RadioBanter - blocking radio waves

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blocking radio waves

If you are working surrounded on all sides by earth potential and you
touch
a mains live wire, then the current has a path through you to earth. In
theory, only the phase (or live) wire represents a serious hazard but in
the
event of a chassis going live, this could be fatal.

This is exactly the recommended situation in a 'normal' UK mains enviroment
where all exposed metal must be bonded to mains earth (equi-potential
bonding). An RCD is used to provide protection.

By having mains voltages floating, earthing either side of the isolated
mains accidently to earth should not result in a lethal shock as the
potential on the touched wire only will move down to earth. An RCD is not
needed because although one side of the mains has become earthed through
you, there should be no return path unless you are touching the other
mains
wire somehow, in which case you would get a shock whether you were in
contact with earth or not!

Whilst that is true, not having a earth on equipment *inside* the screened
room is still a hazard.For Example; If due to a fault a case goes to mains
potential, the the case of adjacent equipment will be at some other
potential, that could be anything from 'real' ground (the earthed wall
potential of the cage) to perhaps 1/2 rail due the filtering components.
Either way there exists the possiblity of lethal shocks. It is not easy to
control what the case potential or connection will be. Commonly there will
be a connection to the screened room ground on at least some of the
equipment, and that will aslo be external mains ground by default. A lot of
equipment cases will float to 1/2 mains due to the input filtering. Once a
fault develops it is very easy for the isolation barrier provided by the
transformer to be by-passed leaving you with no protection. A neutral to
ground fault could go undetected forever until someone touches something at
line potential. At least with an earth and an rcd there is some protection
in this situation.

110v Isolating transformers with their central point earthed are used on
all
building sites in the UK to limit shock voltages to 55v AC. In this case
the
transformer is earthed because it is impossible to work away from other
natural earth points, girders, trusses, rods, etc. To a large extent this
also prevents the risk of inappropriate equipment designed only for
domestic
purposes being used on site.

True, 55V is considered 'safe', but it is not common to have 110V test
equipment and mains in a screened room in the UK, so it is not really
relavent.

For a Faraday cage installation, the mains supply should be completely
isolated and floating using a double insulated approved transformer, no
autotransformers.

As I said this has never been the case in the dozen or so screened rooms
that I have worked in!!

73
Jeff

blocking radio waves

On 19 Jun, 23:50, "merlin-7" wrote:
When I build my new shack, I was thinking about lining the inside of the
plywood with something, before it is installed, that would block outside
radio transmissions inside the shack.
Mostly in the 2 meter range and higher.

Would simple aluminum foil work?

If so what frequency range could I expect it to work in?

If not any suggestions on material? The cheaper the better.

I know there is a term for a room like this but I can not rember it..faraday
chamber?

The shack will be 12 foot by 16 foot, right next to my 60' tower, with HF
and 2m antennas on it. I have no idea how they will interact with such a
building...

Thanks
Joe

We had very expensive screened rooms at Racal's Bracknell site when I
worked there a few years ago. They were double-screened with copper
sheet on the inside and outside. We had to use double-screened cables
for some measurements, there was too much leakage from standard coax.

Leon

blocking radio waves

On Wed, 20 Jun 2007 09:26:17 -0700, Leon
wrote:

We had to use double-screened cables
for some measurements, there was too much leakage from standard coax.

The term for this is Tri-axial cable. Often each end of the braid is
left open but each connector has one braid. In other words, there is
no braid to braid connection, and there is no shell to shell
connection. The braids overlap.

It is very simple to violate shielding through the wrong sequence of
groundings from the source to the load. The art of shielding is
actually found in the practice of finding your reference (ground).

73's
Richard Clark, KB7QHC

blocking radio waves

merlin-7 wrote:

...

In the end, I would make sure that large of cube of metal sheet
(capacitor plate) was grounded, finally, to some ground close to the
shack--I'd image if that "capacitor plate" was allowed to float it may
well be able to store a lethal charge in itself (if well insulated enough.)

The main ground, tied to close earth ground at the shack, would be my
goal, whether it filled code requirements or not ... damn inspector
can't sleep in my shack!

Of course, you are talking to guy who has worn a grounding strap on his
wrist ...

Regards,
JS

blocking radio waves

"Jeff" wrote in message
. com...

If you are working surrounded on all sides by earth potential and you
touch
a mains live wire, then the current has a path through you to earth. In
theory, only the phase (or live) wire represents a serious hazard but in
the
event of a chassis going live, this could be fatal.

This is exactly the recommended situation in a 'normal' UK mains
enviroment
where all exposed metal must be bonded to mains earth (equi-potential
bonding). An RCD is used to provide protection.

By having mains voltages floating, earthing either side of the isolated
mains accidently to earth should not result in a lethal shock as the
potential on the touched wire only will move down to earth. An RCD is
not
needed because although one side of the mains has become earthed through
you, there should be no return path unless you are touching the other
mains
wire somehow, in which case you would get a shock whether you were in
contact with earth or not!

Whilst that is true, not having a earth on equipment *inside* the screened
room is still a hazard.For Example; If due to a fault a case goes to
mains
potential, the the case of adjacent equipment will be at some other
potential, that could be anything from 'real' ground (the earthed wall
potential of the cage) to perhaps 1/2 rail due the filtering components.
Either way there exists the possiblity of lethal shocks. It is not easy
to
control what the case potential or connection will be. Commonly there will
be a connection to the screened room ground on at least some of the
equipment, and that will aslo be external mains ground by default. A lot
of
equipment cases will float to 1/2 mains due to the input filtering. Once a
fault develops it is very easy for the isolation barrier provided by the
transformer to be by-passed leaving you with no protection. A neutral to
ground fault could go undetected forever until someone touches something
at
line potential. At least with an earth and an rcd there is some protection
in this situation.

110v Isolating transformers with their central point earthed are used on
all
building sites in the UK to limit shock voltages to 55v AC. In this case
the
transformer is earthed because it is impossible to work away from other
natural earth points, girders, trusses, rods, etc. To a large extent
this
also prevents the risk of inappropriate equipment designed only for
domestic
purposes being used on site.

True, 55V is considered 'safe', but it is not common to have 110V test
equipment and mains in a screened room in the UK, so it is not really
relavent.

For a Faraday cage installation, the mains supply should be completely
isolated and floating using a double insulated approved transformer, no
autotransformers.

As I said this has never been the case in the dozen or so screened rooms
that I have worked in!!

73
Jeff

Jeff

You are quite correct in what you say, however working in a Faraday cage is
inherently unsafe. You are surrounded on all sides by earth potential and in
the event of any fault, the path is more than likely going to go through you
to earth. Not a desirable situation. If mains supplies are floating, then
there should be no return path to provide a lethal shock.

Further provisions in the BS 7671 regs suggest that items of equipment are
separated so that a person cannot be in contact with two items of equipment
for precisely the reasons you give, to eliminate the risk of shock if two
items of equipment are faulty with the case of each item being connected to
a different side or phase of the mains.

The Faraday cage is designed to prevent all external influences from
affecting experiments and tests inside the cage. Careful design and
consideration of the inherent risks need to be taken into account at every
stage. A lot of stuff in commercial labs is hidden behind panels or
incorporated into the building design at the planning stage, so may not be
apparent in everyday use. Commercial labs also have the resorces to put
screening materials behind insulated walls, floors and ceilings and to
employ special techniques around window and door openings. These techniques
are simply too time consuming and expensive for most people to use in a
domestic shed type environment. The standard insulation tests for floors
using heavy, sharp, pointy objects with high voltage applied are just not
practical for a shed, the floor would collapse under the stress.

It is quite feasible to build a Faraday cage in a room or shed within a
domestic environment. It will be inherently unsafe to work in the shed with
live equipment exhibiting certain fault conditions unless additional
precautions are taken to ensure that no contact whatsoever can be made with
the screening material. Floating power supplies are a requirement due to the
need to maintain isolation of the interior of the cage from the outside
conditions.

None of these regulations are a statutory requirement in the UK but are
based on best practice as identified by the IET (formerly IEE). My concern
is to point out the safety implications and not dictate how the project is
finally built. Under UK law, electrical contractors are not allowed to
disconnect a customer's supply no matter how unsafe it may be. They can only
advise the customer not to use the installation/equipment and advise the
electrical supply company of the defects. The supply company can disconnect
a supply, but rarely do so as long as they are getting paid.

Mike G0ULI

blocking radio waves

Joe wrote:
"I know there is a term for a room like this but I can not remember
it..''

The term that comes to mind for me is "screened room".

I`ve used them in shortwave broadcasting stations where they provided
excellent isolation. They were constructed of ordinary copper
windowscreen wire which solders well for good continuity. Sometimes the
rooms were double screened with wire on both sides of the supporting
structure. You`ve already been given the caveats on joints and bypassing
every conductor which punctures the structure. That`s necessary.

Best regards, Richard Harrison, KB5WZI

blocking radio waves

Thanks everyone

The shack will be run off of solar power only and 95% of everything it it
will be 12 volt. It will have a 1200 watt inverter to run different AC items
as needed ( a soldering iron comes to mind)

I plan on grounding the sheild (for lack of a better term) of the shack, to
my lightning /RF grounding system. There is no way that I would not ground
the hell out of something like this.

I use LMR 600 and 400 for the coax runs.

The sheilding does not need to be perfect, just good.

thanks again..
joe
KI4ILB

blocking radio waves

The real problem in making a shielded area like you propose isn't so
much the material as the seams and wires. Connection between panels has
to be virtually continuous, or you'll have very ineffective shielding no
matter what material you use. You need a completely sealed box, floor
and ceiling included, with no unbonded hole or seam larger than a very
small fraction of a wavelength in size. Aluminum is hard to bond well,
so that's a poor choice. Doorways are particularly problematic - make a
visit to a local EMC lab to see what measures are typically taken. The
sealing requirements become more stringent as frequency increases.

An equally or even more difficult problem is getting power, telephone,
and any other conductors through the shield without allowing unwanted
signals to come in with them.

Roy Lewallen, W7EL

merlin-7 wrote:
When I build my new shack, I was thinking about lining the inside of the
plywood with something, before it is installed, that would block outside
radio transmissions inside the shack.
Mostly in the 2 meter range and higher.

Would simple aluminum foil work?

If so what frequency range could I expect it to work in?

If not any suggestions on material? The cheaper the better.

I know there is a term for a room like this but I can not rember it..faraday
chamber?

The shack will be 12 foot by 16 foot, right next to my 60' tower, with HF
and 2m antennas on it. I have no idea how they will interact with such a
building...

Thanks
Joe

Quote:

Originally Posted by merlin-7 (Post 576094)

When I build my new shack, I was thinking about lining the inside of the
plywood with something, before it is installed, that would block outside
radio transmissions inside the shack.

Ignoring for the moment whether it could be done (yes it could, but it would be very expensive to do it effectively), the question arises as to why you'd want to? Putting your equipment into some "tempest hardened" screen room doesn't (on the surface) provide any particular operational benefit, since any interfering signals will arrive via the antenna in any case.

blocking radio waves