Antenna cable loss and increasing of the power level

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Mar 31st, 2009
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Hi board,

Does someone of you have a calculation for an antenna cable loss? For example:

I have an antenna-cable that absorbs 3dB - how much AP output power is needed to balance that absorbation?


In the document http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008008883b.html there is a table (search for: "Table 1. Decibel Values and Corresponding Factors"). Would this table be correct as a reference?


For example: If the cable loss is 3dB, the factor is 0,5 (see table in the linked document) - so if I configured 50mW at the AP, only 25mW will be availabe at the antenna because of the cable loss, right?

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jeff.kish Tue, 03/31/2009 - 07:52
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You're correct about your calculation, and that data sheet is a very good document to read through to gain a solid understanding of wireless communication.


One thing to keep in mind - A 3dB loss does indeed knock your power in half (50mW to 25mW, for example). However, your coverage area isn't decreased in half. The reason we use dB in the world of wireless is because this accurately reflects how wireless signal propagates. If you have a coverage of 200 feet at 20dB, and you lose 3dB for any reason, your coverage will drop to 170 feet, not 100 feet.


This should explain why an AP's power settings generally go from 100mW to 50mW to 25mW, etc... One might wonder why on earth you'd ever configure an AP to operate at 4mW, a tiny fraction of the full power. In reality, this is closer to 30% power.


So again, you're correct to say that a 3dB loss can be countered by a 3dB increase in power at the source. Always try to think in terms of dB, not mW. If you think in terms of mW, you'll make errors when estimating coverage.

Scott Fella Thu, 04/02/2009 - 16:32
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Here is a website that shows you loss per foot depending on what cable you use.


http://www.timesmicrowave.com/cgi-bin/calculate.pl


Also what you need to look at is the gain of antenna you are using. You need to find you total TX out from the antenna.


http://www.csgnetwork.com/antennaecalc.html

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