With Andy on this one - any good domestic appliances workshop should be able to check with a microwave calibration unit to test for leakage.
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What\'s for you won\'t pass you by
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Such calibration units have to be tested annually every year and a certificate issued to ensure that they are accurate.
That should give some idea of the importance attached to their accuracy...:-)
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What's for you won't pass you by
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"That should give some idea of the importance attached to their accuracy...:-)"
Not really - its a bs5750 or iso2000 requirement for any electronic test equipment to be calibrated.
But back to the point.
A microwave oven is possibly one of the most powerful and disruptive things mere mortals are allowed to have without a license. Modern 800watt+ microwaves are very powerful tranmitters. It could easily disrupt car electronics.
For this reason they are very well shielded and should have minimal leakage. If yours is leaking enough* to get through a brick wall then its baking your eyeballs.
*Its most unlikely, the sheilding on a microwave is pretty foolproof.
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The microwave oven cooking volume presents a 'resonant cavity' to the magnetron and a standing wave pattern is set up in the cavity. In other words (RF-wise) the magnetron is working into a high impedance. If there is a lot of microwave leakage (enough to get through a double-brick cavity wall) then the magnetron would be overloaded and would fail.
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I can confirm that the same thing happens to my mate's Mazda 6.
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I reckon this is a wind-up.
Doesn't Bluetooth and WiFi also use frequencies around 2.5GHz? Mazda alarms would be going off all the time.
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Bluetooth 2.45ghz @ 1 milliwatt
Wifi 2.4 ghz (some on 5ghz) 500milliwatt
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My guess is it could be the surge via the wiring when the thing goes on and off that is generating some RF by the wiring ringing up. Such frequency would be lower than microwaves. You are, after all, switching a kilowatt or so of power on and off. Though if it is radiating like that it should not have got past the interference tests.
Another point is, the usual 2.5 GHz oven frequency is dependant on the cavity and magnetron tuning, and so could go off frequency during switch on and switch off -- it is known as mode jumping. It is the sort of thing that has to be taken into account during design.
But without doing some tests it is difficult to say more. You could try watching things like the television and your car to see if there are interference splurges when it goes on and off.
Hope your oven is NOT leaking. I would not expect even quite a large oven leak to stop the microwave working as a lot of power is absorbed when you put food in and it still keeps going.
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The internal cooking area is too big to the given wavelength to provide any resonance or impedence and anyway the waves are dispersed to the cooking area by the wave guide. (which is a resonant cavity)
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The internal cooking area is too big to the given wavelength to provide any resonance or impedence and anyway the waves are dispersed to the cooking area by the wave guide. (which is a resonant cavity)
Think that's all mixed up.
The cooking chamber is a resonant cavity. There is a standing wave pattern set up in it. The food absorbs some of the microwaves. If there is no food in there then there is nothing to absorb the enegry and it gets reflected back into the waveguide (which is not a resonant cavity) - this is why it is bad to operate a microwave without food in.
If there is a lot of leakage then the standing wave pattern cannot be set up and the microwave will not work properly.
I had a friend who worked on magnetrons for a company in Lincoln. He reckoned that if you left the door of a microwave open (having overridden interlocks) then it would not work - either just not work, or the magnetron would be damaged.
Microwaves operate on 2450MHz, which as noted in another post is similar to Bluetooth and WiFi.
I think bricks would be very opaque to microwaves!
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"Think that's all mixed up."
Well think again.
"The cooking chamber is a resonant cavity."
Its not
"the waveguide (which is not a resonant cavity)"
It has to be or it would not be guiding waves anywhere.
"gets reflected back into the waveguide"
very minute amounts, A wave guide being a wave guide means that a wave would have to be just in the right place to get back in. After bouncing around the cooking cavity its very unlikely.
A cavity magnetron gains all its impedence internally from the strapped cavity funny enough thats why its called a cavity magnetron.
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Just had a quick look at our old microwave;
The oven measures approximately 24cm x 36cm x 36cm
I can see a number of ways for waves of length 12cm to bounce around, and close upon themsleves, i.e. potential ways to give a resonant condition. All of these are higher modes, rather than being a fundamental - the cavity is too large in each dimension for a single wavelength to span between walls. If any particular resonant mode is excited or not depends upon the position and direction of the source with respect to the nodal pattern inside the cavity.
A waveguide does not have to be at resonance to carry power. The main criterion to be met for adequate transmission is that the change in impedance for the wave upon entering or exiting the waveguide must not be large (the reverse of this principle is how the step changes in exhaust size at the silencers strip off acoustic waves, reflecting them back towards the engine, rather than allowing them to be guided right out of the back of the pipe - in addition there may be 'fluff' in the silencer cavity to give some 'resistive' loss). If the change in impedance is large at any interface traversed by a wave, then significant power may be reflected back.
As it is possible that a standing wave may be set up in the cavity, and that at the point of entry of the wave into the cavity, the amplitude of the excited standing wave cannot be zero, then it is possible for some energy to be 'reflected' back towards the magnetron.
>>A cavity magnetron gains all its impedence internally ...
RF, Sorry, I don't follow this bit - please could you explain?
number_cruncher
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"Think that's all mixed up." Well think again. "The cooking chamber is a resonant cavity." Its not
Go and read a few books. The cooking chamber is a resonant cavity and a standing wave pattern is set up in it. That's why you get 'hot spots' and 'cold spots' in a microwave - you need a 'stirrer' to agitate the standing wave pattern or a turntable to move to food through it.
"the waveguide (which is not a resonant cavity)" It has to be or it would not be guiding waves anywhere.
What nonsense!
"gets reflected back into the waveguide" very minute amounts, A wave guide being a wave guide means that a wave would have to be just in the right place to get back in. After bouncing around the cooking cavity its very unlikely.
Because there is a standing wave pattern in the cavity *lots* of power can be reflected back into the waveguide if there is nothing in the oven to absorb the power. Enough to cause sparking in the waveguide. This is way you should never run an oven empty, or if there is just a small amount of food in there you put a cup of water in to absorb some power.
A cavity magnetron gains all its impedence internally from the strapped cavity funny enough thats why its called a cavity magnetron.
??
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Please excuse the pedantry...
At resonance, the impedance is very low - the reactive parts cancel, and only the real, 'resistive' part remains. At resonance, one obtains a large output amplitude for a given source strength.
pedantry over - thankfully!
I think that the oven cavity is likely to have quite a number of possible resonances, considering that the wavelength is about 12cm at this frequency.
Is it possible that the walls of the house are transparent at this wavelength? If the wall were conductive, then an electromagnetic wave is very quickly attenuated - hence ECUs get put into metal 'Faraday' cages or enclosures. However, unless the OP lives in a steel container!, the walls will not necessarily absorb the wave.
One possible course of action, is to move the oven around the house a bit. If it only disturbs the car when it is in close proximity, then the leakage of microwave radiation may be responsible.
number_cruncher
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Don't alarm sensors use ultrasound?
Andy
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Don't alarm sensors use ultrasound?
Not all. Ragtops are prone to falsely going off due to the movement of the canvas - hence why a microwave sensor is better.
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I doubt the 2.45Ghz microwave leakage from the oven is penetrating through a cavity brick wall and then into the car!
I was guessing the kitchen would have a window close to the car, and that any leakage would reflect around the kitchen walls with some going through the window.
In terms of the quatity of leakage considered "acceptable" in the URL I posted earlier:
ADVICE TO OCCUPIERS
"10 It is considered reasonably practicable to control stray radiation leakage to below 5 mW/cm . Leakage rates in excess of this figure should be viewed as an early warning that some deterioration is taking place and that the equipment may require checking and adjustment."
It's my understanding of zoned microwave car alarms that if the pattern of microwaves inside the car is disturbed/absorbed (by an intruder) the alarm is activated. The quantity of RF inside the car presumably conforms to "safe levels", and will be quite low, and this static pattern could be altered by another source of RF on the same frequency.
Regarding Bluetooth, and other transmit/recieve equipment in this band, I'd expect them to find an empty channel before they start transmitting - so you don't have problems with multiple devices in close proximity - but the oven just blasts away at 2.54Ghz.
Regards
John H
P.S. I am not a physicist.
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Having worked in microwaves (and got a few MW patents) you have not got that right RF.
Wave guides are what they say. The waves reflect off the walls as part of the guidance. As Numbercruncher says, the aim is no reflections so the power passes smoothly along. Although, as shown below, wave guides are often used to make resonant cavities.
Wave guides can be explained to the non-microwave person by starting with co-ax cables. Infinitely long ones look at their input like a resistor with a characteristic resistance determined by the inductance and capacitance per unit length. If you cut the cable and put a resistance of this value across the far end of a length of it, the power continues to flow smoothly into it and none comes back. Wave guides behave somewhat similar. Get a feed putting power into it, in the correct wave mode, and terminate it properly in a load that matches it, and you can have a low loss transmission medium with no reflection.
However, you can produce a resonant cavity, (or cable equivalent of it) by shorting/closing the far end. Waves then bounce back and if the length is just right, and the driven end is made right, you can pump it (push the swing) to store up energy.
At lower powers (and this MAY be made use of in microwave ovens) one end reflection is total, the other is partially opaque or has a hole in it known as an iris -- same effect. The pump can be somewhere inside spaced a predetermined distance off an end wall. Power will build up in the cavity (like the swing) and, with suitable design, will control the frequency of the microwaves.
An iris, at the oven end, allows a predetermined fraction of the stored power to flow out, whilst keeping enough in to maintain frequency control. In the oven the power can set up more standing waves, as has been described. If the iris is not well designed, this can upset the frequency -- cause mode jumps.
I see my MWO has a plastic bung in the WG at the oven end but I have not examined it in detail. I have seen the stirrer that has been mentioned in early oven designs. May be the rotating turntable, these days, is enough, without it. And the rotating plate has a certain amount of loss to control 'empty oven' problems.
I would expect dry walls to let through some of the 2.5 gig microwave power, depending on their construction. But we are talking very low power here, not enough to pulse a semiconductor junction to produce a DC pulse. Different story if the microwave/RF power is able to mix with an existing frequency in the car's system. But where?
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Yes I know about wave guides, having worked on Radar I designed and used them as feeders (in the megawatt range too).. I was trying to explain how a wave guide is resonant (as is an aerial)
Perhaps you can explain to a certain person, how it is claimed that the microwaves bouncing around in the cooking area could be called a "cavity" when A: it is too big to be so, b: would have to be precisely sized in percentages of wavelengths to do so (a very short wavelength at that), and c: redirect a significant portion of the output power back up into the magnetron through a precisely sized waveguide aperture.
you say
"However, you can produce a resonant cavity, (or cable equivalent of it) by shorting/closing the far end. Waves then bounce back and if the length is just right, and the driven end is made right, you can pump it (push the swing) to store up energy."
agreed. Is this not how the cavity INSIDE the magnetron works? and is strapped to prevent frequency drift?
To make the cooking cavity "resonant" or able to bounce waves back up the waveguide (or even make the waveguide look closed)the cooking cavity would need to be sized in %proportions of 3cms. All designers would need to do is produce a cooking cavity outside this proportions and "closure" and therefore overloading of the magnetron and the waveguide feed to the oven does not happen.
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Anyway - as this has the makings of a feud I am bouncing out of this thread.
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Although very interesting for some of you, please remember that this site is primarily for motoring.
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Cavities can have dimensions of (N+0.25)wavelengths and still work [N=whole number]. Bye.
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Cavities can have dimensions of (N+0.25)wavelengths and still work [N=whole number]. Bye.
So is that the concise version of your last thread then?
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You can check for leaks by holding the tip of one of those electrical testing screwdrivers with an LED or neon light outside the oven and see if it lights up. Alternatively, if you're really lazy, wave a fluorescent lamp near it.
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>>You can check for leaks by holding the tip of one of those electrical testing screwdrivers with an LED or neon light outside the oven and see if it lights up. Alternatively, if you're really lazy, wave a fluorescent lamp near it.
I heard.Bottle of lemonade in front of door.If any leaks the bottle will boil..Fail to see where this will affect a car alarm though.. I thought the transmission was short range not long
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Steve
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Hi Steve,
If the microwaves get out, they can propagate in air or other non-conducting materials very well. The non-conducting bit was why I asked about the transparency of walls further up - just because we can't see through walls doesn't mean that microwaves can't get through! As mentioned above, this is the frequency range used for bluetooth, radar, and, not mentioned above, some colleagues of mine are currently looking to try to detect *gravitational waves in the microwave region of the spectrum which came from times very early in the formation of the universe, shortly after the big bang.
These waves can travel! - although they are usually well trapped inside the oven.
The bottle of lemonade test sounds like it could be quite sensitive - I might have a go. Our oven is really quite old and manky, I could imagine it isn't working optimally!
I suppose another test on the problematic car alarm could be to 'foil' the alarms microwave emitters/detectors by temporarily surrounding them with some tin foil, and trying the oven.
number_cruncher
* gravitational waves of any form haven't been detected yet, but many researchers all over the world are trying to detect them using an amazing range and array of kit!
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Not sure about tin foil - I once wrapped my phone in the stuff to see what would happen and it worked fine.
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Hi Steve, If the microwaves get out, they can propagate in air or other non-conducting materials very well. These waves can travel! - although they are usually well trapped inside the oven.
I agree with that - 2.54Ghz was allocated to a UK Broadcaster for terrestrial vision links until around 30 years or so ago.
2.5 Watts with a 4 foot dish at either end would be used up to 20 odd miles, and have a signal to noise ratio better than 46dB end to end.
We stopped using that channel when a rubber processing plant in Trafford Park started to interfere with it - 4 volts of hum on a 1 volt signal when they used their "oven".
It's that history which made me consider the RF being the cause of the car alarm trip.
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last thread then? -- Just on microwave ovens.
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Wow - thanks for all the comments and conjecute - I didn't mean to cause any trouble! Its good to know there is such a pool of knowledge in this forum. I think buzbee is on the right lines.. Its a newish (>12 months) Samsung 900 Watt microwave, so I don't think its leaking. As the electric meter consumer unit is also in close proximity I wonder if its the electric field being created by all that current flowing setting off the cabin detectors in some way(I'm not sure if these are microwave or ultrasound).
Oh and no I don't live in a steel container...yet
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