. Set the camera on " bulb" if using film and open the shutter. These. Days you'd use "integrate, average, or time exposure" on a digital camera. 

Leave the shutter open. Sweep the arm,
Move the cart back, sweep the arm,  then repeat as needed. 
Close shutter when done.

The wave will show up as a series of light and dark spaces proportional to amplitude and phase of the signal in your time exposure.

Makes really neat near field plots with short wavelengths.

If you Google "koch bell labs sound " in "Googlr images" something will pop up.


Steve

[cdev]

Something like that seems like a useful technique in a number of different kinds of measurements.

For example, "poor mans thermography" might be done with a non contact thermometer in a manner something  like that, using a RGB LED, maybe to display the color mapped output.