I Think My Faraday Cage Is Leaking All Over The Place

[Qfwfq]

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.

It doesn't to be a full quarter wave and I think JMJ makes a good point about the design being specifically for the purpose.

[Turtle]

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.

It doesn't to be a full quarter wave and I think JMJ makes a good point about the design being specifically for the purpose.

 

mmmm....this still does not explain why those of us who tried a phone in an oven went from bars outside to no bars inside. :confused: door seals do degrade as hinges loosen over time and/or debris accumulates.

 

modest, have you tried your phone in other ovens yet? that might rule out the phone. if you don't want to buy a detector, maybe some appliance dealers or the local utility have detectors you can borrow? :shrug:

[JMJones0424]

What door seal? Unless your microwave is 30 years old, you have an RF choke, not an EMI seal. And unless you tested your microwave at the same location in modest's house, I don't think you can say that the RF environment was the same for your test. Modest may well have a leaky microwave, but the fact that a non harmonic of 2.54 GHz gets through is no proof. There are many different RF choke designs, it may be that the one used in modest's microwave is substantially different than freeztar's, yours, and mine. I have a sharp, my choke is simply an empty void with a series of metal protrusions spaced along the center of the channel. Perhaps modest has some dielectric filled choke that is more transparent to off harmonic frequencies. The RF choke is usually located underneath a plastic debris cover that snaps into the outer edge of the inside of the door. If the frame has a choke, you may need to remove the cabinet to see it.

 

Any appliance repairman that works on microwaves should have either a signal meter or a cheap leak detector.

 

EDIT:

Though not exactly the same frequency, there are devices that operate in the 12 cm band that you may have. Try placing the phone in the microwave and synching a bluetooth device outside the microwave to the phone. If this is successful, then I'd be concerned enough to have the oven leak tested.

[modest]

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.

 

Ahhhhhh!

 

I see what you're saying now. You are absolutely correct. I was assuming that the door formed a conductive seal with the rest of the cage. I actually looked at it and mistook the plastic lining covering the screen on the inside of the door for metal. Looking again I see it is indeed plastic. So, there is a gap around the door which is a choke for that particular frequency. That makes perfect sense :D

 

Thank you, JM.

 

~modest :)

 

EDIT--->

 

Found a great description:

 

Leakage of microwaves round

the edge of the oven door is prevented

by a design feature known

to radio engineers as a ‘quarter

wave choke’ (see, for example,

www.freepatentsonline.

com/3956608.html). The operation

of such a choke is

explained in various sources

for microwave engineers, such

as www.microwaves101.com/

encyclopedia/quarterwave.cfm.

The basic idea is that a quarter

wavelength of transmission line

will transform an open circuit

(infinite impedance) to look like

a closed circuit (0 impedance) at

the other end. This is a commonly

used technique when joining

sections of waveguide together

because it avoids depending on

the reliability of metal to metal

contact. The same technique

is used in the microwave oven,

where the path between the door

and the body is designed to be a

quarter wavelength from inside

to outside at the operating frequency

of 2.45 GHz. This means

that, from both the inside and the

outside, the door and the oven

body look like continuous metal,

but only at this frequency.

 

http://iopscience.iop.org/0031-9120/42/5/M04/pdf/0031-9120_42_5_M04.pdf

 

I never would have guessed it worked this way. Very good info. Very much appreciated :)

[Turtle]

What door seal? Unless your microwave is 30 years old, you have an RF choke, not an EMI seal. And unless you tested your microwave at the same location in modest's house, I don't think you can say that the RF environment was the same for your test. Modest may well have a leaky microwave, but the fact that a non harmonic of 2.54 GHz gets through is no proof. There are many different RF choke designs, it may be that the one used in modest's microwave is substantially different than freeztar's, yours, and mine. I have a sharp, my choke is simply an empty void with a series of metal protrusions spaced along the center of the channel. Perhaps modest has some dielectric filled choke that is more transparent to off harmonic frequencies. The RF choke is usually located underneath a plastic debris cover that snaps into the outer edge of the inside of the door. If the frame has a choke, you may need to remove the cabinet to see it.

 

Any appliance repairman that works on microwaves should have either a signal meter or a cheap leak detector. ...

 

well, by "seal", i mean the gap between door & cabinet. presumably the size of the gap is as critical a measure as is the diameter of the holes in the screen and their distance apart. i searched "what makes microwave ovens leak?" and found the door "seal" referred to often as a source of leakage. :shrug: i'm certainly no expert. here's one reference and the google results link:

 

Canadian Centre for Occupational Health and Safety

Can microwaves leak radiation?

 

Old or faulty door seals are the most common causes of microwave radiation leakage. Mechanical abuse, a build-up of dirt, or simple wear and tear of continued use can cause door seals to be less effective. Theoretically, there will be small amounts of leakage through the viewing glass but measurements have shown this to be insignificant. ...

 

google: what makes microwave ovens leak?

[modest]

mmmm....this still does not explain why those of us who tried a phone in an oven went from bars outside to no bars inside. :confused:

 

I've just taken JM's advice and tested the hypothesis by rotating the oven. After rotating it about 20 degrees the signal strength goes to zero.

 

~modest

[freeztar]

I've just taken JM's advice and tested the hypothesis by rotating the oven. After rotating it about 20 degrees the signal strength goes to zero.

 

~modest

 

Cool, now you know about which direction the cell tower is in, eh? :)

 

I'll try and give mine a full rotation tomorrow and see if I can expose the door leak on my own microwave.

[Qfwfq]

...this still does not explain why those of us who tried a phone in an oven went from bars outside to no bars inside.

More prolixly, what I meant was that, in order for the gap to constitute a leak, it needn't be a quarter wavelength.

[modest]

Cool, now you know about which direction the cell tower is in, eh? :)

 

Yes! And, not only that, I think I know exactly what tower it is

 

In the line along the center of my microwave and a tower which I'm quite familiar with lies the usual gap in my microwave's door. When I pivot the thing out of the way of that tower I get a loss of signal. I was really quite excited to discover this. My girlfriend was somewhat less excited, I might say, to discover my new-found enthusiasm for putting phones in the microwave, turning the thing about, and calling myself like a madman. :)

 

~modest :lol:

[freeztar]

Yes! And, not only that, I think I know exactly what tower it is

 

In the line along the center of my microwave and a tower which I'm quite familiar with lies the usual gap in my microwave's door. When I pivot the thing out of the way of that tower I get a loss of signal.

 

Nice! :)

 

I was really quite excited to discover this. My girlfriend was somewhat less excited, I might say, to discover my new-found enthusiasm for putting phones in the microwave, turning the thing about, and calling myself like a madman. :)

 

 

I can totally empathize! I show my wife quirky things like this from time to time and she just says "That's nice honey" while at the same time I'm sure she's questioning my sanity and wondering what she got herself into. :hihi:

[Qfwfq]

My girlfriend was somewhat less excited, I might say, to discover my new-found enthusiasm for putting phones in the microwave, turning the thing about, and calling myself like a madman.

Yup I can just picture her, triumphantly exulting over your discovery :lol: Dem gals just don' know what dey's missin'!

 

Modest, you could ven test whether the incident wave has horizontal or circular polarization. It's already apparent that it ain't vertical, unless your micro in ordinarily in a rather unconventional position for these appliances.

[modest]

Modest, you could ven test whether the incident wave has horizontal or circular polarization. It's already apparent that it ain't vertical, unless your micro in ordinarily in a rather unconventional position for these appliances.

 

Yeah, I've heard that in situations where you have a loss of signal it can help to rotate the phone just as much as it might help to move around a bit... which I would assume means that cell phone antennas can be linearly polarized.

 

I don't know much about this sort of thing... I'm maybe not following how the microwave's position determines that the wave is not vertical. Is it that the microwave serves to eliminate signals from all but one tower meaning that a vertical wave from the tower and a horizontally-placed phone antenna would give no signal noise?

 

~modest

[Qfwfq]

I'm maybe not following how the microwave's position determines that the wave is not vertical.

When the sides of the gap are vertical they damp out any vertical component of the electric field oscillation. What gets through is the component at right angles to them (like the field between a capacitor's armatures).

 

I imagine the repeaters use circular polarization, so as to have less cases in which the receiver antenna doesn't detect it. A simple dipole antenna would fail if pointed straight toward the tower. If the incident wave were linear polarized, the dipole would also have to be reasonably at right angles to the plane of polarization, which would be more troublesome. So, you could test their wisdom by swivelling the micro around the appropriate horizontal axis. It might be a bit better to start with the micro on its side, so as to use the longer gap, except that the side with the hinges is prone to be a bit damped.

[modest]

When the sides of the gap are vertical they damp out any vertical component of the electric field oscillation. What gets through is the component at right angles to them (like the field between a capacitor's armatures).

 

Really? So, when the oscillation is parallel to the gap it is dampened? I wouldn't have guessed that. Is it something to do with a microwave's gap specifically or gaps in general? It just seems like... when the gap is larger than the wave... neither waves with a parallel or perpendicular oscillation should be blocked. But, I admit, this is a subject with which I have very little knowledge.

 

I imagine the repeaters use circular polarization, so as to have less cases in which the receiver antenna doesn't detect it.

That makes sense

 

A simple dipole antenna would fail if pointed straight toward the tower. If the incident wave were linear polarized, the dipole would also have to be reasonably at right angles to the plane of polarization, which would be more troublesome. So, you could test their wisdom by swivelling the micro around the appropriate horizontal axis. It might be a bit better to start with the micro on its side, so as to use the longer gap, except that the side with the hinges is prone to be a bit damped.

 

This might take some doing. I'll see what I can accomplish...

 

~modest

[Qfwfq]

It just seems like... when the gap is larger than the wave... neither waves with a parallel or perpendicular oscillation should be blocked.

In this case you ought to have a mighty fine leak, even at the frequency the choke is designed for.