The VHF and UHF frequencies  are likely to be in the range of 146 MHZ (VHF) and 444.000 MHz (UHF).  These are WAY different from frequencies in GigaHertz. To modify it, you would be reducing the size considerably.  MFJ sells a Yagi designed specifically for WiFi frequencies, it may be simpler to buy it, or copy their design and build your own. View the antenna at http://www.mfjenterprises.com/products.php?prodid=MFJ-1800

One can be built from rigid copper wire using foam board as "boom".  As with any Yagi array, you will need to get each element precise, the element spacing too.  If you are thinking more omni-directional, a simple vertical would be what you need.  However .at the Ghz frequencies , an omni-directional antenna is probably not what you want.

Andy.