[EE] RF absorbers for LOW frequencies

[David Van Horn]

Is there an absorber that isn't crazy expensive, that will give me 4 or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

[Jean-Paul Louis]

Look for the material used in anechroic chambers. Should still work a 500kHz
Jean-Paul N1JPL

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On Thu, Nov 1, 2018 at 12:35 PM, David Van Horn<***@backcountryaccess.com> wrote: Is there an absorber that isn't crazy expensive, that will give me 4 or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

[David Van Horn]

It doesn’t. Those guys don’t even look at anything below 30 MHz.
One outfit said they could make a version for me but the cones would have to be something like 100’ long.
That means that their absorber is about as good as air.

From: Jean-Paul Louis <***@yahoo.com>
Sent: Thursday, November 1, 2018 11:02 AM
To: Microcontroller discussion list - Public. <***@mit.edu>; David Van Horn <***@backcountryaccess.com>; Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] RF absorbers for LOW frequencies

Look for the material used in anechroic chambers. Should still work a 500kHz

Jean-Paul
N1JPL
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On Thu, Nov 1, 2018 at 12:35 PM, David Van Horn
<***@backcountryaccess.com<mailto:***@backcountryaccess.com>> wrote:
Is there an absorber that isn't crazy expensive, that will give me 4 or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

[Sean Breheny]

Have you tried IC ESD foam (the conductive black kind)?

On Thu, Nov 1, 2018, 12:34 PM David Van Horn <

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4 or 5
dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.
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[David Van Horn]

Not yet. I know it works well at VHF and above. I was actually in a bit of an argument over this stuff years ago, where I suggested it and someone was insisting it wouldn't work, then someone who does stealth coatings stepped in and commented it worked quite well but was rather unsuitable for aircraft skin. 😊

-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of Sean Breheny
Sent: Thursday, November 1, 2018 11:11 AM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] RF absorbers for LOW frequencies

Have you tried IC ESD foam (the conductive black kind)?

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4
or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.
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[AB Pearce - UKRI STFC]

I would be tempted to try and make some cones out of a soft wood like pine (deal) or balsa.

The 100' long figure mentioned earlier would be to make a 1/4 wavelength pyramid for an anechoic chamber, as you are looking for <6db attenuation I suspect that something lossy like soft wood will do.

But that brings up the nature of what you want such low attenuation for? If it is to introduce a loss between transmitter and receiver you might be better doing a lossy labyrinth out of softwood or IC foam. This may also work to be a better way of getting the loss you want than a pyramid anyway.

I know that there is an epoxy called Ecosorb around, but I don't know what the cost would be or how lossy it would be at such a low frequency. They are using it here to make an anechoic set of pyramids with around 45 db reflection loss at >50GHz.



-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of David Van Horn
Sent: 01 November 2018 17:18
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: RE: [EE] RF absorbers for LOW frequencies

Not yet. I know it works well at VHF and above. I was actually in a bit of an argument over this stuff years ago, where I suggested it and someone was insisting it wouldn't work, then someone who does stealth coatings stepped in and commented it worked quite well but was rather unsuitable for aircraft skin. 😊

-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of Sean Breheny
Sent: Thursday, November 1, 2018 11:11 AM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] RF absorbers for LOW frequencies

Have you tried IC ESD foam (the conductive black kind)?

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4
or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.
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[John Ferrell]

Long, Long ago when I was a field engineer with IBM in Ohio a customer 
placed a call that their computer room smelled hot all over the place.

Since they were (are) a large installation with several main frames in a
computer room that was big enough for half a dozen basketball courts no
one was laughing.

Much of the power used was supplied by 400hz rotary generators in three
phases.  After pulling a lot of raised floor panels the odor was well
distributed throughout.

It seems that someone decided that the 400hz stuff needed to be placed
in metallic conduits. The induced current was enough to warm the
conduits and stink. The following weekend

the electricians removed the the metallic conduits.  The time frame was
the mid 1980's.  There are probably a lot of better ways.

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4 or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

--
John Ferrell W8CCW
Julian NC 27283
It is better to walk alone,
than with a crowd going the wrong direction.
--Diane Grant

[John Gardner]

!

Post by John Ferrell
Long, Long ago when I was a field engineer with IBM in Ohio a customer
placed a call that their computer room smelled hot all over the place.
Since they were (are) a large installation with several main frames in a
computer room that was big enough for half a dozen basketball courts no
one was laughing.
Much of the power used was supplied by 400hz rotary generators in three
phases.  After pulling a lot of raised floor panels the odor was well
distributed throughout.
It seems that someone decided that the 400hz stuff needed to be placed
in metallic conduits. The induced current was enough to warm the
conduits and stink. The following weekend
the electricians removed the the metallic conduits.  The time frame was
the mid 1980's.  There are probably a lot of better ways.

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4 or 5
dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

--
John Ferrell W8CCW
Julian NC 27283
It is better to walk alone,
than with a crowd going the wrong direction.
--Diane Grant
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[m***@tutanota.com]

Possibly epson salts, magnesium sulphate heptahydrate.  You could probably fill plastic pyrimids with it.  Basically anything with polarized molecules and or water that would make a lossy capacitor dielectric.  PVC or CPVC might also work well, I'd think lots of plastics.
Democracy now!

--------------------------------------------------

Post by John Ferrell

Post by David Van Horn
Is there an absorber that isn't crazy expensive, that will give me 4 or 5 dB attenuation in a thickness of around a foot or less, at 0.5 MHz?
Not a reflector like metal, but an absorber that will give the energy somewhere to die.

--
John Ferrell W8CCW
Julian NC 27283
It is better to walk alone,
than with a crowd going the wrong direction.
--Diane Grant
--
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[Sean Breheny]

I think the problem with water is that it will reflect a lot. Dave really
needs something that has a complex dielectric constant (like water) but
where the magnitude of the constant is close to 1, at the frequency of
interest.

Possibly epson salts, magnesium sulphate heptahydrate. You could probably
fill plastic pyrimids with it. Basically anything with polarized molecules
and or water that would make a lossy capacitor dielectric. PVC or CPVC
might also work well, I'd think lots of plastics.