Help with log formula

1. Sep 18, 2007

mcalves

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. Sep 18, 2007

neutrino

The logarithm is not defined for negative numbers.

3. Sep 18, 2007

HallsofIvy

Staff Emeritus
Are you sure that "log-1" means log(-1)?? Look more closely at the formula.

4. Sep 18, 2007

mcalves

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

5. Sep 18, 2007

neutrino

$$\log^{-1}x = \frac{1}{\log{x}}$$

Click on the LaTeX to see the code.

Last edited: Sep 18, 2007
6. Sep 18, 2007

mcalves

thanks neutrino, but what about that (-3/20) that will give me a negative. Logs dont work that way right?

7. Sep 18, 2007

neutrino

They don't. Are you sure db is negative?

8. Sep 18, 2007

mcalves

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 ^^

9. Sep 18, 2007

neutrino

Sorry, I'm clueless. Maybe someone familiar with this topic (at the Electrical Engineering forum) will be able to answer your question.

10. Sep 18, 2007

mcalves

Thanks neutrino. I'll give it try. :)