RF Math

8 posts by 3 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: June 25, 2004:
  • Hi all,

    Have my CWNA exam tomorrow, and am starting to panic about RF math, as I'm still weak on it, particularly the logarithmic side of things.

    Can anyone recommend any sites which might help? I've got the study guide and some practice tests.



  • By (Deleted User)

    Memorize the following:

    0 dBm = 1 mW
    10 dBm = 10 mw
    20 dBm = 100 mW
    30 dBm = 1000 mW = 1 W

    +3 dB = 2x mW
    +10 dB = 10x mW

    -3 dB = 1/2x mW ( one half )
    -10 dB = 1/10x mW ( one tenth )

    This will give you most of what you need to know in life.

    As for the test.... well, I can't say... ;)

  • I would memorize the dBm-to-mW conversion for gain/loss in both 3dB and 10dB increments:

    0.0001mW = -40dBm
    0.001mW = -30dBm
    0.01mW = -20dBm
    0.1mW = -10dBm
    1mW = 0dBm
    10mW = +10dBm
    100mW = +20dBm
    1000mW = +30dBm
    10000mW = +40dBm

    0.0625mW = -12dBm
    0.125mW = -9dBm
    0.25mW = -6dBm
    0.5mW = -3dBm
    1mW = 0dBm
    2mW = +3dBm
    4mW = +6dBm
    8mW = +9dBm
    16mW = +12dBm

    Also make sure you know how to use the 10’s and 3’s of RF math:

    • -3dB is half the power in mW. Adding -3dB halves the wattage value (10mW – 3dB = 5mW).
    • +3dB is double the power in mW. Adding +3dB doubles the wattage value (10mW + 3dB = 20mW).
    • -10db is one-tenth the power in mW. Adding -10dB divides the wattage value by 10 (200mW – 10dB = 20mW).
    • +10dB is ten times the power in mW. Adding +10dB multiplies the wattage value by 10 (10mW + 10dB = 100mW).

    Also know all of the max power levels allowed by both the FCC and IEEE at the antenna (EIRP) and at the intentional radiator (mW).

    And you are correct, there are no good web pages for simple RF math that I've found. I've been haunting many of the amateur (HAM) radio pages, and most of the tutorials seem to jump from "What is a decibel?" to trig/calculus math for wave propogation. Yeesh!

  • By (Deleted User)

    John, perhaps you have found your calling. Start a web page of your own for this topic! :)

    ( I like your tables, BTW. Nice work! )

  • Thanks. I may put together some sort of tutorial/workbook type document and submit it here.

    And I'm James ;)

  • Thanks guys for all your help.

    Although this came in extremely useful, I still flunked with 64%.

    I found that understanding what was actually being asked a problem for some questions, and that I couldn't be 100% sure of correctness.

    However, I still need to obviously address some areas of my learning. Will try again.

  • Sorry to hear about that. With that score it does seem like you needed more study time. At least your chance of success is much higher the 2nd time around.

    Do they give you a printout of the areas that you missed questions? They do that for the CompTIA exams.

  • Unfortunately there is no print out supplied. This would be really useful in terms of knowing which areas I still need to focus on.

    Other exams I've taken e.g. CCNA, CCDA work in much the same way I believe (managed to pass these OK!).

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