I have installed and am running a wireless bridge less then a km apart with crystal clear line of site. While configuring the bridge I had some old antennae from some intel wireless AP's and tested them side by side and got about 19-20Mbs. When I traded them out with the Cisco yagi antennae that rate dropped to 15Mbs, when I did the actual install the bandwidth dropped further to 10Mbs.
I am wondering if I am missing some optimization that can boost the performance, it seems weird that the using an old antennae for a different AP does better then the Cisco Yagi unless I just missed something. I have the set the antennae to 14 db gain(antnennae says 13.5) and it is set to external as well as set to use the least congested channels.
you failed to mention what the old antennas were, but in terms of yaggi antnnea's they have an exteremely narrow beamwidth and depending on the distance that can have an effect on the speeds it provides. A yaggi antenna needs to be carefully aligned to assure the greatest level of signal, reliability, and performance.. If the other antenna's are an omni or sector/patch antenna, they are far more forgiving in terms of the pointing and alignment of the antennas to each other. From the inital information you have provided, I'd say that your antennas are not properly installed or pointed, also keep in mind the orientation of the antenna, since it's relatively easy to invert or rotate a yaggi antenna and not realize you have changed it's beam pattern and polarization.