In article ,
Avery Fineman wrote:

I've seen a fair amount of electronic hardware over the last half
century and haven't encountered any equipment operating below
1 GHz that used any sort of "RF-absorbing" material.

I've seen ads in an EMI-related trade magazine for a ferrite (or
ferrite-loaded polymer I suppose) material, in the form of an
adhesive-backed sheet which can be cut and then stuck onto the tops of
ICs (CPUs, DSPs) or placed between adjacent circuit boards, to help
reduce unwanted emissions. I don't know whether this stuff's useful
frequency range goes low enough for the OP's needs, but it might be
worth a look.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Dave Platt" wrote in message
...

In article ,
Avery Fineman wrote:

I've seen a fair amount of electronic hardware over the last half
century and haven't encountered any equipment operating below
1 GHz that used any sort of "RF-absorbing" material.

I've seen ads in an EMI-related trade magazine for a ferrite (or
ferrite-loaded polymer I suppose) material, in the form of an
adhesive-backed sheet which can be cut and then stuck onto the tops of
ICs (CPUs, DSPs) or placed between adjacent circuit boards, to help
reduce unwanted emissions. I don't know whether this stuff's useful
frequency range goes low enough for the OP's needs, but it might be
worth a look.

From my few weeks in the microwave absorber business :)) , IIRC an absorber
has to be ELECTRICALLY at least nearly a quarter-wave thick to be really
good. The ferrite loading helps accomplish this. A good impedance match to
the 377 ohm impedance of free space helps avoid reflections. For a broad
band, this encourages the use of the deep pyramid absorbers to taper the
impedance mismatch.

Some hams have found that absorbing material inside to covers of preamps in
the __hundreds of MHz region___ reduces the likelihood of self oscillation.
I've never seen a serious suggestion that absorbers would help in the
few-MHz region.

73 de bob w3otc

"Dave Platt" wrote in message
...

In article ,
Avery Fineman wrote:

I've seen a fair amount of electronic hardware over the last half
century and haven't encountered any equipment operating below
1 GHz that used any sort of "RF-absorbing" material.

I've seen ads in an EMI-related trade magazine for a ferrite (or
ferrite-loaded polymer I suppose) material, in the form of an
adhesive-backed sheet which can be cut and then stuck onto the tops of
ICs (CPUs, DSPs) or placed between adjacent circuit boards, to help
reduce unwanted emissions. I don't know whether this stuff's useful
frequency range goes low enough for the OP's needs, but it might be
worth a look.

From my few weeks in the microwave absorber business :)) , IIRC an absorber
has to be ELECTRICALLY at least nearly a quarter-wave thick to be really
good. The ferrite loading helps accomplish this. A good impedance match to
the 377 ohm impedance of free space helps avoid reflections. For a broad
band, this encourages the use of the deep pyramid absorbers to taper the
impedance mismatch.

Some hams have found that absorbing material inside to covers of preamps in
the __hundreds of MHz region___ reduces the likelihood of self oscillation.
I've never seen a serious suggestion that absorbers would help in the
few-MHz region.

73 de bob w3otc

Thanks to everyone who has responded. I think the conclusion is that at the
frequencies I'm interested in, and the available space inside the radio (a
little RS DX-394 table radio), it's impractical to absorb the 455 kHz
crosstalk energy from 2nd IF to frontend. Better to attempt to
compartmentalize the radio. That may prove to be impractical also as it
would appear very difficult to make small (removable) shields over the IF
section that would not have gaps. However, I'll examine the pcb layout more
closely to see if there are any viable paths for the sides of the shield
box.

73, Tom

Thanks to everyone who has responded. I think the conclusion is that at the
frequencies I'm interested in, and the available space inside the radio (a
little RS DX-394 table radio), it's impractical to absorb the 455 kHz
crosstalk energy from 2nd IF to frontend. Better to attempt to
compartmentalize the radio. That may prove to be impractical also as it
would appear very difficult to make small (removable) shields over the IF
section that would not have gaps. However, I'll examine the pcb layout more
closely to see if there are any viable paths for the sides of the shield
box.

73, Tom

R J Carpenter wrote:

"Dave Platt" wrote in message
...

In article ,
Avery Fineman wrote:

I've seen a fair amount of electronic hardware over the last half
century and haven't encountered any equipment operating below
1 GHz that used any sort of "RF-absorbing" material.

I've seen ads in an EMI-related trade magazine for a ferrite (or
ferrite-loaded polymer I suppose) material, in the form of an
adhesive-backed sheet which can be cut and then stuck onto the tops of
ICs (CPUs, DSPs) or placed between adjacent circuit boards, to help
reduce unwanted emissions. I don't know whether this stuff's useful
frequency range goes low enough for the OP's needs, but it might be
worth a look.

From my few weeks in the microwave absorber business :)) , IIRC an absorber
has to be ELECTRICALLY at least nearly a quarter-wave thick to be really
good. The ferrite loading helps accomplish this. A good impedance match to
the 377 ohm impedance of free space helps avoid reflections. For a broad
band, this encourages the use of the deep pyramid absorbers to taper the
impedance mismatch.

Some hams have found that absorbing material inside to covers of preamps in
the __hundreds of MHz region___ reduces the likelihood of self oscillation.
I've never seen a serious suggestion that absorbers would help in the
few-MHz region.

73 de bob w3otc

You are right of course.
My fault. I didn't read the OP's question thoroughly enough. I was
thinking of freq. above UHF. Absorbing material for lower freq. has
to be a LOT thicker than 1 cm. This can easyly be seen in any test
chamber.
With wavelengths longer than the compartment of the circuit I do
not see the necessity of absorbing material. Common construction
practice as pointed out will be suficient. Or just bury the device
6 feet deep in the soil ;-))

Kind regards, Eike

R J Carpenter wrote:

"Dave Platt" wrote in message
...

In article ,
Avery Fineman wrote:

I've seen a fair amount of electronic hardware over the last half
century and haven't encountered any equipment operating below
1 GHz that used any sort of "RF-absorbing" material.

I've seen ads in an EMI-related trade magazine for a ferrite (or
ferrite-loaded polymer I suppose) material, in the form of an
adhesive-backed sheet which can be cut and then stuck onto the tops of
ICs (CPUs, DSPs) or placed between adjacent circuit boards, to help
reduce unwanted emissions. I don't know whether this stuff's useful
frequency range goes low enough for the OP's needs, but it might be
worth a look.

From my few weeks in the microwave absorber business :)) , IIRC an absorber
has to be ELECTRICALLY at least nearly a quarter-wave thick to be really
good. The ferrite loading helps accomplish this. A good impedance match to
the 377 ohm impedance of free space helps avoid reflections. For a broad
band, this encourages the use of the deep pyramid absorbers to taper the
impedance mismatch.

Some hams have found that absorbing material inside to covers of preamps in
the __hundreds of MHz region___ reduces the likelihood of self oscillation.
I've never seen a serious suggestion that absorbers would help in the
few-MHz region.

73 de bob w3otc

You are right of course.
My fault. I didn't read the OP's question thoroughly enough. I was
thinking of freq. above UHF. Absorbing material for lower freq. has
to be a LOT thicker than 1 cm. This can easyly be seen in any test
chamber.
With wavelengths longer than the compartment of the circuit I do
not see the necessity of absorbing material. Common construction
practice as pointed out will be suficient. Or just bury the device
6 feet deep in the soil ;-))

Kind regards, Eike

"Tom Holden" wrote in message
...
Thanks to everyone who has responded. I think the conclusion is that at
the
frequencies I'm interested in, and the available space inside the radio (a
little RS DX-394 table radio), it's impractical to absorb the 455 kHz
crosstalk energy from 2nd IF to frontend. Better to attempt to
compartmentalize the radio. That may prove to be impractical also as it
would appear very difficult to make small (removable) shields over the IF
section that would not have gaps. However, I'll examine the pcb layout
more
closely to see if there are any viable paths for the sides of the shield
box.

Small gaps won't matter as long as they are shorted out at one or both ends.
A well-fitting cover would do that. You'll observe that commercial gear uses
very thin tinned steel? for shield boxes. The lid edges are bent into spring
fingers to hold them in place.

"Tom Holden" wrote in message
...
Thanks to everyone who has responded. I think the conclusion is that at
the
frequencies I'm interested in, and the available space inside the radio (a
little RS DX-394 table radio), it's impractical to absorb the 455 kHz
crosstalk energy from 2nd IF to frontend. Better to attempt to
compartmentalize the radio. That may prove to be impractical also as it
would appear very difficult to make small (removable) shields over the IF
section that would not have gaps. However, I'll examine the pcb layout
more
closely to see if there are any viable paths for the sides of the shield
box.

Small gaps won't matter as long as they are shorted out at one or both ends.
A well-fitting cover would do that. You'll observe that commercial gear uses
very thin tinned steel? for shield boxes. The lid edges are bent into spring
fingers to hold them in place.

On Sun, 9 Nov 2003 16:00:21 -0500, "Tom Holden"
wrote:

Thanks to everyone who has responded. I think the conclusion is that at the
frequencies I'm interested in, and the available space inside the radio (a
little RS DX-394 table radio), it's impractical to absorb the 455 kHz
crosstalk energy from 2nd IF to frontend. Better to attempt to
compartmentalize the radio. That may prove to be impractical also as it
would appear very difficult to make small (removable) shields over the IF
section that would not have gaps. However, I'll examine the pcb layout more
closely to see if there are any viable paths for the sides of the shield
box.

73, Tom


You'll never know before you have tried, and remember that Racal
solved a similar problem with their famous RA-17 series receivers
using a hacksaw to make a little mark in the chassis to stop unwanted
radiation from one point to another. Believe I've seen the application
of those carbonized foam used in the lids of boxes which were
definitely not microwave equipment

Such things are impossible to predict

It is also some definite requirement for the thickness of the walls to
act as screen on certain frequencies, as an example could be mentioned
that pcb laminates are not thick enough for good screening on 80m in
an application with two oscillators which need good screening to avoid
coupling to be used for third order IP measurements

73
Jan-Martin, LA8AK
Amateur radio techniques http://home.online.no/~la8ak/c.htm
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