I Think My Faraday Cage Is Leaking All Over The Place

[modest]

I just noticed, oddly enough, that my cell phone gets 2 bars of reception sitting right outside my microwave and 3 bars inside with the door closed. I think cell phones use radio waves which are larger than microwaves, so I wouldn't expect it to be leaking a lot of EM radiation that my phone would be picking up on (or, at least, I would expect the radiation to be attenuated and not amplified).

 

Can anyone explain this? It seems a rather peculiar observation to me—has me scratching my head.

 

~modest

[Turtle]

I just noticed, oddly enough, that my cell phone gets 2 bars of reception sitting right outside my microwave and 3 bars inside with the door closed. I think cell phones use radio waves which are larger than microwaves, so I wouldn't expect it to be leaking a lot of EM radiation that my phone would be picking up on (or, at least, I would expect the radiation to be attenuated and not amplified).

 

Can anyone explain this? It seems a rather peculiar observation to me—has me scratching my head.

 

~modest

 

after considrable online searching for pertinent frequencies, calculating, researching, & head-scratching, i checked a cell phone in the microwave. :doh: :mobile_ph 3 bars outside the microwave, 0 bars inside. sounds like a leak to me. :lightning

 

might as well give my notes. i thought maybe the phone wavelength might get through the holes, but they are way smaller than even a 1/4 wave.

 

microwave ovens:2.45 GHz oven (2450 MHz) wavelength in mm 122.44897959183673 1/4 wave 30.612244897959183

http://en.wikipedia.org/wiki/Microwave_oven

 

...For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in the ITU Region 2 while 2.45 GHz is available worldwide.

...

Most microwave ovens allow users to choose between several power levels. In most ovens, however, there is no change in the intensity of the microwave radiation; instead, the magnetron is turned on and off in duty cycles of several seconds at a time. This can actually be heard (a change in the humming sound from the oven), or observed when microwaving airy foods which may inflate during heating phases and deflate when the magnetron is turned off. For such an oven, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. Newer models have inverter power supplies which use pulse width modulation to provide effectively-continuous heating at reduced power so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating.

 

The cooking chamber itself is a Faraday cage which prevents the microwaves from escaping. The oven door usually has a window for easy viewing, but the window has a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh are much less than the microwaves' wavelength, most of the microwave radiation cannot pass through the door, while visible light (with a much shorter wavelength) can. ...

 

oven door mesh specs:http://www.steellong.cn/Perforated-Metal/Perforated-Microwave-Oven-Screen-851-1.htm

...an aperture diameter of 1.2 mm or less and a ratio of the aperture diameter to a center-to-center distance of an aperture of 0.67 to 0.85.

 

cell phones:http://en.wikipedia.org/wiki/Cellular_frequencies

 

...In the U.S., the analog AMPS standard that used the Cellular band (800 MHz) was replaced by a number of digital systems. Initially, systems based upon the AMPS mobile phone model were popular, including IS-95 (often known as "CDMA", the air interface technology it uses) and IS-136 (often known as D-AMPS, Digital AMPS, or "TDMA", the air interface technology it uses.) Eventually, IS-136 on these frequencies was replaced by most operators with GSM. GSM had already been running for some time on US PCS (1900 MHz) frequencies. ...

 

1900 MHz wavelength mm 157.8947368421053) 1/4 wave 39.47368421052632

[modest]

after considrable online searching for pertinent frequencies, calculating, researching, & head-scratching, i checked a cell phone in the microwave. :doh: :mobile_ph 3 bars outside the microwave, 0 bars inside. sounds like a leak to me. :lightning

 

:blink: ...hummm.... :unsure:

 

cell phones:http://en.wikipedia.org/wiki/Cellular_frequencies

 

...In the U.S., the analog AMPS standard that used the Cellular band (800 MHz) was replaced by a number of digital systems. Initially, systems based upon the AMPS mobile phone model were popular, including IS-95 (often known as "CDMA", the air interface technology it uses) and IS-136 (often known as D-AMPS, Digital AMPS, or "TDMA", the air interface technology it uses.) Eventually, IS-136 on these frequencies was replaced by most operators with GSM. GSM had already been running for some time on US PCS (1900 MHz) frequencies. ...

 

1900 MHz wavelength mm 157.8947368421053) 1/4 wave 39.47368421052632

 

Yeah, that all confirms that something peculiar seems to be going on. After reading what you wrote it occurred to me that my phone may be at fault, so I put my girl's phone in. 3 bars inside and 3 outside.

 

I'm at a loss. There's no chance, I should add, that the phones' antennae are touching the metal of the cage as they are sitting on a piece of glass at the bottom that turns about. Also, I guess I should add as I don't think I've explicitly stated it, the microwave was not in operation.

 

It's fairly new. There are no obvious defects in the metal lining.

 

I don't know. This seems very strange to me.

 

~modest :eek_big:

[Don Blazys]

Quoting Modest:

Also, I guess I should add as I don't think I've explicitly stated it, the microwave was not in operation.

 

Good.

 

One should never cook a cell phone!

 

Don.

[Ken]

I just noticed, oddly enough, that my cell phone gets 2 bars of reception sitting right outside my microwave and 3 bars inside with the door closed. I think cell phones use radio waves which are larger than microwaves, so I wouldn't expect it to be leaking a lot of EM radiation that my phone would be picking up on (or, at least, I would expect the radiation to be attenuated and not amplified).

 

Can anyone explain this? It seems a rather peculiar observation to me—has me scratching my head.

 

~modest

 

I can't explain it but when I was doing electophysiological research - mostly eletroretinograms and some single cell recording we had a 6x10 Faraday cage with double metal walls, and extensive grounding and power isolation. I would often pick up elevator transients from about 50 ft. away.

 

That was really annoying. :rolleyes:

[modest]

I can't explain it but when I was doing electophysiological research - mostly eletroretinograms and some single cell recording we had a 6x10 Faraday cage with double metal walls, and extensive grounding and power isolation. I would often pick up elevator transients from about 50 ft. away.

 

That's a very good point. I should add that my microwave is grounded

 

~modest

[Don Blazys]

Here's some more data.

 

I put my "Verizon LG" cellphone in two different microwave ovens (a "Sharp" and a "Sanyo")

and in both cases, it didn't lose any bars at all. (Both ovens are grounded.)

 

Don.

[modest]

Quoting Modest:

 

 

Good.

 

One should never cook a cell phone!

 

Don.

 

Well, depends on whose phone it is I should think ;)

 

~modest :lol:

[modest]

Here's some more data.

 

I put my "Verizon LG" cellphone in two different microwave ovens (a "Sharp" and a "Sanyo")

and in both cases, it didn't lose any bars at all. (Both ovens are grounded.)

 

Don.

 

Oh, good. So, it's not just me.

 

I guess there must be a good explanation as to why a microwave can, at least on occasion, receive good EM signals inside the cage... I just can't figure why that would be.

 

~modest

[Ken]

Oh, good. So, it's not just me.

 

I guess there must be a good explanation as to why a microwave can, at least on occasion, receive good EM signals inside the cage... I just can't figure why that would be.

 

~modest

 

I'm not sure that a simple grounded plug and receptacle would really suffice as a Faraday cage. I'm thinking of unnoticed ground loops, etc.

 

In fact the microwave may actually be serving as an extended antenna for the cell phone frequencies. But I'm not an electrical engineer and I don't play one on television. B)

[Don Blazys]

Well, depends on whose phone it is I should think ;)

 

~modest :lol:

 

 

Good point!

 

Perhaps the cell phones of most lawers and politicians should be cooked...

right along with the lawers and politicians themselves! :lol:

 

Don.

[CraigD]

I just noticed, oddly enough, that my cell phone gets 2 bars of reception sitting right outside my microwave and 3 bars inside with the door closed. I think cell phones use radio waves which are larger than microwaves, so I wouldn't expect it to be leaking a lot of EM radiation that my phone would be picking up on (or, at least, I would expect the radiation to be attenuated and not amplified).

 

Can anyone explain this? It seems a rather peculiar observation to me—has me scratching my head.

 

~modest

I’m likewise puzzled by this. :unsure:

 

My first hunch would be to mistrust the bars display on your phone(s), and suspect the phone's actually got practically no signal, but continues to display 2 or 3 bars.

 

Have you tried actually calling the phone when it’s closed up in the oven?

[modest]

I’m likewise puzzled by this. :unsure:

 

My first hunch would be to mistrust the bars display on your phone(s), and suspect the phone's actually got practically no signal, but continues to display 2 or 3 bars.

 

I actually feel relieved that you're puzzled, if that makes sense.

 

Have you tried actually calling the phone when it’s closed up in the oven?

 

In fact, the reason I checked the bars is because a call got through. I was getting an incessant, almost a crank, call while my phone sat on top of the microwave. Rather annoyed, I skipped the rational process of turning the ringer or the phone off and decided to just throw it in the microwave expecting that would put an end to things.

 

But, yeah, it kept ringing with a couple subsequent calls.

 

~modest :shrug:

[freeztar]

Weird.

 

I just tested my phone and I get 3 bars outside and 0 inside, as expected.

 

You might consider investing in an rf meter to check the microwave for leaks. I saw a few on ebay for around $20.

 

It's one thing if your bars do not change at all, but the fact that they increase is intriguing. It does seem logical to conclude that not only is there a leak, but it is acting as a signal amplifier. Perhaps you can move the microwave to a different location for testing. Also, I would try it unplugged and also while moving the power cord around in case it is acting as an antenna.

 

What are the make/model of your cell phone and microwave?

[JMJones0424]

Tried a quick google search for relevant links, but didn't immediately find anything worth posting. However, I hope this will spur the discussion.

 

I think the answer lies in examining why a microwave oven is not a true faraday cage, specifically the door structure. Unlike a real faraday cage designed to block all RF signals, a microwave oven has a door designed with a 3 cm (1/4 wave of 2.45 GHz) choke rather than EMI gaskets, so it is only effective at blocking RF that is resonant to 2.45GHz.

[modest]

Weird.

 

I just tested my phone and I get 3 bars outside and 0 inside, as expected.

 

Hey Freezy

 

Yeah, your observation, as well as Turtle's, makes me want to get to the bottom of this more than ever.

 

You might consider investing in an rf meter to check the microwave for leaks. I saw a few on ebay for around $20.

 

Good idea. I don't have one and don't think I know anyone who would have one. I'll see what I can do...

 

It's one thing if your bars do not change at all, but the fact that they increase is intriguing.

 

I've done a bit more experimenting on that front. My initial observation was that outside it had 2 bars and inside it had 3. I now realize that if I position my phone just right or stand away from it, it can easily have 3 bars outside.

 

I've also now called 3 different phones and looked at 3 different bar levels while in the thing. In all three cases, the phone rang and the bars were not reduced by being in the microwave. So, I don't think there can be any doubt... my microwave has absolutely no effect at blocking EM radiation at the frequency Turtle gave earlier.

 

This seems absolutely mind boggling to me.

 

rhaps you can move the microwave to a different location for testing. Also, I would try it unplugged and also while moving the power cord around in case it is acting as an antenna.

 

I unplugged it with one phone with no effect. I didn't move it as there is limited counter space.

 

What are the make/model of your cell phone and microwave?

 

The microwave is an Emerison mw8126w and my phone is a Treo 750. The other two phones, I don't know at the moment. But, I think the fact that all 3 pones have acted similarly would pretty well rule out that it is a phone-related issue.

 

Tried a quick google search for relevant links, but didn't immediately find anything worth posting. However, I hope this will spur the discussion.

 

I think the answer lies in examining why a microwave oven is not a true faraday cage, specifically the door structure. Unlike a real faraday cage designed to block all RF signals, a microwave oven has a door designed with a 3 cm (1/4 wave of 2.45 GHz) choke rather than EMI gaskets, so it is only effective at blocking RF that is resonant to 2.45GHz.

 

No doubt, a microwave is not a true Faraday cage. I was mostly trying to be clever with the title of the thread.

 

I'm not sure we can blame that gap in the door. Turtle gave the usual frequency of a digital phone as 1900 MHz with a 1/4 wavelength of 39 mm... more than an inch... there's just no way the door would have that kind of a gap.

 

I don't know... this all seems very strange to me. In one aspect, I'm somewhat intrigued. In another aspect, I'm rather frustrated that what I'm seeing id contrary to that which I would think I know.

 

I guess... yeah, I just don't know :shrug:

 

~modest

[JMJones0424]

No doubt, a microwave is not a true Faraday cage. I was mostly trying to be clever with the title of the thread.

 

I'm not sure we can blame that gap in the door. Turtle gave the usual frequency of a digital phone as 1900 MHz with a 1/4 wavelength of 39 mm... more than an inch... there's just no way the door would have that kind of a gap.

 

I don't know... this all seems very strange to me. In one aspect, I'm somewhat intrigued. In another aspect, I'm rather frustrated that what I'm seeing id contrary to that which I would think I know.

 

I guess... yeah, I just don't know :shrug:

 

~modest

 

I am not refering to a gap in the door. There isn't a 3 cm gap in the door either. If the microwave oven were a true faraday cage, there would need to be a metallic gasket to bridge the gap between the door and the rest of the metal box. The problem with this is that any debris or worn spots in the gasket could cause a short over the gap.

 

To get around that design flaw, instead of using a metal gasket, they use a 1/4 wave RF choke that prevents ONLY harmonics of 2.45 GHz from passing through. It is transparent to any other frequency. At 1.9 GHz, you have a metal box on 5 sides with a screen in the front, and a 6 cm (3cm on the perimeter of the door, and the another 3 cm around the door opening) gap running around the perimeter of the door side of the box. Thus, because of the RF choke, a microwave acts as a faraday cage ONLY to frequencies with integer divisors or multiples of 2.45 GHz.

 

Additionally, if your phone reports signal to noise ratio instead of actual signal strength as "bars", then it is easy to see why you could have more "bars" inside the oven rather than outside. A neat test would be to use the microwave as a primitive homing gun, since the RF can only come in from one side of the 6 sided box, you should be able to rotate the microwave and get a general direction to the tower, assuming your bar meter is sensitive enough to show a difference in strength when pointed towards the tower rather than away.