Forum

i do not understand very well this quiestion:

Q:Given: You are Transmiting data using an 802.11g AP connected to an 18 dBi Omnidirectional antenna through a cable producing 3dB loss.

If you wanted to transmit at the maximun allowed EIRP, what would be the dBm rating at the IR?

The correct answer is 18!

Explanation:
If the Maximun is 36 dBm and the system uses an 18 dBi antenna then
36-18=18 dBm of maximum EIRP.
-------------------------------------------------------------

i do not undestand very well that, for me is:
a 802.11g AP the defalut Tx is 20dBm+18 dBm is 38dB minus the 3dBd of cable loss the relust is 35dB!!!!

Can someone explain this for me??????

Thanks.

Hi ZURC,

I will give this may best shot; maybe someone can jump in with a better explanation.


For a PtMP link, the FCC limits the intentional radiator power to 1 watt (+30dBm), and the EIRP to 4 watts (+36dBm) in the 2.4 GHz ISM band.

(802.11g OFDM operates in the 2.4 GHz ISM band)

From: CWNP Wireless LAN Administration Courseware v3.0

FCC 2.4 GHz PtMP Rules:

1) Point to Multipoint (PtMP) links consist of a central connection point and two or more remote links.

2) If an omni-directional antenna is used then the link is considered point to multipoint regardless of the number of links.

3) Maximum 2.4 GHz PtMP power from the intentional radiator (IR) is 1000mw if using an antenna capable of 6dBi gain.

4) For each 3dBi the antenna gain is increased, IR power must be reduced by 3dB.

5) PtMP EIRP in the 2.4 GHz band my not exceed 36dBm.
(1000mw = +30dBm and 4000mw = +36dBm)

The 1:1 rule for this states that for each additional 3dBi of antenna gain, the power out of the IR (this is the power in to the antenna) must be decreased by 3dB to keep the EIRP at 4 watts.

The intentional radiator is the transmitter and cable and connectors up to the antenna connection.

The power out of the intentional radiator is the power out of the transmitter minus the power lost in the coax and connectors and arriving at the input to the antenna.

So if the antenna gain is 18dBi (12dBi above 6dBi) and the power out of the IR is 1000 mw, the power out of the IR must be deceased by 12dB to keep the EIRP at 4 watts. (Keeping in mind that every 3dB increase will double the power and every 3dB decrease will half the power)

+12dB = 3dB + 3dB + 3dB + 3dB

(-3dB = half power)

1000 mw -3dB = 500 mw
500 mw - 3dB = 250 mw
250 mw - 3dB = 125 mw
125 mw - 3dB = 62.5 mw
(62.5mw out of the IR in to the antenna, but if we have -3dB of cable and connectors loss the power out of the transmitter must be 125mw.)

You want to TX at MAX PWR in the 2.4GHz ISM band using an 18dBi gain antenna.
Knowing that 4 watts (+36dB) is MAX PWR allowed.
Knowing you have 3dB power loss in cable and connectors.

1) 125mw power out of transmitter
2) 62.5mw loss due to cable and connector is 3dB (-3dB = half power)
3) 62.5mw power in to antenna
4) 4 watts EIRP after 18dBi gain from antenna

+18dBi = 3dBi + 3dBi + 3dBi + 3dBi + 3dBi + 3dBi (antenna gain)

(+3dB = double power)

62.5mw * 2 = 125mw * 2 = 250mw * 2 =500mw * 2 = 1000mw * 2 = 2000mw * 2 = 4000mw (+36dBm)



I hope I got this right. Anyone, Please jump in and correct me if I have made any mistakes or it is not clear.

**** In a previous post I said EIRP was Effective Isotropic Radiated Power and I should have said Equivalent Isotropic Radiated Power. Sorry about the mistake.****

73 mike

If you are to transmit at the Maximum EIRP of 36dBm using an antenna of 18dBi then the IR = EIRP - Antenna Gain (36-18) = 18dBm.

IR+Antenna Gain = EIRP

The tricky part in the question is it is asking you for the IR (which includes AP and cable/connector loss) and not the Maximum Transmission Power of the AP.

The real scenario for this question is like this.

AP (21dBm) + 3dB cable loss + Antenna Gain (18dBi) = 36dBm Max EIRP

so IR = AP (21) + cable loss (-3) = 18dBm

The 3dB cable loss mentioned in the question is to tickle your mind giving more information than required. You should be looking for such things in th exams as they test your ability to figure out those things.