You are about to learn RF maths, without having to use logarithms. The rule of 10s and 3s provides approximate values, not necessarily exact values. If you are an engineer creating a product that must confirm to RF regulatory guidelines, you will need to use logarithms to calculate the exact values. I would like to introduce you to an easy way to calculate or you can use this method to create quick cheat sheet for calculation which can be used during CWNA exam.
Rule of thumb:
- 0 dBm = 1 mW
- +3 dB increase = (mW x 2) Double the Power
- -3 dB decrease = (mW ÷ 2) Half the power
- +10 dBm = mW x 10
- -10 dBm = mW ÷ 10
Step 1:
Make the table and fill it with known values. as we know from the rule of thumb that 0 dBm = 1 mW. Now if we add 10 dBm to 0 dBm its mean we will multiply 10 mW to 1 mW, So we get 10 dBm = 10 mW. Now use rule of 10 again and add 10 dBm to 10 dBm and multiply 10 mW with 10 mW, It will give us 20 dBm value and 100 mW. Now we have 3 entries as shown below and we can take it from here.
| dBm | mW | dBm | mW | dBm | mW |
| 0 | 1 | 11 | 22 | ||
| 1 | 12 | 23 | |||
| 2 | 13 | 24 | |||
| 3 | 14 | 25 | |||
| 4 | 15 | 26 | |||
| 5 | 16 | 27 | |||
| 6 | 17 | 28 | |||
| 7 | 18 | 29 | |||
| 8 | 19 | 30 | |||
| 9 | 20 | 100 | |||
| 10 | 10 | 21 |
Step 2:
Now we perform normal addition and we will get our values. Starting from first entry go down 3 boxes and double the mW entry. For example 3 dBm box will get value from doubling the mW 1 +1 = 2mW. We will keep going 3 boxes below and double the mW entry.
| dBm | mW | dBm | mW | dBm | mW |
| 0 | 1 | 11 | 22 | ||
| 1 | 12 | 16 | 23 | ||
| 2 | 13 | 24 | 256 | ||
| 3 | 2 | 14 | 25 | ||
| 4 | 15 | 32 | 26 | ||
| 5 | 16 | 27 | 512 | ||
| 6 | 4 | 17 | 28 | ||
| 7 | 18 | 64 | 29 | ||
| 8 | 19 | 30 (10+10+10) | 1000(10x10x10) | ||
| 9 | 8 | 20 | 100 | ||
| 10 | 10 | 21 | 128 |
Step 3:
On same principle now start from our second entry point in step 1. From 10 dBm box go down 3 boxes and half the mW value. For example 10mW will be 5mW and keep going down until you hit the last box. Once one side is completed now go upwards from 10 dBm boxes and double the mW value as show below.
| dBm | mW | dBm | mW | dBm | mW |
| 0 | 1 | 11 | 22 | 160 | |
| 1 | 1.25 | 12 | 16 | 23 | 200 |
| 2 | 13 | 20 | 24 | 256 | |
| 3 | 2 | 14 | 25 | 320 | |
| 4 | 2.5 | 15 | 32 | 26 | 400 |
| 5 | 16 | 40 | 27 | 512 | |
| 6 | 4 | 17 | 28 | 640 | |
| 7 | 5 | 18 | 64 | 29 | 800 |
| 8 | 19 | 80 | 30 (10+10+10) | 1000(10x10x10) | |
| 9 | 8 | 20 | 100 | ||
| 10 | 10 | 21 | 128 |
Step 4:
In this step we should be able to complete our conversion box cheat sheet. You can create this sheet before your CWNA exam and use it for your exam questions.
Our 3rd entry point in step 1 was 20 dBm = 100 mW. So same as step 3 we go 3 boxes below and half the mW value and we go up and double the mW value.
| dBm | mW | dBm | mW | dBm | mW |
| 0 | 1 | 11 | 12.5 | 22 | 160 |
| 1 | 1.25 | 12 | 16 | 23 | 200 |
| 2 | 1.56 | 13 | 20 | 24 | 256 |
| 3 | 2 | 14 | 25 | 25 | 320 |
| 4 | 2.5 | 15 | 32 | 26 | 400 |
| 5 | 3.12 | 16 | 40 | 27 | 512 |
| 6 | 4 | 17 | 50 | 28 | 640 |
| 7 | 5 | 18 | 64 | 29 | 800 |
| 8 | 6.25 | 19 | 80 | 30
(10+10+10) |
1000
(10x10x10x10) |
| 9 | 8 | 20 | 100 | ||
| 10 | 10 | 21 | 128 |