Excellent question! I don't have any problem using a store's wifi - whether I patronize the business or not. I don't sit and use it for hours - just to check mail or whatever and be on my way. Like the OP, there are times when I don't care to go inside and would prefer to be able to access it from the parking lot.
With all of that in mind, I tried a 16 element Yagi antenna (1 reflector element, one driven element, and 14 director elements) for marketed for wifi frequencies on a recent trip and was very disappointed in its performance since it only gave the equivalent performance, over a distance, when compared to the results which an omni-directional (rubber duck type) antenna produced. I don't know if it was a design defect by the manufacturing company or if the individual unit was defective due to mishandling in transit or otherwise. Also, I was going through a heavy/coarse metal window screen (not fine gauge copper or fabric, etc), which I would think would absorb some signal, but that would be true for the omni-directional antenna that I used for comparison, too. I did find that the Yagi was very directional, as one would expect, and would quickly lose signal strength if moved to one side or the other.
I've used Yagis for 10, 15, and 20 meter wavelengths and found them very effective there - so I'm satisfied that the Yagi concept/design is a sound and effective one - leaving me inclined to suspect a defect in the unit I was using.
I would love to know of a specific product, whether a yagi antenna (they're not all are the same quality!) or a panel/grid-type directional antenna used in conjunction with something to amplify signals, or something else, that would allow easy access from a bit further back in the parking lot rather than up front in the handicap spaces. Size matters, in a vehicle, of course, so the smaller the solution would be, the better and more useful it would be!
Peterson
