Most wifi antenna are omni directional I think and all that means is that you pick up from all around you. If you want have better reception try getting a 2.4ghz 1.8db omni directional antenna.
The reason why iBurst promotes Omni antennae is to spread the load on towers also should a tower go down you will still be able to connect to another that is in range.
If you want to purchase a directional antenna that would isolate the signal of an individual tower and improve the signal considerably rather have a look at these directional antennae.
Omni antenna work well for RECIEVING of the signal from the Tower, but not well for transmitting of YOURS weak signal to the Tower.
Work well if you have strong signal from more then one tower - can change connection to the less loaded one.
In case of the bad signal the only solution - directional antenna.
Don't forget you would need to purchase a pigtail - best price at phoenix.
If you are thinking of purchasing a USB / Notebook antenna, shop around there are some suppliers that offer it at a third less than what iBurst supplies it at.
Provide me with more specific information about what you want to achieve, so I can give you more detailed advice.
If you do want to purchase the iBurst planar "Desktop Antenna", shop around iBurst sells it for R 550, however most suppliers sell it for around R 300.
Does it use the exact same plugs? The plug on the USB iburst modem is a tiny little push-in job while in other gsm applications they've always been like those oldschool BNC connections.
Just off the top of my head, have a look at digital planat and simplicity online if I recall correctly they stock it at R 300. delivery should be no more that R 65.