Absorptive Shielding?
 

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

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




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

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 , "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!

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
--
remove ,xnd to reply (Spam precaution!)

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
--
remove ,xnd to reply (Spam precaution!)

Most novice designers get burned somewhere along the line by failure to
realize what Ian is describing -- that all amplifiers are differential
amplifiers, and calling one of the inputs "ground" doesn't impart magic
properties. Once that lesson has been learned (alas, usually the hard
way), the probability of making circuits work as designed increases
dramatically.

Even so, though, a surprise sometimes comes along at the last minute. I
know of one expensive, high quality test instrument that had a ground
path broken at the last minute because of ground current that was
induced in the signal path by the field from the display CRT deflection
yoke. Forewarned, I discovered 100 mA of induced current in the shield
of a multi-turn delay line from the same source, in an instrument in
whose design I was participating. Simply soldering the turns of
semi-rigid coax together reduced the effective transformer secondary to
one turn from 20 or so, making it possible to reduce the coupling and
its effect to an acceptable level by other means.

But I remember that shortly after I went to work at Tektronix, I was in
the main lobby admiring the example of the first oscilloscope model
produced by that company, which was on display there. Its success and
the reputation it gathered as a quality test instrument was instrumental
in launching the company. It had two chassis, one above the other,
supported by four L - cross section aluminum legs between the chassis.
One of those four legs had very neatly been cut, leaving a small gap. A
sturdy piece of phenolic was mounted between the gapped ends, again very
neatly, with countersunk flathead screws. I remember that seeing this
gave me a good feeling that ground currents could be a problem for the
best of 'em.

The 'scope in the lobby was a production model, but somewhere, I'm sure,
there was a prototype much like the Racal device -- with a hacksaw cut
across the leg.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
J M Noeding wrote:


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.

[...]

More than a "little mark" - a big cut, deliberately intended to
interrupt an unwanted current path through the chassis.


Such things are impossible to predict

If you stop to think about it, such things often *can* be predicted...
or at least explained with the benefit of hindsight.

When we connect a signal from point A to point B, we often forget about
the return path through 'ground'. But every signal path must also have a
return path - that's why we call these things "circuits". Even if we
shield the signal conductor to provide a preferred return path, there
are still other return paths available in parallel with that, through
the box.

Return currents will divide between these paths according to the RF
impedance that each path presents (Ohm's law), so you will always get
some RF currents in the body of the box. The box is just another RF
conductor, and when currents flow through it, voltages will appear.

The key thing is to remember that there's no magic that makes the box a
perfect zero potential. For all us Sixties children: "'Ground' is a
delusion, Grasshopper."

Therefore in a complex project you will always get some interaction
between different stages. What's hard to predict is how much effect this
will have on overall performance.

The problem that plagued Racal was spurious receiver responses. Racal
was a startup company with a revolutionary all-band receiver concept...
but it involved several oscillators in different stages, that were
getting into the wrong parts of the receiver and causing "birdies". They
couldn't sell a single unit until that problem was fixed.

They had already gone to the huge expense of a diecast chassis divided
into many shielding compartments... and it wasn't working. Very soon,
the money would run out and Racal could cease to exist. Hence the
desperate solution with the hacksaw - and it saved the company. Legend
has it that the prototype receiver (complete with saw cut) was on
display in Reception for many years... maybe they shoulda kept the
hacksaw too?


I once had a problem of this type with spurious responses from a
receiver that was constructed in three (apparently) shielded diecast
boxes with coax signal interconnections. The boxes were stacked side by
side inside the case of the receiver. Remembering the Racal experience,
something made me shove a piece of folded paper under one of the boxes,
to insulate it from the receiver case. The spurs disappeared!

Looking further into this, the real problem was with the power
interconnections between the boxes, which I'd been ignoring until then.
RF was getting into the power rails, and contact between the boxes
formed a ground loop. Unwanted signals were being transferred between
the two boxes via this 'back channel'. With better bypassing of the
power rails, it was possible to re-connect the boxes to the case of the
receiver.

Most novice designers get burned somewhere along the line by failure to
realize what Ian is describing -- that all amplifiers are differential
amplifiers, and calling one of the inputs "ground" doesn't impart magic
properties. Once that lesson has been learned (alas, usually the hard
way), the probability of making circuits work as designed increases
dramatically.

Even so, though, a surprise sometimes comes along at the last minute. I
know of one expensive, high quality test instrument that had a ground
path broken at the last minute because of ground current that was
induced in the signal path by the field from the display CRT deflection
yoke. Forewarned, I discovered 100 mA of induced current in the shield
of a multi-turn delay line from the same source, in an instrument in
whose design I was participating. Simply soldering the turns of
semi-rigid coax together reduced the effective transformer secondary to
one turn from 20 or so, making it possible to reduce the coupling and
its effect to an acceptable level by other means.

But I remember that shortly after I went to work at Tektronix, I was in
the main lobby admiring the example of the first oscilloscope model
produced by that company, which was on display there. Its success and
the reputation it gathered as a quality test instrument was instrumental
in launching the company. It had two chassis, one above the other,
supported by four L - cross section aluminum legs between the chassis.
One of those four legs had very neatly been cut, leaving a small gap. A
sturdy piece of phenolic was mounted between the gapped ends, again very
neatly, with countersunk flathead screws. I remember that seeing this
gave me a good feeling that ground currents could be a problem for the
best of 'em.

The 'scope in the lobby was a production model, but somewhere, I'm sure,
there was a prototype much like the Racal device -- with a hacksaw cut
across the leg.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
J M Noeding wrote:


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.

[...]

More than a "little mark" - a big cut, deliberately intended to
interrupt an unwanted current path through the chassis.


Such things are impossible to predict

If you stop to think about it, such things often *can* be predicted...
or at least explained with the benefit of hindsight.

When we connect a signal from point A to point B, we often forget about
the return path through 'ground'. But every signal path must also have a
return path - that's why we call these things "circuits". Even if we
shield the signal conductor to provide a preferred return path, there
are still other return paths available in parallel with that, through
the box.

Return currents will divide between these paths according to the RF
impedance that each path presents (Ohm's law), so you will always get
some RF currents in the body of the box. The box is just another RF
conductor, and when currents flow through it, voltages will appear.

The key thing is to remember that there's no magic that makes the box a
perfect zero potential. For all us Sixties children: "'Ground' is a
delusion, Grasshopper."

Therefore in a complex project you will always get some interaction
between different stages. What's hard to predict is how much effect this
will have on overall performance.

The problem that plagued Racal was spurious receiver responses. Racal
was a startup company with a revolutionary all-band receiver concept...
but it involved several oscillators in different stages, that were
getting into the wrong parts of the receiver and causing "birdies". They
couldn't sell a single unit until that problem was fixed.

They had already gone to the huge expense of a diecast chassis divided
into many shielding compartments... and it wasn't working. Very soon,
the money would run out and Racal could cease to exist. Hence the
desperate solution with the hacksaw - and it saved the company. Legend
has it that the prototype receiver (complete with saw cut) was on
display in Reception for many years... maybe they shoulda kept the
hacksaw too?


I once had a problem of this type with spurious responses from a
receiver that was constructed in three (apparently) shielded diecast
boxes with coax signal interconnections. The boxes were stacked side by
side inside the case of the receiver. Remembering the Racal experience,
something made me shove a piece of folded paper under one of the boxes,
to insulate it from the receiver case. The spurs disappeared!

Looking further into this, the real problem was with the power
interconnections between the boxes, which I'd been ignoring until then.
RF was getting into the power rails, and contact between the boxes
formed a ground loop. Unwanted signals were being transferred between
the two boxes via this 'back channel'. With better bypassing of the
power rails, it was possible to re-connect the boxes to the case of the
receiver.