I’m having difficulty coming up with the EIRP of one of our APs. We’ve got the card’s output power, then 145 ft of LMR400, then an amplifier to make up for signal loss, and then a 3-way splitter, each going to a 120 degree sector.
I definitely want to learn how to figure this right to comply with regulatory yada yada…
Could someone step me through this?
The Eirp or total system output is quite simple to measure.
It is essentially a mesurment of the TOTAL amount of power that leaves the radiating element minus the loses… (I.e coaxial length, connectors … etc)
Example scenario.
Fcc permits 4 Watts for ptmp (34dbm)
1 Watt amplifier = 30 (dbm)
6 dbi antenna= 6
15ft of lmr 400 = -1
… You simply add all of the above, including the negative (same as subtracting in the latter) and you will get 35. Because you are 1 dbm over the legal limit, you have a few choices … decrease amplification power, decrease size of antenna or increase length of antenna… Well you get the drift.
The rules are a bit more lengthy and technical but the above should be helpful.
Regards.
Ps. do not get confused at you will see 2 Watts differs from 4 watts by only 3 db. In a perfect world, (2 watts = 33dbm which means 4 would equal 66dbm) but that isnt the case, rf power is not linear but rather logarithmic as a 6db increase would yeild 10x greater output power…
Further info … http://www.fab-corp.com/pages.php?pageid=1
Be careful with giving incorrect advice! Your numbers don’t add up warwick9
35 dBm is not 1dBm over the 4W limit, but 1 under, and what about the power output of the radio before the amplifier?
Ps. do not get confused at you will see 2 Watts differs from 4 watts by only 3 db. In a perfect world, (2 watts = 33dbm which means 4 would equal 66dbm) but that isnt the case, rf power is not linear but rather logarithmic as a 6db increase would yeild 10x greater output power…
actually, 10dB is 10x the power, and 6dB is 4x the power
I thought that if I use a .5 Watt amplifier, that I will have .5 Watts of output regardless of the input power. Is this not correct?
Also, I would think that running 3 sectors off of one card using a 3-way splitter would enable me to divide the total output power (as if I were using an omni) by 3. Am I right in this assumption?
Not to mention, the -1dBm for just 15’ of LMR-400 seems like an awful amount of loss. Maybe it includes the connector or pigtail loss as well?
Thanks for the replies btw, I do understand more now. (I think 
1dB loss is about right for 5m (15ft) of LMR400 cable. Note that losses are also frequency dependent. This is the loss at 2.4GHz, at 5.8GHz loss is approx. 60% more. The loss stated doesn’t include connector losses.
warick09 also incorrectly states 4W = 34dBm, actually 4W = 36dBm.
I believe you are right about the power being split equally by the splitter, but you also have insertion losses for the splitter itself, which for examples at my supplier can be in the region of 5 to 7dBm.
With your amplifier I would be inclined to believe you are right. If the power level is stated in Watts this would normally be the maximum output power. One amplifier at my supplier states an input power range of 0 to 20dBm (1mW to 100mW), gain is automatically adjusted to give an output of 1W (30dBm). Really you need to refer to the manufacturer of your equipment or your supplier for more information.
Basically you first need to understand how to convert between power levels in Watts and dBm. It’s really easy once you know how:-
for every +/- 3dBm, power in Watts doubles or halves.
for every +/- 10dBm, power in Watts increases/decreases 10 fold.
0dBm = 1mW
3dBm = 2mW
6dBm = 4mW
10dBm = 10mW
20dBm = 100mW
30dBm = 1W
Then you have to refer to the specification of the equipment you have, to determine the various losses.
Then it’s simple arithmetic.
Argh, do forgive my error stricken notings … < Waz real early in the morning> 
..at the guys cleaned up things.
Regards.