5.2dBi omni antenna has a stronger signal coverage than 12dBi omni antenna

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Jun 17th, 2009
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Hi,


We currently have a problem with our 12dBi, 2.4GHz Cisco antenna. We don't know if the antenna is defective or not. We have more than 10 1242AG LAPs in an outdoor site and eath of them has 5.2dBi antennas. We are using a 4402-50 WLC. We replaced one LAP with a 12dBi antenna to test how strong the signal coverage would be. The result was it has a lower signal coverage than the 5.2dBi antenna. We don't know why it was like that. We are suspecting that the antenna maybe faulty.


Hope someone here can help.


Thanks.

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Leo Laohoo Wed, 06/17/2009 - 20:27
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It's really hard to test these parts if you don't have the right testing equipments. If you have doubts about them, RMA.

George Stefanick Thu, 06/18/2009 - 03:42
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Did you check the TX power of the AP on the 5.2 vs 12 dBi antenna? Is it possible that the 5.2 antennas are at a higher TX level then they 12?

ericgarnel Thu, 06/18/2009 - 06:11
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How are you testing the coverage? Keep in mind that as you increase the gain, the signal becomes more focused.

mat.edwards Sun, 06/28/2009 - 01:45
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You need to think about how antennas are used to increase dB.


Antennas passively increase signal by focusing and squashing the RF energy emitted. Think of a low gain omni antenna (5.2dBi) as a beach ball of coverage. To passively increase the coverage emitted you would need to squash it (Sit on it) this would push the signal out to the side but there would be less above and below.


As you are using the setup in an outdoor environment I imagine you have the antennas installed quite high. What you will find is the antennas will provide very low signal directly below them, but as you move further away it will begin to get higher, say -75dBm to -65dBm, but will never reach much higher. If you do have the antennas quite high I would either try replacing them with 2.2dBi antennas as a trial or if you have them on posts, re-install the AP's to walls and use diversity patch antennas.


Also make sure the antennas are polarized vertically only.


Thanks

Mat


scottmac Sun, 06/28/2009 - 05:46
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To add to Mat and Eric's point about passive gain being the result of intentionally distorting the radiation pattern:

What you'll find in many cases is that the radiation pattern, in addition to getting flatter (to spread out more) is that it also tend to radiate more upwards, like a cone with the pointy end pointing down (or like an upside-down umbrella).


That's why the Smith charts are important (the circular graphs that show what the pattern looks like from above and the right side if the pattern.


Gain isn't free, you either have to add power actively (an amp) or passively (distort the pattern to concentrate it).


If this is an outside setup, you might actually do better with a tri or quad of sector antennas and a power divider. That arrangement give you the added reach, omni-directionality, and a workable pattern.


Keep in mind though, that anything other than a "stock" setup pre-certified from the vendor usually requires a certification process to meet local laws and regulations in most countries.


Good Luck

abpsoft Sun, 07/05/2009 - 05:35
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And then there's regulation.


In addition to all the good points about directional antennae being a zero sum game of increasing the signal somewhere at the price of decreasing it somewhere else, there is regulation of EIRP. This is less of a problem in FCC land, but in the ETSI domain for instance, there is no rule to deal antenna dBi for transmitter dBm. Hereabouts it's "20dBm EIRP - and we mean it" (simplifying things to the 2.4GHz ISM band only and ignoring spectral density). So you are not allowed to gain any EIRP (independently from the shape of your radiation pattern, even with a highly directional yagi or dish) beyond 20dBm, which means TX power has to be lowered accordingly. So if you don't lie about the gain of your external antenna, in the "wrong" regulatory domain, the WLC will have to limit TX power,

down to 8dBm with a 12dBi antenna, per the law.


The only gain we have from directional antennae is on the receive side, but that requires symmetrical setups, making "frisbee flat omnis" mostly useless for coverage - the STAs cannot reach them on the way back.


HTH,

Andre.

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