What's the reason microwave oven frequencies are at 2.45 Ghz?

[brajesh]

TL;DR Summary

How are microwave oven frequencies chosen?

Hi,

A little background before my question:

I read that microwave oven frequencies are at 2.45 Ghz, or about a 12cm wavelength.
Lately, 5G is using frequencies called millimeter waves, and these frequencies can be as high as 40Ghz.

So, linguistically to me, it seems as microwave oven waves are a smaller wavelength than 5G, because millimeter wave sounds longer than microwave to me --- but it's actually the reverse!

Ok aside from the fright of thinking about that, coming to my question, I was wondering why what's the reason that 2.45 Ghz was chosen for microwave oven frequencies? I'm guessing that was the state of the art technology in the 1950's when microwave ovens were invented?

And my next question, can microwave ovens use higher frequencies, maybe in the 5G range, and would that cook food better or faster?

thanks

 

[Frabjous]

It‘s a tradeoff. While you want water to absorb as much energy as possible, you also want it to absorb it throughout the thickness. If it absorbed ”too well” the absorbtion would be localized at the surface which would cause heating in the volume by thermal transport which is like “normal” cooking.

 

[Baluncore]

2.45 GHz is in a band reserved for ISM; Industrial, Scientific and Medical radiation.
Assignment of RF power applications to the ISM bands, keeps them from interfering with communications services.
https://en.wikipedia.org/wiki/ISM_radio_band#Frequency_allocations

 

[Drakkith]

And my next question, can microwave ovens use higher frequencies, maybe in the 5G range, and would that cook food better or faster?

Microwaves can indeed use different frequencies other than 2.45 GHz. Commercial microwave ovens often use a frequency 915 MHz instead. Why they do so I don't know.

As for cooking food faster, the primary determination in how long food takes to cook is how much power the food is absorbing from the microwaves. A 1000-watt microwave cooks faster than a 600 watt microwave (assuming both are used at full power) because the former is higher in power output than the latter.

Another concern is that you can cook too fast, creating spots that are burnt next to spots that are nearly frozen. This is because the power isn't evenly distributed throughout the field inside the microwave. The microwaves bounce around and interfere with each other, creating a standing wave inside the cavity, which results in hot and cold spots where the waves either constructively or destructively interfere. Putting too much power into the field can overcook the food at the hot spots before the heat has time to diffuse into the rest of the food through conduction.

 

[sophiecentaur]

Summary: How are microwave oven frequencies chosen?

So, linguistically to me, it seems as microwave oven waves are a smaller wavelength than 5G, because millimeter wave sounds longer than microwave to me -

You've got a point there. This is a linguistics question; the nuts and bolts are not really relevant. The terms and classification of EM radio wavelengths followed the history of the technology. We went from long and medium wave, through High Frequency, Very High Frequency and Ultra High Frequency. The term "microwave" was often used for less than 1GHz, although those wavelengths are many cm (more like decimetres; a long way from microns) and it's a good marketing term for selling a new technology.
So you could say that the "micro' term was used up too early, because there are a lot of powers of ten (Octaves) available on the way to optical wavelengths of hundreds of nm. We talk of cm, mm, sub mm, Far Infra Red, Near Infra Red on the way to visible.
But the calculations will mostly use the metre, these days so we all know where we are.

 

[vanhees71]

It's just a good range of frequencies for water to absorb the em. energy. Here's a nice didactical article about the details:

https://doi.org/10.1088/0031-9120/39/1/006

 

[anorlunda]

Summary: How are microwave oven frequencies chosen?

aside from the fright of thinking about that, coming to my question,

Your question implies that high frequencies are scarier than low frequencies. Not true until you get into X,rays.

Shine a flashlight on your hand and see if it burns. Light has higher frequencies than 5G

 

[brajesh]

Your question implies that high frequencies are scarier than low frequencies. Not true until you get into X,rays.

Shine a flashlight on your hand and see if it burns. Light has higher frequencies than 5G

yes, thanks for alleviating my fear :)

 

[brajesh]

It's just a good range of frequencies for water to absorb the em. energy. Here's a nice didactical article about the details:

https://doi.org/10.1088/0031-9120/39/1/006

sounds like a nice article but I can't access it

 

[brajesh]

You've got a point there. This is a linguistics question; the nuts and bolts are not really relevant. The terms and classification of EM radio wavelengths followed the history of the technology. We went from long and medium wave, through High Frequency, Very High Frequency and Ultra High Frequency. The term "microwave" was often used for less than 1GHz, although those wavelengths are many cm (more like decimetres; a long way from microns) and it's a good marketing term for selling a new technology.
So you could say that the "micro' term was used up too early, because there are a lot of powers of ten (Octaves) available on the way to optical wavelengths of hundreds of nm. We talk of cm, mm, sub mm, Far Infra Red, Near Infra Red on the way to visible.
But the calculations will mostly use the metre, these days so we all know where we are.

Thanks, I'm glad you see the linguistic muddle too :)

 

[brajesh]

Microwaves can indeed use different frequencies other than 2.45 GHz. Commercial microwave ovens often use a frequency 915 MHz instead. Why they do so I don't know.

As for cooking food faster, the primary determination in how long food takes to cook is how much power the food is absorbing from the microwaves. A 1000-watt microwave cooks faster than a 600 watt microwave (assuming both are used at full power) because the former is higher in power output than the latter.

Another concern is that you can cook too fast, creating spots that are burnt next to spots that are nearly frozen. This is because the power isn't evenly distributed throughout the field inside the microwave. The microwaves bounce around and interfere with each other, creating a standing wave inside the cavity, which results in hot and cold spots where the waves either constructively or destructively interfere. Putting too much power into the field can overcook the food at the hot spots before the heat has time to diffuse into the rest of the food through conduction.

cool, this is helpful, thank you

 

[sophiecentaur]

Thanks, I'm glad you see the linguistic muddle too :)

You will find that problem at every twist and turn of Engineering. Add that to fanciful ideas from the informally educated and you just have to stick with the textbook.

 

[davenn]

Another concern is that you can cook too fast, creating spots that are burnt next to spots that are nearly frozen. This is because the power isn't evenly distributed throughout the field inside the microwave. The microwaves bounce around and interfere with each other, creating a standing wave inside the cavity, which results in hot and cold spots where the waves either constructively or destructively interfere. Putting too much power into the field can overcook the food at the hot spots before the heat has time to diffuse into the rest of the food through conduction.

which is why the microwave oven has a rotating plate to alleviate these problems

 

[sophiecentaur]

which is why the microwave oven has a rotating plate to alleviate this problems

Three of the (high end) ovens I have owned did not have a turntable. The alternative is a ‘mode stirrer’ which alters the phase from the feed. Using a turntable restricts the size of the pot.
Another detail: that content of the food makes a big difference to where the heating takes place. In my experience, stews and soups tend to cook on the outside (see the bubbles) and dryer food can have hotspots inside.
It’s complicated.

 

[robphy]

which is why the microwave oven has a rotating plate to alleviate these problems

I think there should be a second off-centered rotating plate [with a different angular velocity] to further alleviate problems:
think epicycles.

 

[Baluncore]

I think there should be a second off-centered rotating plate [with a different angular velocity] to further alleviate problems:

I have not noticed further problems.
An epicycle would reduce the size of the dish that could be used.
A metal fan could function as a mode scatterer if it was needed.

 

[Drakkith]

Eh. I'll just stir my food halfway through instead of pay double the price for a microwave.

 

[hutchphd]

Commercial microwave ovens often use a frequency 915 MHz instead. Why they do so I don't know.

Early diathermy machines I know used 915 MHz also. Perhaps anything higher was too difficult to produce in the day. At the end of WWII I think 1 GHz radar was the absolute max.
Also I believe the absorption depth for 2.4 GHz in liquid water is less than a cm.and shorter with increasing frequencies. As one increases frquency , you will create a broiler at some point, not a uniform heater. Already been done.

CORRECTION: (after prompting and actual research) the magnetrons available at the start of the war could produce 1GHz and beyond. thanks to @Baluncore

 

[Baluncore]

At the end of WWII I think 1 GHz radar was the absolute max.

During WWII, H2S at 3 GHz, was operational from early 1943.
https://en.wikipedia.org/wiki/H2S_(radar)

During WWII, H2X at 10 GHz, was operational from Oct 1943.
https://en.wikipedia.org/wiki/H2X#Pathfinder_missions

 

[sophiecentaur]

Also I believe the absorption depth for 2.4 GHz in liquid water is less than a cm.and shorter with increasing frequencies. As one increases frquency , you will create a broiler at some point, not a uniform heater. Already been done.

I've always been skeptical about the advantage in 'quick cooking'. The 'busy lives' idea is specious for most people. If you want quick then have a sandwich. It's only in petrol stations and the like where sub five minute cooking times are relevant and those ovens spoil what was, at the start, pretty manky food. I always find that a few minutes with the appropriate power gives even cooking and avoids 'broiling' and pots that are difficult to clean.

 

[tech99]

A frequency band of 2450 +/- 50 MHz, allocated to Industrial, Scientific and Medical, was allocated at the 1947 ITU Conference at Atlantic City. It applied to ITU Region 2 so did not include the Americas.

 

[Baluncore]

It applied to ITU Region 2 so did not include the Americas.

I thought that ITU Region 2 always was the Americas.
I believe the 2450 MHz ISM band was originally proposed by the USA, who lobbied other ITU Regions to make the 2450 MHz ISM allocation worldwide.
https://en.wikipedia.org/wiki/ISM_radio_band#History

 

[tech99]

My mistake as I misread the document. It says,
"In Region 2, Australia, New Zealand, Northern Rhodesia, Southern
Rhodesia, the Union of South Africa, the territory under mandate of Southwest
Africa, and the United Kingdom, the frequency 2 450 Mc/s is designated for
industrial, scientific and medical purposes. Emissions must be confined within
the limits of ±50 Mc/s of that frequency. Radiocommunication services oper-
ating within those limits must accept any harmful interference that may be
experienced from the operation of industrial, scientific and medical equipment"

 

[Baluncore]

I agree that the ITU allocations are confusing in many dimensions.

Microwave oven magnetrons are tuned by their physical size. They are designed to start cold, close to the high-frequency end of the ISM band. As they operate, the copper body heats and expands, so they rapidly chirp downwards, until they reach a thermal equilibrium in the bottom half of the band. At the end of each cooking cycle, they simply disappear. At meal times near the city, the spectrum analyser display looks like gannets diving and feeding.

 

[anorlunda]

At meal times near the city, the spectrum analyser display looks like gannets diving and feeding.

Excellent description of visual images. Thanks.

 

[sophiecentaur]

At meal times near the city, the spectrum analyser display looks like gannets diving and feeding.

Excellent description of visual images. Thanks.

I have a Panasonic oven which has an 'inverter' and that seems to modulate the energy over a short timescale - no obvious on-off-on-off. Also, it gives fast, irregular bursts of power (so called Chaos) which is supposed to give better and faster de-frost times. I would say that it does seem quicker. I guess the spectrum analyser display could be different - once all ovens are like that- because there would not be the amount of temperature cycling.

PS aren't magnetrons magic things? They seem to go on for ever.

 

[tech99]

They are magic. When Randell and Boot made the first one at Birmingham University, they guessed the dimensions, sealed it with sealing wax and put it on a vacuum pump. They were staggered when it worked straight away, and rather than one watt of power of previous tubes, they could draw sparks an inch long from the RF output, corresponding to about 10 kilowatts.

 

[Baluncore]

Randall and Boot first successfully adapted and applied the cavity magnetron to RADAR. They did not invent it, nor did they build the first example. R&B were good researchers and read the literature. They were prepared, and in the right place at a critical time. Britain provided examples of the R&B cavity magnetron to the USA.
There is an open copy of a history available here.
https://ieeexplore.ieee.org/ielx7/74/6735456/06735528.pdf
Y. Blanchard, G. Galati and P. van Genderen,
"The Cavity Magnetron: Not Just a British Invention [Historical Corner],"
in IEEE Antennas and Propagation Magazine, vol. 55, no. 5, pp. 244-254, Oct. 2013, doi: 10.1109/MAP.2013.6735528.

 

[fluidistic]

It's just a good range of frequencies for water to absorb the em. energy. Here's a nice didactical article about the details:

https://doi.org/10.1088/0031-9120/39/1/006

I can't access to the article right now. However I thought it had nothing to do with water (which absorbs well anywhere except in the visible range, out of memory). Wikipedia links to https://www.wtamu.edu/~cbaird/sq/mo...icrowaves-in-a-microwave-oven-tuned-to-water/ where it is stated that ''There is a popular myth that explains microwave ovens as operating at a special resonance of water molecules. In reality, this myth is just that, a myth. Referring to the Figure 15.2, you can see that there is no resonance of water at this frequency. The first resonant peak occurs above 1THz, and the highest loss occurs well into the infrared. There is no special significance of 2.45 GHz, except that it is allocated by the FCC as being allowable for microwave oven usage.''

 

[cjl]

Frequency choice does relate to penetration depth into the food though, and it's important to pick one that isn't just deposited right on the surface if you don't want to just sear the surface while leaving the center raw. As can be seen if you look at a [chart of water absorbtion vs frequency], 2.4GHz gives around a cm for a reduction by a factor of e, with salty water (obviously very common in food) shortening this a bit. This is a nice balance where it still will deposit an adequate amount of energy into small items while not only heating the surface of larger items. If you decided to use 100MHz instead, much of the energy would pass through the food rather than being absorbed, and if you decided to use 50GHz, you'd end up with something very good at searing the surface without heating the middle very much, which would be annoying as you'd have to rely purely on conduction to heat up the middle, greatly slowing the cooking process and eliminating one of the main advantages microwaves have over ovens.

This also potentially explains the commercial ovens using a lower frequency - in commercial use, you'd tend to expect the typical item being microwaved to be larger, and thus an increased penetration depth is beneficial for being able to heat the food at a higher power level while still avoiding the broiling or searing issue (and you wouldn't care about the reduction in efficiency and effectiveness when heating small items).

 

[Baluncore]

If you decided to use 100MHz instead, much of the energy would pass through the food rather than being absorbed, ...

Because a microwave oven is a cavity, it does not matter if the radiation bounces off the highly reflective walls many times before it is attenuated by the item being heated in the cavity. A high-Q resonant dipole is a similar situation, it will radiate eventually.

RF heaters operating below 100 MHz are bigger than magnetrons and need much bigger enclosures, so the oven cavity will not look like a short or open circuit to the generator. It will always be a compromise. Obviously, the oven cavity needs to be the size of a big dinner plate, while the RF generator needs to be smaller, so the microwave oven will be mostly enclosure, that will fit in a kitchen.

HF heaters employ a resonant LC circuit. Dielectric heaters place the material in the electric field of the capacitor, while induction heaters place the material in the magnetic field of the inductor. But neither of those extremes works well for a plate of prepared food. The economic solution comes about when an RF heater has a multiple wavelength cavity, that can develop a diagonal complex standing wave pattern. For a large dinner plate of about 400 mm, the wavelength comes out at about 100 mm = 3 GHz. The 2.450 GHz ISM band is conveniently close.