An installation uses a 21dBi parabolic dish. It is connected to the 802.11n radio via low loss antenna coax rated at 4dB loss/100 feet. After testing, it has been determined that 2mW of power will be lost in the cable. An EIRP producing 200mW is desired. What should the transmit power be set to?
A. 8mW
B. 3dBm
C. 3.16mW
D. 5mW
@V
You use dbi to calculate the EIRP. Have a look here:
https://www.everythingrf.com/rf-calculators/eirp-effective-isotropic-radiated-power
If you enter following values:
Output power: 0.00316 (for3.16mW)
Cable loss: 3
Antenna gain: 21
you get 22.99687083 dBm as result, which is the calculated 23dB we use in our example.
how about the fact that antenna gain is given in dBi? so it is 21+2.14 = 23.14
x – 3 + 23.14 =23
x = 2.86
the closest is 3, so the answer is B.
Basically you have to know this table:
0dB = 1mW
3dB = 2mW
6dB = 4mW
9dB = 8mW
10dB = 10mW
20dB = 100mW
23dB = 200mW
Calculation:
We have to translate the 2mw of cable loss into dB. This is 3dB. Then we have to translate the desired output to dB. This is 23dB. The EIRP is calculated like follows:
Transmit Power – cable loss + Antenna gain = EIRP
So in our example:
x -3 + 21 = 23
If we resolve for x we get 5.
Now convert that 5dB back to mW.
0dB = 1mW
3db = 2mW
6db = 4mW
So, 5 dB is somewhere in between 2 and 4 mW. Closest answer is 3.16mW
Very helpful JohnDoe, much appreciated.
Or just go reverse from 200mW with a rule of 3dBm:
200mW=23dBm
100mW=20dBm
50mW=17dBm
25mW=14dBm
12.5mW=11dBm
6.25mW=8dBm
3.125=5dBm