What can be used to soak flexible foam so it become DIY microwave absorber?

[Terminator3]

Can we soak flexible foam with some copper paint or something like that so it will start to absorb microwaves in 2 to ~10GHz range? Maybe it is silver paint, carbon paint, some silicates or all things put together? I have doubts about copper and silver paintings, maybe they will only reflect and not absorb. Although commercial absorbers use some nano-particles, carbon, silver, etc. But it must be very different from copper paint. There is a paper called "Properties of sodium silicate bonded sand hardened by microwave heating" and some silicate-based carbon EMI shielding paints, so it seems that silicate absorb microwave and produces heat. Idea is to make anechoic chamber or place for DIY measurements wich too sensitive to reflections.

 

[FvM]

Review properties of existing absorber products, e.g. from Laird https://www.lairdtech.com/products/category/581

Most RF absorbers are resistive (ideally matching the free space impedance), some are lossy dielectric or magnetic materials.

Metals resistivity is too low unless you manage to produce very thin layers. Carbon isn't bad. It's used by some absorbers.

 

[Terminator3]

Thank you for the reply.

update: found some explanation here: https://en.wikipedia.org/wiki/Radar-absorbent_material
and some patents that clarify on used materials.

In this brochure: https://www.leadertechinc.com/assets/files/catalogs/absorbers.pdf
i read that

Absorbers are composed of either iron-infused silicone or carbon-coated polyurethane foam.

What that carbon coating means? Carbon poweder mixed with paint, or carbon particles in foam to some depth?

also from this page: **broken link removed**

If the primary application is for millimeter wave frequencies, it is recommended to ask for absorber to be
supplied without paint, as it can have a negative effect on the reflectivity performance.

Although all i have seen before was blue colored absorber (black inside). Why blue color is used, is it some carbon-filled absorber paint?

 

[biff44]

it means the foam, when it is still liquid, has carbon added to it, so that when it dries in its pyramid shaped molds...the carbon is ~uniformly distributed thru it.

it is pretty hard to get the foam to be nonreflective any other way. they pyramidal shape is important, but you do not want it so heavily loaded (like a surface finish only) so that you do not have a big surface reflection.

I experimented a little once with iron filled paint. they make a paint for use in children's bedrooms, where you paint the walls, and they become magnetic so you can hang things on the walls. I thought I could paint things with it, like wooden tables, etc, to make them ~ non reflective for antenna testing. turns out...it did not help much at all.

 

[jiripolivka]

it means the foam, when it is still liquid, has carbon added to it, so that when it dries in its pyramid shaped molds...the carbon is ~uniformly distributed thru it.

it is pretty hard to get the foam to be nonreflective any other way. they pyramidal shape is important, but you do not want it so heavily loaded (like a surface finish only) so that you do not have a big surface reflection.

I experimented a little once with iron filled paint. they make a paint for use in children's bedrooms, where you paint the walls, and they become magnetic so you can hang things on the walls. I thought I could paint things with it, like wooden tables, etc, to make them ~ non reflective for antenna testing. turns out...it did not help much at all.

The reason why it is not easy to build a good microwave absorber is that one needs to meet strict specifications. Good products are expensive.
If you want to play, try to fold multiple layers of newspaper sheets, and wet the stuff. Similarly, wetted wood plank is also good.
If you have a corrugated polyurethane foam block or sheet, you can try apply rubber glue and sprinkle graphite powder over it.
The good absorber must absorb an incident wave with a reflection less an 20 dB down, and over a wide incidence angle. Difficult for a wide frequency band. Certain absorbers are made of layered dielectric/insulator/absorber stuff, tuned to a particular band.

 

[volker@muehlhaus]

For EM simulators that use a simple resistive sheet for approximate absorbing boundary, the sheet resistance is 377 Ohm/square. Yes, this is for flat absorbing sheets only, but it might give an idea what resistivity we are looking for.

 

[biff44]

here is a 250 ohms/square carbon paint. You could paint various flat layers of foam, and experiment around with getting a broadband absorption.

**broken link removed**