I have tried several different wifi antenna's including both directional and omni-directional. The directionals worked well but not on anchor. A high gain antenna is very important and a good quality cable is a must. The following is the list of three component we used we create the best system:
1) HG2415U-PRO HyperGain® 2.4 GHz 15 dBi Professional Omni Antenna $99.95
2) CA4NMRSF030 Custom Cable Assembly, N-Male to Reverse Polarity SMA-Plug (Female) 400-Series 30 ft $44.99
3) EnGenius EUB-362 EXT 250mW 8002.11g USB $44.00
The antenna and cable I purchased from www.HyperLinkTech.com.
For the Engenius unit I just did a google search on the part number and looked for the cheapest price.
We've meet many other cruisers and our system works as good as any we've seen and much better than most. It's not unusual to receive a usable signal miles offshore. The system works of off one USB connection and no other external power is required.