Microwave absorber

[alan.bain]

I know it as the black stuff found in many microwave enclosures.  It's very useful when an LNA turms into an oscillator when it goes in the metal box.  But there's a vast choice and it is very expensive (getting more so) and bad things seem to happen over time to it over the years ...

The black foam becomes a sticky gooey mess (a bit like the foam in boxes of VNA cal kits)  and some the black rubber sheet like stuff seems worse. I've recently found some in a 2GHz IF amp with little droplets of liquid on the surface of the absorber and the board underneath showed severe verdigris style corrosion on the copper (as if from acetic acid) and some gold traces have lifted off their alumina substrate.  Does the absorber degrade?  Or could it be the glue?    Not having one of those handy Ramen spectometers I don't know what the droplets were....

I'm wondering what to replace it with - what should go into selection of absorber?  In the past I've used Eccosorb MCS which I know is silicone based and quite nice to work with (buy bad on the wallet when Mouser insist on selling whole sheets for several hundred dollars). 

But I realise I don't know nearly enough about selection of this stuff or indeed how it works other than by black-magic.  Is there any useful information about and are there any lower cost options (I am reluctant to get involved with trying black antistatic foam because I have seen what happens to that over the years in trays of EPROMS).

Any pointers welcome!

Alan

[paul@yahrprobert.com]

I once had a microwave cavity at 1.5 GHz and I was measuring its Q, and for fun I experimented with putting various substances in it to see what would kill the Q.  One of the best was steel wool. Also what worked well was a powdered iron toroid, I believe it was 2 material. Also taking an ohm or two resistor and soldering its leads to make a 1 cm^2 loop worked pretty good.

[tggzzz]

I've occasionally wondered if something like this "microwave plate warmer" might be sufficient. https://wave-ware.com/products/microwave-plate-warmers?variant=33457197547658

No, I don't understand why silicone might work, unless it is loaded with something else. Anyway, if it gets hot then it must be absorbing some energy, and that might be sufficient. If nothing else it is a quick cheap experiment, and failure might be useful in other ways.

In case it isn't obvious, this concept is worth what you paid for it.

[coppercone2]

silicon carbide stones will do it too. like machinist stones, you can get them white hot. coarse ones

[Georgy.Moshkin]

It is possible to target specific frequency range and absorb radiation using EBG arrays etched on a FR4 substrate. There are plenty of papers with various configurations, with and without chip resistors. Such PCB can replace the top cover of the aluminum shielding enclosure. I did some simulations few years ago and  I remember that 1.6mm FR4 was good frequencies 10GHz and lower, and 1.0mm FR4 for 20GHz and higher. Substrate height directly influences via height, so to create optimal absorptive "mushroom" need  to choose via diameter, substrate thickness and patch size. There are some pros and cons, and of the problems is that with a large number of EBG elements via count can add to the cost. Via-less EBG structures are usually not very robust, but maybe there are some good configurations. For example, absorptive element created using two radial stubs with a chip resistor in-between, or something like that.

[Randy222]

Everything absorbs Rf to some level.
May I ask, how much "attenuation" is the goal for the incident Rf?
Is "scatter" in your boat of "absorption", or are you literally wanting to absorb and dissipate Rf energy?

[EggertEnjoyer123]

https://ieeexplore.ieee.org/document/8541699

Maybe graphite (or some composite) would work.

[Randy222]

https://www.mwtmaterials.com/products/rf-functional-coatings/

or make your own
https://americancarbonyl.com/product/carbonyl-iron-powder/

I use this powder for making my own pressed epoxy cores for various things.

[coppercone2]

what is the mix ratio typically? its not cheap

[mr ed]

Try iron oxide (rust) in silicon.

[Randy222]

what is the mix ratio typically? its not cheap

I don't have all the details, but you yourself have to do some homework.
The stuff is not cheap, but you're asking to do something that is not easy, relatively speaking.
Some digging and reading should get you some knowledge. How expensive it becomes depends on your requirements.

https://iopscience.iop.org/article/10.1088/2053-1591/ab392a

[coppercone2]

at least a general range would be nice, when it comes to custom RF components I can stick my money in like at least 20 different random process technologies at random based on sketchy publishings. having a real person actually claim that one of the machines has a chance helps!



strange polymer requirements? mysterious consistency problems? japanese company won't talk to you? involves the indium corporation? end up requiring a optometrist license? research hides the fact that way more then 10000 samples were processed by a hired company and the theoretical data and models claiming to later guide the three iteration design was actually later fabricated while playing research paper madlibs using JMP?  patent has more decoys then a rouble crusher? secret shelf life?

[Randy222]

50g $162(US), shop around, maybe less in EU or Asia.
https://ssnano.com/inc/sdetail/carbonyl-iron-powder--fe--4-8um-/261

How you mix it and apply it is on you to do homework.

[coppercone2]

I was just curious because it looked mildly interesting, I don't need to go on another wild goose chase with materials right now

maybe someone else will try your idea out