Questions about finding dBm values

[Mohamed Abdul]

Homework Statement

1. A sinusoidal source with RMS voltage V = 11Volts across a resistor R = 50Ohms, determine the dBm value. Hint: you will first have to determine the power delivered and then check the formula for dBm calculations.

2. A type of coaxial cable has a loss of 12dB/100m at some frequency. Determine the ratio of the input voltage to the output voltage for 100m of this cable.

Homework Equations

dBm = 10logbase10(P/1mW)
dB = 20logbase10(V/Vo)

The Attempt at a Solution

For the first one, I found power to be 2.42. I then put 10logbase10(2.42/1000) into my calculator and got -26.16.

For the second 1 I set 12 = 20logbase(V/Vo), divided both sides by 12, and got .6 = logbase10(V/Vo). I then took 10 to the .6 power and got 3.98.

I'm just wondering if my math is right; getting a negative answer for the first one doesn't seem right to me, and I'm unsure if I did the log properly for the second one.

 

[TSny]

For the first question, you need to check your conversion of mW to W.

Your solution of the second question looks correct to me.

 

[Mohamed Abdul]

For the first question, you need to check your conversion of mW to W.

Your solution of the second question looks correct to me.

Wait do get things into standard units wouldn't I just divide by 1000 since there are 1000 mW in one Watt?

 

[TSny]

Wait do get things into standard units wouldn't I just divide by 1000 since there are 1000 mW in one Watt?

In the formula where you have P/(1 mW), you need to make sure the numerator and denominator are in the same units. So, either you can convert P to mW or you can convert 1 mW to Watts.

If you leave P in Watts, then you need to replace the 1 mW by its equivalence in Watts. 1 mW = ? W.

 

[Mohamed Abdul]

In the formula where you have P/(1 mW), you need to make sure the numerator and denominator are in the same units. So, either you can convert P to mW or you can convert 1 mW to Watts.

If you leave P in Watts, then you need to replace the 1 mW by its equivalence in Watts. 1 mW = ? W.

So I'd be dividing by .001 then, thank you for that.

 

[TSny]

So I'd be dividing by .001 then, thank you for that.

If you mean that you would end up dividing P by .001, yes.