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.
1
3
10
+
-
*
÷
dBm
0
2
10
mW
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
2
3
10
+
-
*
÷
dBm
Unknown
0
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.
2
10
mW
30
1
You need to determine if by
using * and ÷, and 2 and 10,
you can change from 1 to 30
mW.
Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain
3
3
10
+
-
*
÷
dBm
Unknown
-3
2
10
mW
30
Unknown-3
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.
Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain
4
+
-
3
10
-3
*
÷
dBm
Unknown
mW
30
Unknown-3
=15
2
10
÷2
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
5
3
10
-3
+
-
*
÷
dBm
Unknown
mW
30
Unknown-3
15
2
10
÷2
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
6
3
10
+
-
*
÷
dBm
Unknown
mW
30
-3
Unknown-3
15
÷2
+10
Unknown+7
150
*10
+10
Unknown+17
1500
*10
Now you can add the 20 dBi gain of
the antenna. You do this by adding
10 twice.
2
10
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
7
3
10
+
-
*
÷
dBm
Unknown
mW
30
-3
Unknown-3
15
÷2
+10
Unknown+7
150
*10
+10
Unknown+17
1500
*10
2
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
8
3
10
+
-
*
÷
dBm
14.7712
2
10
mW
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
9
3
10
+
-
*
÷
dBm
14.7712
mW
30
-3
11.7712
15.0356
+10
21.7712
150.3561
+10
31.7712
1503.5616
2
10
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.
Rule of 10s and 3s Example 4: 30 mW access point, 3 dB cable loss, 20 dBi antenna gain
10