In general, the microwave oven employs 2.45 GHz and most microwave ovens have similar cavity. As I guess, the transmitting length isn't finite. However, how about valid distance of the conventional microwave? Some people said that radio waves expand out using the inverse square law, meaning that the strength is 1/(distance squared). Then, how much distance is valid to increase temperature of target if there is no door and just with waveguide? If there is any scholar equation or base data about it, I hope to know that. For example, using the governing equation concerning with energy transmition of microwave, quantitatively calculate them.
Furthermore, recent commercialized microwave ovens have a stirrer in the wave guide. The stirrer do a role of microwave diffusion as I know. But it may interrupt microwave transmission or decrease the energy for target. Isn't it?
Thanks
Dongkyu Lee
The inverse square law holds for any point source. For microwave ovens or any other kind of RF propagation, what you are interested in determining is the power density at a certain distance, which is a function of the area of a sphere. If the source is a point, the power becomes distributed over the surface area of the sphere, A = 4*pi*r^2, so power density, such as watts per square centimeter, is reduced by the square of the distance from the source.
If the source is a line source, rather than point source, then power density is reduced as a function of 1/r, as opposed to 1/r^2, because in this case, we are talking about a cylinder, not a sphere. Area of a cylinder is 2*pi*r*l.
If the radiation is focused, e.g. by a directional antenna, we still use the inverse square law, but we introduce this "fudge factor" called "antenna gain." So it works out anyway, by pretending that the antenna is actually creating more signal. When in fact, the antenna is merely concentrating the signal, rather than allowing it to be distributed over the entire surface area of the sphere.
Microwave ovens can either rotate the beam (a beam created essentially by "antenna gain"), or they have a rotating plate under the fixed beam. Either way, the intent is to more uniformly distribute the "signal" on the food.
Anyway, for propagation loss in RF transmission, there are many models. In your case, I think you want to use the free space propagation loss model, which is derived theoretically from the inverse square law, not empirically. Scaled for RF radio transmissions,
loss(dB) = 32.45 + 20*log10(freq. MHz) + 20*log10(dist in km)
That assumes spherical distribution. If you have antenna gain, it would be experessed in dB, and you can add that antenna gain to the negative dB that expresses loss based on distance.
Thank you for your kind reply.
It will help my work. Thank you again Best regard Sincerely Dongkyu Lee
The microwave oven is a closed metal cavity with microwave input from a waveguide in part of one side. Unless there is very strong absorption, the microwave fields will bounce about in the cavity and build up into a resonant standing-wave pattern in the cavity, with parts where there are strong electric fields and parts where the electric fields are small. This can be seen by putting grated cheese on a slice of bread and putting it in the microwave without the stirrer (and not close to the walls).
https://www.youtube.com/watch?v=kp33ZprO0Ck
The energy level will build up in the cavity until the power heating the food, and the power going back to the magnetron, are equal to the magnetron power input. If there is no food then most of the power will go back to the magnetron, which may not be good for it!
If there is a lot of food in the oven, then the energy levels in the cavity will be small, and if there is not much food in the oven, the energy levels in the cavity will be high. In both cases the power input to the food will be roughly equal to the power emitted from the magnetron.
The stirrer or turntable is to move the energy pattern around, or move the food through it, so that the food is cooked evenly, and not like the pattern on the cheese on bread.
If you took the magnetron and waveguide out of the microwave oven, so the cavity wasn't joined onto the end of the waveguide, then the open end of the waveguide would make a fairly good microwave antenna, with 30 degrees or so beamwidth and a far-field distance of about 10 cm.
You would need to be careful. The mains power supply will be dangerous if it isn't in a box and 800 W of microwaves can cause serious injury to eyes,, skin, fingers, hands and limbs, with burning, blindness and lasting nerve damage. It may not all go forward, so being close to it and behind it may not be safe.
There was a criminal who bypassed the interlock of an microwave oven and let it radiate through a wooden or brick wall (putting it with open or removed door very close to the wall) to the sleeping room of the neighbor who has his head just at the other side of the wall. This is REALLY dangerous and can lead to serious injury maybe even death. Apart from this for the 1/r dependency of the field strength (power goes with 1/r^2) the usual far fiield rule hols (2 D^2/lamda) where D is the largest diameter of the radiating aperture (may 30 cm for a typical microwave oven) and lamda is 12 cm for 2.45 Ghz
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