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Why do we need a waveguide to move microwave power from the magnetron to the microwave cavity?

Can't we just push magnetron antenna into the cavity?

Why does it have to go through a waveguide?

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  • \$\begingroup\$ Might it be that the antenna inside the cavity won't work properly due to the "nearness" of the cavity? \$\endgroup\$
    – Andy aka
    Commented Apr 27, 2015 at 7:35
  • \$\begingroup\$ @Andyaka: you mean like standing waves? \$\endgroup\$
    – PlasmaHH
    Commented Apr 27, 2015 at 8:34
  • \$\begingroup\$ You could regard this question as the answer to your other question : you can insert other devices in the waveguide : perhaps an attenuator to vary the power level delivered. Or a diplexer so you can divert power between your target (dinner?) and a dummy load. \$\endgroup\$
    – user16324
    Commented Apr 27, 2015 at 9:16
  • \$\begingroup\$ I understand many microwave systems needs to be tune, and for that they need waveguide, but why microwave which is relatively fixed non changing instrument. my guess is to reduce reflected power, but how ? \$\endgroup\$
    – iamgopal
    Commented Apr 27, 2015 at 9:17
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    \$\begingroup\$ @PlasmaHH I think the unfortunate use of the word "cavity" sent me down the lines of the cavity magnetron whereas the OP may well be referring to the cavity of a microwave oven - see Dave Tweed's answer. \$\endgroup\$
    – Andy aka
    Commented Apr 27, 2015 at 12:56

1 Answer 1

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There are many reasons that the magnetron antenna does not stick directly into the oven cavity of a microwave oven.

  • If it did, it would physically interfere with objects (food and dishes) inside the cavity, and it would be subject to contamination by food spatter.

  • The magnetron and its antenna get quite warm during operation. Having the antenna exposed where it could be touched would expose the manufacturer to liability issues because of burns. The waveguide also functions as an air duct to help cool the mangetron.

  • The impedance of the cavity is not a good match to the impedance of the antenna. The waveguide functions as an impedance transformer so that energy is coupled more effectively from the antenna to the cavity.

  • The impedance of the oven cavity varies with what is placed into it. The waveguide's impedance transformation characteristics help to reduce the variation that the magentron "sees".

  • The use of a waveguide gives the mechanical designer a lot more flexibility with regard to the placement of the magentron, making it possible to make the overall size of the appliance more compact.

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  • \$\begingroup\$ one more question, does the waveguide used in oven, behave like an isolator or circulator ? or how it prevent /reduce reflected power. \$\endgroup\$
    – iamgopal
    Commented May 2, 2015 at 7:07
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    \$\begingroup\$ It doesn't. It's still possible to damage the magnetron if there's no load placed in the oven cavity. \$\endgroup\$
    – Dave Tweed
    Commented May 2, 2015 at 10:35
  • \$\begingroup\$ @DaveTweed isnt the impedance matching thing usefull to prevent reflections ? \$\endgroup\$ Commented Sep 30, 2019 at 7:24
  • \$\begingroup\$ @ManudeHanoi: Yes, that was my third bullet point. \$\endgroup\$
    – Dave Tweed
    Commented Sep 30, 2019 at 15:31
  • \$\begingroup\$ @DaveTweed therefore the wave guide prevents reflecting power (see the initial question above) \$\endgroup\$ Commented Oct 1, 2019 at 15:49

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