Standing Waves in a microwave

  1. Hey,

    I am looking for information on the relationship between microwave wattage and standing waves. Does the wattage increase/decrease the size of the wave? Does wattage increase the frequency of the wave?

    I am also looking the relationship between the time a microwave is operating and the standing waves produced during that time (Does time have any affect on the standing waves in a microwave)?

  2. jcsd
  3. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    The only thing that the wattage will affect is the amplitude of the waves. For all intents and purposes, time is not a factor with the standing waves either. They will achieve steady-state after a very minute amount of time.
  4. Born2bwire is correct the frequency is governs the wavelength of the standing wave in accordance with λ=c/f (where c is the speed of light and f is the frequency of the microwave). Those little circles you see on the door of your microwave are called waveguides. The door of a microwave oven is carefully designed to reflect microwaves so that they can't escape from the oven. That mesh that you see in the door isn't plastic, it's metal. Metal surfaces reflect microwaves and, even though the mesh has holes in it to allow you to observe the food, it acts as a perfect mirror for the microwaves. Basically, the holes are so much smaller than the 12.2-cm wavelength of the 2.45-GHz microwave that the microwave cannot propagate through the holes. Electric currents flow through the metal mesh as the microwave hits it and those currents re-radiate the microwave in the reflected direction. Since the holes aren't big enough to disrupt that current flow, the mesh reflects the microwaves as effectively as a solid metal surface would.
  5. To move this to a more familiar analogy - the power (wattage) of a microwave is akin to the "brightness" of a light. You can imagine a light of a particular color, let's say green. Take a green LED, and run .01mW through it. It will shine dimly at 530nm. If you run more wattage through it, it will shine more brightly at the same 530nm, it is just sending more photons your way.
  6. Additional insights here:

    and later in that same reference:
    "...Another hazard is the resonance of the magnetron tube itself. If the microwave is run without an object to absorb the radiation, a standing wave will form. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to 'cook' itself and burn out./..."
    Last edited: Oct 17, 2010
  7. I have a question. The RF in microwave is continuous and over a period of seconds to minutes?

    For RF used in a MRI study it is pulsed. Things don't cook up inside MRI because of the pulse duration in milliseconds? So is pulse duration a factor here?

    The MRI scanners are rated for 3kW - 4kW power. Microwave ovens are around 1000 watts. Any other important factors to elaborate this difference w.r.t. MRI? thanks.
  8. Pulse duration is everything. I designed a 100 volt pulsing circuit of sub nano second rise and fall time and pulse width programmable from 2 nsec to 100 nsec and rap rate of about 1 kHz. If you look at the pulse into 50 ohm, that is 2A peak current and 100V. That is a lot of peak power. But because the duty cycle is so short (100nS in 1 mS) of about 0.01%. I don't even use have to use big heat sink or large load resistor.
  9. Interesting question, I know that the reason why microwaves heat certain materials is because of the frequency and wave length at whcih they propagate at. Consider this as the (micro)wave alternates between positive and negative, it is the same as a magnet flipping back and forth.
    All liquids and food products, such as this turkey, are made up of molecules. These molecules have positive and negative particles, so they tend to behave like microscopic magnets. As the positive half cycle of the microwave penetrates the food, the negative particles of the molecules are attracted and attempt to align themselves with this positive field of energy. Then, when the microwave energy alternates to the negative half cycle, the opposite occurs -- The negative particles are repelled and the positive particles are attracted, causing a flipping motion (actually, this reaction is the movement of the particles within each molecule, so, technically, they reverse polarity).

    Now, consider that the actual frequency of the RF energy used in microwave ovens is 2450 million cycles per second! Moreover, consider that within the course of one of those cycles, the molecules would actually change their direction (polarity) twice - once for the positive half-cycle and once for the negative half-cycle. This red-hot rate of vibration causes tremendous friction within the food, and - just as rubbing your hands together makes them warm - this friction produces heat.

    Fourier dictates that any waveform or pulse can be constructed by a series of superimposed sine waves. In order words any pulse can be reconstructed using the right type of sine waves. I suspect if the pulse had sinusoidal components similar to the microwave wave it would head the material. However the pulse frequency needs to rival that of the microwave in order to generate a similar type of heating affect....someone correct me if I am wrong
  10. You get the high frequency component by making the rise and fall time short. I forgot the formula of highest frequency component vs rise time. Also I think not only the pulse has to have the high frequency component, the pulse rate has to be high to keep the RF component going strong. You cannot have a pulse frequency of 100 Hz even the rise time is short. Using your example, if you rub your hand super fast for a second and rest for a minute, your hand is not going to get warm.

    Like the example of the circuit I designed, I made use the fact that the pulse rate is low, even the pulse energy is high, but I gave it a lot of time to cool down, turn out the circuit is very compact and no large components required.

    I don't know anything about micro wave cooking though!!!
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