external antennas and gain settings

Unanswered Question
May 19th, 2008

Cisco AIR-AP1242AG-E-K9.

In the web interface, if one goes to:

Network Interfaces/Radio0-802.11G/Setting tab

half way down the page there's a section called External Antenna Configuration and you can enter an Antenna Gain in the box.

The access point came originally with two small plastic antennas, about 6 inches long but I took these off and connected two external antennas, one is 3db, the other 6db. Should I enter 6db in the box or does the access point set the gain automatically? At the moment all antenna settings are at default.

I have this problem too.
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srosenthal Mon, 05/19/2008 - 06:01

That field is informational only. It does not have any effect on the operation of the AP or the power output. People generally use it to let other who manage the AP know what gain antenna is connected to it.

Gain is built into the antenna only.


scottmac Mon, 05/19/2008 - 06:42

Those values are used by devices / applications like the WLSE to draw the "heat patterns" or estimate coverage on the map.

They don't affect the operation of the AP in any way.

Using different antennas is not helping, and may be hurting your coverage. The antennas are for "diversity" only one antenna is active at a time.

Good Luck


scottmac Tue, 05/20/2008 - 04:29


Electrically, it doesn't matter, you're not going to hurt the AP by having different antennas, but you also not really getting any advantage ... and it's possible it it reducing the effectiveness of the AP's coverage.

Good Luck


tonyspcrepairs Tue, 05/20/2008 - 07:04

ok so if I have one antenna better than the other should I only use the the better one and leave the other disconnected? The reason I ask is because I was thinking of getting a CISCO AIR-ANT3338 dish antenna. I can only afford one so would it reduce the effectiveness of the dish if I connected the 6db omni antenna to the other side?

scottmac Tue, 05/20/2008 - 08:10

In the case of a Dish, yes, it would absolutely impact your throughput.

It would be better to put the dish on one port and disable the other port.

The dual ports are for diversity; in an inside environment, having two antennas a small distance apart reduces the effects of "multipath" and "nulls" ... multipath is when there are several images of the same transmission (from multiple reflections), nulls occur when you combine a series of waves (like a primary wave and a near reflection) and the combined waves, added together, result a greatly diminished signal (like a "plus" 35 meeting a "minus" 35 ... the result is a null, or no signal).

For point-to-point, or point-to-multipoint diversity is generally not desired or needed.

The way the antennas are used does not permit efficient operation for coverage of more than one specific area.

Good Luck



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