Same EIRP with different antenna types

[Emmanouil Patiniotakis]

Hello,

I was wondering what is the difference to use a directional antenna instead of an omni if the total EIRP is the same?I mean that if you calculate the free space loss tha same RSSI will be received by the client if the EIRP is the same in both cases. So what is the gain if using directional antenna instead of an omni? A friend said me that the RSSI will be different but what is the scientifical explanation to this...

[Scott Fella]

In short, a higher gain antenna gives you a more directed beam pattern and a narrow beam width. That being said, a directional antenna does the same. It focuses the beam so the beam width would be narrow when compared to an omni directional antenna. When calculating EIRP, you take the TX and the gain into consideration. So having the same EIRP on an omni and a directional doesn't mean that at a given point that the rssi is the same from the RX device. RSSI varies depending in your antenna type and the overall EIRP. Most use or high gain directional in 802.11 is for point to point or point to multipoint deployments. There are many references regarding antenna gain and beam width you can reference.

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[Emmanouil Patiniotakis]

Thanks for the answer. I know what you mean but how someone figure out the exact RSSI if the EIRP calcilation give the same result in both cases?

[Scott Fella]

You can't really calculate that. RSSI is the received signal strength which is determined by the receiving device. RSSI is calculated by the RSSI Max and the SNR.

http://www.ehow.com/how_7823781_calculate-average-rssi-80211.html

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[Emmanouil Patiniotakis]

So you cannot calculate how far your signal will travel, am I right? You can have a minimum estimation based on EIRP but the actual distance cannot be known.

[Scott Fella]

In a point to point you can ESTIMATE but as far as wireless access you have to look at how far can the client device travel. Over powering your clients EIRP can produce one way communication. So you can blast the signal from the antenna as far as the country regulation allows you, but that doesn't mean you client signal can reach the AP. that is what you need to understand. You increase coverage by adding access points not increasing EIRP.

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[Emmanouil Patiniotakis]

I understand what you mean..That is not my point. How can you estimate?If you use your EIRP in your calculation (e.g. free space loss) you get an underestimated distance your signal can travel given a specific received level? I mean that you can travel 50m and for signal will have for example -75 dBm but your actually signal if you use a directional antenna will be -65 dBm (all calculations are arbitary). Am I clear about what I want to mention?

Thanks for this helpful conversation..