Omni antenna, no gain

Hello,

I bought a 15dbi omni antenna to connect to my university wireless network (I live nearby), which I am planning to setup later on the roof.

The problem is, the antenna I bought seems pretty weak, the signal of the strongest random AP from my room is the same as with 2dbi rubber thingy I got with the card (-70).

Some random AP-s are maybe 2-4 dbi better, and some are actually worse (but the stumbler shows few extra AP-s I don't get with rubber one). I scanned with Network Stumbler from the balcony. Anyway, it's far from promised 15dbi of the antenna.

My question is, is it the faulty antennna (It's Airlive 15dbi outdoor), or maybe large omni antennas are more sensitive to height differences than the rubber ones?

The card used for testing is Edimax EW-7318USg (RT73).

consider the bagel.

a bagel is approximately the shape of the radiation pattern from a standard 2.1 dBi rubber ducky.

consider a bagel that has been sat on by Roseanne Barr. or a pancake, with a hole in the mddle. that is the radiation pattern of a 15 dBi omni.

same amount of bagel material, same amount of RF energy; distibuted differently. not an increase in energy, an alteration of the shape of that energy.

yes, you lose gain close in with a gain omni. you may not be able to connect at all under a gain omni.