Date: Tue Feb 13 16:17:58 2007

Author: Bernard Cleyet

Subject: Re: Cell Phone in a Faraday Cage

Post:
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Yes, my rule of thumb is < 1/10 wavelength for significant attenuation,
but evidently not enuf. A vertical copper comb w/ tines spaced ~ 13 mm
is a 50% beam splitter *[E vertical polarization] for X-band (32 mm; 9
+ GHz) radiation. However, transmission thru slits in conductive
materials is quite complicated. In some cases increasing the aperture
results in reduced transmission.

Boris should try different ovens. Some use holed metal plates while
others use foil or film. The difference (OTB=) between 0.85 GHz and 2.4
is only ~ 70%. [inverse square root ratio of frequencies] I suggest
checking that oven for leaks.

* knows from constructing a Michelson.

bc

Stan Dodds wrote:

> The phone picks up a signal because the cage leaks radiation, as you
> have clearly shown. Since cell phones are very sensitive receivers, to
> maintain good service, it won't take much for them to detect the
> leaked radiation.
>
> In your first cage, the holes are probably too big. Cell phones work
> in the 824-849 MHz band, or a wavelength of about 12 cm. Your results
> suggest that a factor of 10 in lambda/opening is not enough to
> attenuate the signal to undetectability.
>
> With the microwave oven, and perhaps the plastic areas of your first
> cage, the metallization may be too thin relative to its skin depth to
> stop penetration. Skin depth in a good metal is a few microns at 1
> GHz, but it might be larger in the sprayed conductive coatings
> sometimes used for shielding. Presumably the microwave oven is good
> enough at its operating frequency of 2.4 GHz, but not at lower
> frequencies where the skin depth is greater.
>
> The window screen cage presumably works because the metal wires are
> thick enough and the holes small enough to reduce the signal from the
> cell tower below detectability. It will, of course, become transparent
> at sufficiently high frequencies. Commercial shielded rooms are
> usually built from solid copper sheeting, with elaborate metallic
> contacts at doorways. They can give more than 100 dB of attenuation
> over very broad bands.
>
> Stan
>
> On Feb 12, 2007, at 12:32 PM, Borislaw Bilash wrote:
>
>> You may recall the demo of placing a transistor radio in a small Faraday
>> cage -- the radio waves are effectively blocked by the cage.
>>
>> Since few of my students know what a transistor radio is I figured that
>> it would be more appropriate to place a cell phone in the cage and use
>> another to call the caged phone. I've gotten inconsistent results -- I
>> am not sure why. The cage is an inexpensive commercial type with a
>> 1cm^2 openings on side, a plastic inner top and bottom cover and a metal
>> outer top and bottom cover.
>>
>> Exp A] Phone sits on inner plastic base of cage; metal covers are also
>> in place; antennae does not touch cage -- sometimes phone rings,
>> sometimes it doesn't. At times the cell phone indicates no service. (I
>> use Verizon).
>>
>> Exp B] Just like Exp A, but the entire cage is covered by a second
>> Faraday cage made from metal window screen -- courtesy of Dave Maiullo's
>> suggestion. Seems to work consistently (ie: doesn't ring).
>>
>> Exp C] Placed the phone is a microwave. (Seems reasonably).. It
>> DOESN'T work.... that is, the phone rings despite being in the closed
>> microwave. (Microwave was off.... and I don't plan to....)
>>
>> Any suggestions would be appreciated.
>>
>> Borislaw
>>
>> Borislaw Bilash II
>> Physics Teacher
>> Pascack Valley High School
>> 200 Piermont Ave.,
>> Hillsdale, NJ 07642
>> tel: 201-358-7060 ext 5007
>> fax: 201-358-7102
>>
>> !DSPAM:45d0b4909888971013301!
>
>
>
>

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Yes, my rule of thumb is < 1/10 wavelength for significant
attenuation, but evidently not enuf. A vertical copper comb w/ tines
spaced ~ 13 mm is a 50%
beam splitter *[E vertical polarization] for X-band (32 mm; 9 + GHz)
radiation. However, transmission thru slits in conductive materials is
quite complicated. In some cases increasing the aperture results in
reduced transmission.

Boris should try different ovens. Some use holed metal plates while
others use foil or film. The difference (OTB=) between 0.85 GHz and
2.4 is only ~ 70%. [inverse square root ratio of frequencies] I
suggest checking that oven for leaks.


* knows from constructing a Michelson.

bc

Stan Dodds wrote:
The phone picks up a signal because the cage leaks
radiation, as you have clearly shown. Since cell phones are very
sensitive receivers, to maintain good service, it won't take much for
them to detect the leaked radiation.

In your first cage, the holes are probably too big. Cell phones work in
the 824-849 MHz band, or a wavelength of about 12 cm. Your results
suggest that a factor of 10 in lambda/opening is not enough to
attenuate the signal to undetectability.

With the microwave oven, and perhaps the plastic areas of your first
cage, the metallization may be too thin relative to its skin depth to
stop penetration. Skin depth in a good metal is a few microns at 1 GHz,
but it might be larger in the sprayed conductive coatings sometimes
used for shielding. Presumably the microwave oven is good enough at its
operating frequency of 2.4 GHz, but not at lower frequencies where the
skin depth is greater.

The window screen cage presumably works because the metal wires are
thick enough and the holes small enough to reduce the signal from the
cell tower below detectability. It will, of course, become transparent
at sufficiently high frequencies. Commercial shielded rooms are usually
built from solid copper sheeting, with elaborate metallic contacts at
doorways. They can give more than 100 dB of attenuation over very broad
bands.

Stan

On Feb 12, 2007, at 12:32 PM, Borislaw Bilash wrote:

You may recall the demo of placing a
transistor radio in a small Faraday
cage -- the radio waves are effectively blocked by the cage.

Since few of my students know what a transistor radio is I figured that

it would be more appropriate to place a cell phone in the cage and use
another to call the caged phone. I've gotten inconsistent results -- I

am not sure why. The cage is an inexpensive commercial type with a
1cm^2 openings on side, a plastic inner top and bottom cover and a
metal
outer top and bottom cover.

Exp A] Phone sits on inner plastic base of cage; metal covers are also
in place; antennae does not touch cage -- sometimes phone rings,
sometimes it doesn't. At times the cell phone indicates no service.
(I
use Verizon).

Exp B] Just like Exp A, but the entire cage is covered by a second
Faraday cage made from metal window screen -- courtesy of Dave
Maiullo's
suggestion. Seems to work consistently (ie: doesn't ring).

Exp C] Placed the phone is a microwave. (Seems reasonably).. It
DOESN'T work.... that is, the phone rings despite being in the closed
microwave. (Microwave was off.... and I don't plan to....)

Any suggestions would be appreciated.

Borislaw

Borislaw Bilash II
Physics Teacher
Pascack Valley High School
200 Piermont Ave.,
Hillsdale, NJ 07642
tel: 201-358-7060 ext 5007
fax: 201-358-7102

!DSPAM:45d0b4909888971013301!








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