High Gain Wifi Help!

Hello,

I have a Linksys WRT54GS wireless router, connected to comcast Zyxel prestige 900 cable modem router, and a 15DBi omni-directional antenna hooked to the linksys router.

My dilemna is that the High gain antenna seems to be giving me little or no signal. I've attempted the following.
-disable/reneabled DHCP on Linksys
-Changed through all channels on Linksys
-disabled mixed frequency mode on Linksys
-Tried two different Antennas on Linksys(to include the ones it came with)

Here's the thing, I can stand right next to the router with the high gain antenna hooked up, walk 30 feet away in a direct line of sight and then the signal drops to two bars. 50 feet away signal is 1 bar and not enough strength to get on the net.

Any clues/help will be greatly appreciated. This one is throwing me for a loop. I've set up wireless networks in the past and haven't this one before!

Hey how is the antenna connected to the linksys because you can lose alot of gain from the cable and connectors. Have you used the cable before and did it work fine?