1

For ‘Rule of 10s and 3s Example 4’ we have an access point

transmitting at 30 mW. The cable and connector between the access point and the antenna create 3 dB

of signal loss, and the antenna provides 20 dBi of gain.

2

dBm mW

+-

*÷

310102

0 1

Start off with the template.

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

3

dBm mW

+-

*÷

310102

Unknown 30

You need to determine if by using * and ÷, and 2 and 10, you can change from 1 to 30

mW.

No matter how hard you try, you will not be able to multiply 1 by 2 and 10 to come up with

30. This does not mean that you can’t calculate the IR or EIRP. It only means that using the rule of 10s and 3s, you cannot set

the dBm to equal 30 mW. In these cases, you can calculate the mW portion of the chart

only. So enter the starting mW value, and set the dBm to unknown.

0 1

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

4

dBm mW

+-

*÷

310102

Unknown

Unknown-3

30

Even though you don’t know the dBm value, you can still calculate the dB change, and

apply the correlative mathematical commands on the mW column. Since the cable and

connector creates a 3 dB loss, enter it in the chart as usual and perform the calculations.

-3

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

5

dBm mW

+-

*÷

310102

Unknown

Unknown-3

30

=15 ÷2-3

Even though you can’t calculate the dBm value, you can still calculate the effect of the dB change and

perform the correlative mathematical commands on the mW column. Since the cable and connector

creates a 3 dB loss, the mW column must be divided by 2.

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

6

dBm mW

+-

*÷

310102

Unknown

Unknown-3

30

15 ÷2-3

You now know that the IR is 15 mW. Since you did not know the original dBm value, you cannot give

the IR value in dBms, only mWs.

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

7

dBm mW

+-

*÷

310102

Unknown

Unknown-3

Unknown+7

Unknown+17

30

15

150

1500

÷2

*10

*10

-3

+10

+10

Now you can add the 20 dBi gain of the antenna. You do this by

adding 10 twice.

And then perform the correlative mathematics on the mW column.

Adding 10 twice in the dBm column means that you must multiply the mW

column by 10 twice.

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

8

dBm mW

+-

*÷

310102

Unknown

Unknown-3

Unknown+7

Unknown+17

30

15

150

1500

÷2

*10

*10

-3

+10

+10

So the EIRP is 1500 mW or 1.5 watts. So even though you could not calculate the actual value for dBms using the rule of 10s and 3s, you

were still able to calculate the mW value.

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

9

dBm mW

+-

*÷

310102

14.7712 30

Don’t forget, instead of using the rule of 10s and 3s, you

can always use the logarithmic formula to

calculate the dBm value.

dBm = 10 * log10(PmW)

14.7712 = 10 * log10(30)

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain

10

dBm mW

+-

*÷

310102

14.7712

11.7712

21.7712

31.7712

30

15.0356

150.3561

1503.5616

If you had used the logarithmic formula dBm = 10 * log10(PmW) to calculate the dBm value for 30 mW, the value would be 14.7712. If you then added and subtracted the dB gains and losses and calculated the corresponding

mW values, this is what you would have come up with.

-3

+10

+10

Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain