# EIRP in a nutshell

Todays topic comes from a sample question that I came across in one of the “Study” questions on the Cisco Learning Network. The question asked how to How do you calculate EIRP? I realized that the last post pretty much covered it, but I failed to explain, or even define, EIRP along with dBm and dBi. So here goes. EIRP stands for Effective Isotropic Radiated Power. This is something that you would use to figure out if a radio solution is within the values allowed by local regulatory bodies such as the FCC or ETSI. For Example; The FCC defines power limitations for wireless LANs in FCC Part 15.247. Any WLAN Manufacturer of 802.11 products must comply with Part 15. So, without going into a rant here is how the formula looks: EIRP = transmit strength + antenna gain – cable loss Thats all well and good, but there is a little more to it. Lets get a little more detail by discussing dBm and dBi. Here are the fun facts: dBm is an abbreviation for "decibels relative to one milliwatt," where one milliwatt (1 mW) equals 1/1000 of a watt. It follows the same scale as dB. Therefore 0 dBm = 1 mW, 30 dBm = 1 W, and -20 dBm = 0.01 mW. dBi on the other hand refers to an isotropic antenna. This type of antenna is theoretical and does not exist in reality. Still, it’s used as a reference for us to compare antennae. Again, the same logarithm progression applies to dBi as seen in the other decibel scales, dB and dBm. Now looking at the formula with a the additional information factored in you would see the following: EIRP = transmit strength (dBm) + antenna gain (dBi) – cable loss (dB) Now you can check out the last post and put the pieces together. For those of you in the Phoenix, AZ. area, I will be doing a CCNA Wireless event in the Ascolta office on Wednesday night, Feb 21st at 6:30pm. You can find more information on the Phoenix location by visiting the Ascolta web site.

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