# 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

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. :)