Coating on metal to minimize the reflection of electromagnetic waves

At what frequency? Light is definitely an electromagnetic wave, and there are interference filters that do a great job.

Longer wavelengths are trickier. A layer of plastic loaded with particles of a ferrite that was lossy at the frequency of interest might work, but I don't think I've seen anybody selling anything like that.

There was a vogue for stealthing military aircraft by minimising the reflection that would comes back from a radar pulse, but it didn't seem to reduce the reflections all that much.

Am 31.12.2020 um 09:42 schrieb Bill Sloman:

This coating reduces the totally reflected electromagnetic RADAR waves to 1/2.

It is the physically possible minimum without distraction.

"distraction"? Who's distracted?

He may have meant diffraction. A native speaker of English might have said interference, The aim is to create destructive - cancelling - inference between the incident and reflected radiation.

It's tricky when the wavelengths get up into the millimetres and longer.

I've seen an anti-reflection coating on a 60GHz antenna.

That was 25 years ago, and the details are hazy, but from memory...

The 20dBi antenna was ceramic, about the size of the last joint of the index finger, and covered in a clear plastic quarter-wave anti-reflection filter equivalent to that you find on optical camera lenses.

In radar, attenuation to one half (3dB) is nothing. The typical round-trip path loss is easily 160 dB, versus 3 dB.

The vogue continues, even if it has dropped out of the mainstream news. A better source is Aviation Week.

Most new platforms (including ships) are designed for some degree of stealth, how much depending on the platform's mission. Most of the stealth effect is due to clever shaping and eliminition of all those little discontinuities. The (optional) RAM (RF Absorbing Material) coating stops edge diffraction and creeping surface modes, and fills the remaining imperfections.

The resulting reduction in surface echo, coupled with various kinds of self-protection jammer to obscure that echo, provides the military benefit.

For ships, look at the Zumwalt class of destroyer.

Joe Gwinn

How come that first graph of attenuation vs frequency only appears to drop by a hundredth of a dB from DC to 1 GhZ ?

And looks like there is a variable or two missing its reference from EQ 1.

I have some nickel paint that I bought for RF attenuation purposes a long time ago.

Am 01.01.2021 um 00:46 schrieb boB:

There is nothing missing ...

How come that first graph of attenuation vs frequency only appears to drop by a hundredth of a dB from DC to 1 GhZ ?

And what is "x" ?

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Leo is more than a few sandwiches short of a picnic.

..... Phil

Am 02.01.2021 um 09:27 schrieb boB:

x is the distance from the antenna (here in water). I set it to 1 m.-

Read the text under the graph.

Am 02.01.2021 um 09:27 schrieb boB:

The translation is bad. I did it with google.-

In this graph You have 3 areas. Conducting area, transition area and wave area.

The 3 areas are considered in a distance 1 m from the antenna under water.

Am 02.01.2021 um 09:27 schrieb boB:

You have to discuss why this is so with the electromagnetic waves of different frequencies and the sea water.

Aquadag is a trade name for a water-based colloidal graphite coating commonly used in cathode ray tubes (CRTs). It is manufactured by Acheson Industries. Used to coat the case of the original Compaq portable.

OK, thanks !

It is kind of interesting.

And Happy new year !

It is remarkably thin, and not all that conductive. Great for getting rid of leakage currents through the glass - which aren't large - but I wouldn't put much faith in it for electrostatic screening at high frequencies.

Faith does nothing electronic. Actually to a certain extent, the higher the frequency, the better. It is lossy.

e:

here are interference filters that do a great job.

cles of a ferrite that was lossy at the frequency of interest might work, b ut I don't think I've seen anybody selling anything like that.

reflection that would comes back from a radar pulse, but it didn't seem to reduce the reflections all that much.

on

id of leakage currents through the glass - which aren't large - but I would n't put much faith in it for electrostatic screening at high frequencies.

It was an expression of scepticism - nothing more.

But not all that conductive. If the electromagnetic radiation doesn't induc e much current in it to start with, it's not going to attenuate much of the magnetic field, and the voltages induced in the thin layer of graphite are going to be visible on the other side so the electric field won't be hit h ard either - so not much screening at all.

One does not need conductivity. Theer are plenty of examples of fertites that are decent insulators that are lossy in the RF region; the powders can be used for attenuation of RF.