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Help with log formula

  1. Sep 18, 2007 #1
    Hello

    Im studying an ATS equipment known as an ILS. One of the formulas to determine the DDM is :

    E2 = E1 log-1(db/20)




    When I try entering log-1, the calculator displays Math Error. What am I doing wrong?

    Thanks for the help in advance
     
  2. jcsd
  3. Sep 18, 2007 #2
    The logarithm is not defined for negative numbers.
     
  4. Sep 18, 2007 #3

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Are you sure that "log-1" means log(-1)?? Look more closely at the formula.
     
  5. Sep 18, 2007 #4
    Its log to the negative power of 1, I dont know how to format that below.


    the funny thing is that there are examples in the book with the answer. For example:


    Ec = 2.5log-1 (-3/20) = 1.77

    Ecs = 1.0log-1(-3/20) = .707

    Ess = 0.155log-1(-3/20) = 0.109
     
  6. Sep 18, 2007 #5
    [tex]\log^{-1}x = \frac{1}{\log{x}}
    [/tex]

    Click on the LaTeX to see the code.
     
    Last edited: Sep 18, 2007
  7. Sep 18, 2007 #6
    thanks neutrino, but what about that (-3/20) that will give me a negative. Logs dont work that way right?
     
  8. Sep 18, 2007 #7
    They don't. Are you sure db is negative?
     
  9. Sep 18, 2007 #8
    its interesting you say that because the its starts out with a 3db then they change it to a -3. I'll type the whole thing below...


    E2 = E1 log-1(db/20)

    Where:
    E2 = before attenuation
    E1 = after attenuation

    The upper attenna signals exiting Z6 at output 2 are attenuated by 3db. Letting "A" equal .31, the Ess signal in the upper antenna would be .5A or .155 prior to Z6. So the equation becomes:

    E2 = .155log-1(-3/20) = .109

    What the heck does that mean ^^
     
  10. Sep 18, 2007 #9
    Sorry, I'm clueless. Maybe someone familiar with this topic (at the Electrical Engineering forum) will be able to answer your question.
     
  11. Sep 18, 2007 #10
    Thanks neutrino. I'll give it try. :)
     
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