Your antenna needs to be the right length which can be determined by :
λ = c/f where λ = wavelength, c = speed of light and f = frequency
If you put the figures in for 802.11b/g wi-fi then you'll find that the wavelength is between 12 and 12.5cm so a suitable value for λ would be 12.25cm. Normally for an omnidirectional antenna you would design a quarter-wave dipole so the length of the antenna element would be 3cm or so. Make the antenna element shorter and it'll work better at the top-end of the band (channel 11/12/13); make it longer and it'll work better at the bottom end of the band (channel 1/2/3).
The coils are what increases the gain of the antenna element and 3dB in this case is a
doubling of signal strength as we are dealing with power and not voltage. Edit - I wouldn't have thought that only one antenna element would produce a 3dB increase in signal strength. I'd have thought you'd need multiple vertical elements (it's called a collinear antenna) to manage this on an omni
Edit - it is of course a collinear antenna as it's 2 quarter-wave dipoles with a meander coil in between
The main question to ask is what sort of antenna does the wi-fi equipment have now?
Edit to correct stupidity
Edit again - I have a set of jpgs with dimensions for a four element collinear somewhere. God knows where they came from but I'll dig them out if you want?