I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom

Any advice on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices
are
packaged in?
How thick would it have to be?
=======
It is perhaps somewhat expensive ,especially for larger areas , but
conductive paint would do the trick.

Frank GM0CSZ / KN6WH

Tom Holden wrote:

I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom




Carbon spray might work at microwave frequencies but I don't think it
will be thick enough
at the frequency you are working at. Tire rubber and bicycle inner tube
material works at microwave
but again I don't know about lower frequencies..

Bill K7NOM

Tom Holden wrote:
I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom



If you need a thicker absorbtive layer you can use the conductive
sponge which is used to pack old style DIL ICs.

Kind regards, Eike

In article , "Eike
Lantzsch, ZP6CGE" writes:

Tom Holden wrote:
I was wondering if there is a radio frequency absorptive or otherwise
lossy
material with which one might line a conductive shield or use instead of
it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in
the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of
the
input circuitry. I suppose this is because the energy that would
ordinarily
escape through the plastic lid is now reflected back and is trapped
inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices
are
packaged in?
How thick would it have to be?

73, Tom

If you need a thicker absorbtive layer you can use the conductive
sponge which is used to pack old style DIL ICs.

I doubt that such will work below VHF. Absorbing foam polymers
have been used for decades on antenna ranges for the microwave
region. Emerson & Cuming have been making various grades of
that for a long time. Search the Web for them and see what they
say and what they have for information.

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.

The best bet is just plain old metal shielding, bypassing feed-thrus,
series inductances, etc.

Len Anderson
retired (from regular hours) electronic engineer person

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

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

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