can protective grid in door fail during normal use? 2

[ProfDrK]

I have a large Panasonic microwave (inverter) and since I check it for leaks using two meters (digital and diode/meter) in the usual way. Well, lately, suddenly, it started to put out an unsafe field from the mid-section of the door (7-10 mW/cm2 max.) that varies over the visible part of the grid and can reach zero at the edges. The seals around the door don't leak measurably at all when firmly seated. I thought the metal grid in the door could never leak or deteriorate. The peaks appear as hotspots and trail off to safe (not zero) limits within a 3 inch circle on the facre of the door. I will continue to use this appliance style microwave and can fit it with copper screen that I have for other microwave equipment if I have to. I am inquiring here about the abrupt onset of leakage because it is baffling to me since I have several much older microwaves that never developed a leak through the door's window.

 

[biff44]

No, it can not fail in the middle of the door, unless you see a melted hole blown thru the metal mesh.

What you are seeing is RF leakage from the door seal, that is simply concentrating (forming a standing wave) at that one point. Go clean the edges of the door from any food, etc, that has built up.

 

[ProfDrK]

I never thought leakage from a foot away could be measured at a higher level than where the leak occured. Anyway, the seals are not dirty as this is never used for food. As I said, the door seal doesn't leak except where all doors leak (1 mW/cm2) but the signal is high in the middle of the door face and it maxes out at the door and falls off as one moves the meter away from the microwave.
I do like the idea of a standing wave but it implies massive leakage from all around the door.

 

[biff44]

Well, something must have changed. I would look at where metal grid connects to the door to see if it worked loose or is corroded.

What is fairly common in microwave circuits is to have energy leaking out of two points at a low level. If you are unlucky, or it is a broadband circuit, if those two points at N/2 wavelegths apart, you get a resonant circuit with a "standing wave". The voltage at the standing wave can typically be 10 times higher than the voltage at the leakage point. And that voltage maximum would not occur at the leakage point, but at least a quarter wavelength away.

In microwave circuit design, that is why you sometimes see absorber material stuck onto the housing cover--it dampens out the resonances so they do not become so large that amplifier circuits oscillate via a leakage feedback path, adjacent circuits do not leak spurious energy into other circuits, etc.

 

[Higgler]

FYI, in an antenna aperture, 0.4 D^2/Lambda is the distance away from the antenna whereby you measure the peak power density. 2D^2/Lambda is the normal distance to measure antenna patterns, but D^2/Lambda can often be used.

Hence, measuring the distance where the peak of leakage radiation occurs depends on aperture size (and phase and amplitude of the rf at the leakage points of course). Considering a small spot or crack leakage, or effectively a tiny aperture, it would still leak or radiate a max at 0.4 d^2/Lambda, but that would be right at the leak point since the calculated distance might be tiny, lets say 0.05 inches.

It's possible that the mesh in the door appears in tact but has a crack that you can't see. Or there are bulges at opposing sides and the peak leakage field adds in phase at the center outside the door.

kch

 

[ProfDrK]

A crack isn't visible in the grid and the 'leak' is gone for now. However, the bulge comment is near phsycic. This oven is operated with the magnetron and powersupply at top and the door hinge at the bottom. This provides me a long throw downwards to heat tall containers. It is conceivable the oven frame and door may not be sealed due to the reorientation and that this distortion is symetrical on the two long sides of the door. Still, just a guess.

 

[Higgler]

Is this an older Amana Radarange (from 1984?), if so, they have a two part rf seal that's different than most single part seals. The distance the door closes may have a large affect on the seal.

I believe the normal seal of a microwave oven doesn't use any actual rf gasketing, but just 1/4 wave open distances (length of overlap on the door) along a low impedance parallel plate transmission path, which forms and open circuit effectively mismatching the door leak path with the open air impedance inside the oven. A slight change in the hinge position may imperceptibly open a gap along the door seal and up the leakage quickly.

kch