1242 AP Antenna Question

[dhopper82]

We are looking into physically securing our LWAP's better than we have them now.  A plan that has been proposed is to place all the AP's in our comm closets and run coax cables to the locations we would like wireless signal to be.  We have the standard AIR-ANT4941 antennas for our ap's.  How long of a coax cable can be run from the AP to the antenna before there is signifigant loss of signal?

[Kayle Miller · Accepted solution]

dhopper,

     So the answer to your question is not a short or easy one to answer, but here is the information you need to formulate some basis to develop an answer.  Are you planning on just 802.11b/g or 802.11a also? What size cable are you planning to use?

Typical Cable options

LMR-200 has a loss of .165db/ft or 1.6db per 10ft @ 2.4Ghz and a loss of .264db/ft or 2.6db per 10ft @ 5Ghz  plus 1db of loss for the connections. LMR-200 cable is approx 1/4" in diameter (typical antenna lead) but the connector is about 1/2" (typical RP-TNC plug)

LMR-400 has a loss of ..066db/ft or 0.6db per 10ft @ 2.4Ghz and a loss of .109db/ft or 1.1db per 10ft @ 5Ghz  plus 1db of loss for the connections. LMR-400 cable is approx 1/2" in diameter but the connector is about 3/4" (typical RP-TNC plug)

LMR-600 has a loss of .043db/ft or 0.4db per 10ft @ 2.4Ghz and a loss of .073db/ft or 0.7db per 10ft @ 5Ghz  plus 1db of loss for the connections. LMR-600 cable is approx 3/4" in diameter (typical antenna lead) but the connector is about 1" (typical RP-TNC plug)

LMR-900 has a loss of .029db/ft or 0.2db per 10ft @ 2.4Ghz and a loss of .049db/ft or 0.4db per 10ft @ 5Ghz  plus 1db of loss for the connections. LMR-900 cable is approx 1" in diameter (typical antenna lead) but the connector is about 1 1/2" (typical RP-TNC plug)

So to calculate the end result here is how you do it.

AP Transmit Power - Connector Loss - Cable Loss + Antenna Gain = Effective Isotropic Radiated Power (EIRP)  *note: EIRP can not exceed 30db

So lets run 2 scenarios with this information: 802.11b power is 100mw (20db), 802.11g power is 30mw (14db), 802.11a power is 40mw (17db), antenna is 6db for 802.11b/g, 5db for 802.11a, and you cable length is 20ft.

No cable scenario

802.11b -->  20db + 6db = 26db = 400mw

802.11g --> 17db + 6db = 23db = 200mw

802.11a --> 17db + 5db = 22db = 150mw

With Cable LMR-200 Cable (numbers are close but not exact)

802.11b -->  20db - 1db - 3.2db + 6db = 22db = 150mw

802.11g --> 17db  - 1db - 3.2db + 6db = 19db = 80mw

802.11a --> 17db  - 1db - 3.2db + 5db = 18db = 60mw

As you can see that 20ft of cable drastically reduced the effective power of the access points. FCC regulations require you stay under 30db max EIRP, and for every 3db that the power is increased it doubles for every 3db it's decreased it is halved as shown below

200mw = 23dbm

100mw = 20dbm

50mw = 17dmm

25mw = 14dbm

I hope this helps answer your questions. If you found this helpful please feel free to rate it.

Thanks,

Kayle

[Leo Laohoo]

Nice one Kayle.  +5

[Kayle Miller]

Thanks leolaohoo!!

[dhopper82]

That's exactly what I'm looking for.  Thank you so much!  I'm thinking now we'll probably run power and Ethernet Cable to an area in the ceiling and then have a small coax cable running to the antennas and attached outside... 

[Kayle Miller]

dhopper, Glad I could help!!

[Scott Fella]

Just to add to the post... Here is a good link that calculates your db loss with various cables:

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

http://www.timesmicrowave.com/cable_calculators/