OT: Microwave oven mystery

[winchman]

We have a L-shaped kitchen counter. There's a counter-top MW oven at a 45-degree angle in the corner. Just to the left there's a little FM radio that my wife listens to when she's cooking. About four feet to the right there's a built-in MW above the range hood for the cooktop.

The radio plays normally when the counter-top MW is cooking, but it develops a lot of static if the door on the other MW is opened. The static goes away when the counter-top MW is off.

Obviously the static is being caused by the counter-top MW, but why do we only hear it when the door is open on the other MW?

[Paul Alciatore]

Sounds to me like you have two things going on. First, the counter top microwave probably has a radiation leak. It may not be very much, but when it gets inside the other microwave, the energy is received by the magnetron there and is re-radiated at a somewhat different frequency. Again, not much energy, but enough for the sensitive front end of the radio to pick up. The two frequencies beat against each other, probably in the magnetron and come out in the radio band and probably in a lot of other bands as well. Your radio is just close enough to get the full benefit of this marriage of microwaves so you hear static or other types of interference.

Whenever two different frequencies of E-M radiation meet in a non-linear device, like the cavity of that magnetron, they will mix and produce four frequencies: the original two, their sum, and their difference. It is the difference signal that is getting into your radio. If one or both of those original frequencies are broad band signals (not a single, discrete frequency), then those sum and difference frequencies can be all over the spectrum. Believe me, I have seen some horrendous examples of broad band spurious signals being generated this way. And I watched the FCC tech. just scratch his head and shrug. The magnetrons in microwaves are made for raw power, not for producing clean, discrete signals. That's why those ovens are well shielded.

[winchman]

Yeah, there must be a leak in the counter-top MW, but I don't see how the magnetron in the second MW will do anything, since it's off while the door is open.

Could the screen in the door glass of the second MW be reflecting the leaked radiation back toward the radio?

[MattiJ]

Sounds to me like you have two things going on. First, the counter top microwave probably has a radiation leak. It may not be very much, but when it gets inside the other microwave, the energy is received by the magnetron there and is re-radiated at a somewhat different frequency. Again, not much energy, but enough for the sensitive front end of the radio to pick up. The two frequencies beat against each other, probably in the magnetron and come out in the radio band and probably in a lot of other bands as well. Your radio is just close enough to get the full benefit of this marriage of microwaves so you hear static or other types of interference.

Whenever two different frequencies of E-M radiation meet in a non-linear device, like the cavity of that magnetron, they will mix and produce four frequencies: the original two, their sum, and their difference. It is the difference signal that is getting into your radio. If one or both of those original frequencies are broad band signals (not a single, discrete frequency), then those sum and difference frequencies can be all over the spectrum. Believe me, I have seen some horrendous examples of broad band spurious signals being generated this way. And I watched the FCC tech. just scratch his head and shrug. The magnetrons in microwaves are made for raw power, not for producing clean, discrete signals. That's why those ovens are well shielded.

I wonder what kind of receiving properties the unpowered magnetron in the oven has?

I'd go for more complicated speculation like that the integrated microwave CPU or whatever is radiating and leaking via open door* at around FM frequencies and 2.4Ghz RF leak from countertop microwave combine at the radio receiver.
* or associated wiring nearby door switches, or change in operating mode

Winchman: what happens if you unplug the built-in MW?
Is the built-in MW with fancy electronics or "stupid" model with mechanical timer?

[dave_r]

Possibly.

I recall seeing a tv show a long time ago, where a q&d test for a microwave leak was to hold a fluorescent tube close to the various openings of the microwave, and if the tube would light up, it was indicative of either needing repair or replacement.

[SirLesPatterson]

I have found numerous situations (half a dozen) where 2.4 GHz wifi is interrupted in peoples homes when the microwave is running. In almost every case the wireless router is on the other side of the wall (next room over) from the microwave and moving the router away solves it.

[MattiJ]

Why I never have strange problems?
Home lab has 2pcs R&S CRTU-RU GSM test monsters with spectrum analyzers to 2.7Ghz and two R&S FSIQ-7 7Ghz spectrum analyzers

[Juiceclone]

If u can light a small florescent tube outside the microwave cavity, u got a serious leak. All microwaves "leak" a bit, just detectable with the correct eq. . We used a "wand" type detector to evaluate microwave safety. Harmonics are sometimes generated within an incorrectly designed oven and propagate directly out or thru the cord and even then the building wiring. There's a kilowatt of rf energy bouncing around in that cavity and can be hard to contain.

[Machine]

I would take a metal cookie sheet of about the same size as the built-in MW's door and try an experiment. Run the countertop MW with radio going and hold the cookie sheet close to the same position as an open door on the built-in MW's door (with its door closed). Move the sheet around in various positions and see if you can repeat the anomaly with the radio. Then move the cookie sheet much closer to the countertop MW and try moving it in various positions to see if you can reflect leakage back to the radio for similar effect. Or also simply move the radio out into the direct presence of the leakage area and see what happens. Also try unplugging the built-in MW (if practical) for further isolation and see what happens. Then maybe just for fun, even wrap the radio in tinfoil and if possible ground the tinfoil (safely). I suppose the antenna would still have to be exposed, so not sure if that'll work, but might be fun to try and see what happens.

[Willy]

I would take a metal cookie sheet of about the same size as the built-in MW's door and try an experiment. Run the countertop MW with radio going and hold the cookie sheet close to the same position as an open door on the built-in MW's door (with its door closed). Move the sheet around in various positions and see if you can repeat the anomaly with the radio. Then move the cookie sheet much closer to the countertop MW and try moving it in various positions to see if you can reflect leakage back to the radio for similar effect. Or also simply move the radio out into the direct presence of the leakage area and see what happens. Also try unplugging the built-in MW (if practical) for further isolation and see what happens. Then maybe just for fun, even wrap the radio in tinfoil and if possible ground the tinfoil (safely). I suppose the antenna would still have to be exposed, so not sure if that'll work, but might be fun to try and see what happens.

This would be along the lines of my thinking as well since microwaves can not penetrate metal and are reflected by it, much like they are inside the metal can of a MW, as well as the metal grid inside the door itself.
Also try covering the opening of the MW with the open door to either validate or invalidate Paul's theory.

[ed_h]

Here's what is likely happening:

2.4 GHZ (or so) leakage from MW1 is getting into tuned circuits in MW2. There it is finding some non-linear passive component such as a diode. The non linear transfer function produces all kinds of harmonics that are re-radiated. Some of these harmonics are in the passband of the radio's front end or IF stages.

The non linear component can also act as a mixer, combining the 2.4 GHz from MW1 with any other RF around, such as WIFI, to produce sum and difference frequencies.

Ed

[CCWKen]

I'd go with Paul's explanation any day but perhaps it more simple. Could be one is wired incorrectly. When the door is opened on the built-in, It's lighting a light, right? Maybe a loose wire in the socket or hard wiring.

[winchman]

Winchman: what happens if you unplug the built-in MW? No static.

And, further testing shows the counter-top MW doesn't really have to be running to get the static.


Is the built-in MW with fancy electronics or "stupid" model with mechanical timer? Fancy electronics circa late 1990s. Definitely not mechanical.



So, it's something to do with the light bulb in the built-in MW. We're just going to live with it. There's no way to get to the bulb without taking it down and opening it up. I don't want to mess with the range hood.

[engineerd3d]

The door of the microwave is made of material that stops RF radiation to some extent. Otherwise people would lobotimise themselves watching the tv dinner cook. With that said, I think this is more of a case of blockage than interferance. I bet the waves are being disrupted by the door. If you move the radio to the other side of the room does the static return with the door opened?

This reminds me of a place I contracted some work with when I was in school. I used to do IT work for small companies near my area for a few extra bucks. Run antivirus etc. I had one customer who said their internet dropped out during lunch time every day. I found that puzzling. I asked to see the network equipment. Turns out he had a switch under his microwave, I turned it on and the internet dropped in the entire office. I told him to move the microwave and he said no. So I said we can move the switch and rewire the entire office. He said ok. 700$ later the problem was fixed as I ran brand new lines to the other side of the office.

[wombat2go]

Any of the speculation about the cause may be the one.
I suggest to use an FM receiver with isolated balanced FM antenna terminals.
Take the advantage of "Common Mode Rejection"

Then make a balun.
The baluns shown in the photo are made from these surplus items:

http://www.surplusshed.com/pages/item/m2199.html

Sorry my baluns are roughly made, because i experiment and measure, and forget about finishing them when they work!
The above core becomes lossy with a roll off above at above 60 MHZ, which i think is good at 95 MHZ,
because it reduces the level of the powerful FM stations close by.

The balun on the left is for FM on a 1972 Pioneer SX 440 in my office in the basement.
About 2: 6 turns with 2 parallel ( not bifilar) primary turns as shown.

The one on the table is for 3.5 to 14 MHz on the vintage 1963 Lafayette HE30 ( vac tube shortwave receiver)

https://app.box.com/s/6uqcjpkc3mn94hccnl0qbggeazktj3ts

Run left-over RG-6 coax to an approximate 1/4 wave on 95 MHz, using the little garden fibreglass flag poles.

Here I mounted the FM antenna right above the zapper. But nary a crackle from the demise of any Midget Mosquito.

https://app.box.com/s/064mhbjqroifbzyz9wq8p2v36b6c66vv

By the way.....
We used these in Australia for weatherproofing extension leads connectors like the zapper

http://www.extensionleadsaustralia.com.au/wheather-proof-leadlock-safety-work-extension-lead-15amp-10amp-contractor-tradesman-home-vacuum-cleaners-power-tools-hedge-electric-chord.html#.WZ9tI3UpA18

I would like one for that zapper , but never could find one for USA.
Anybody used these?