I suspect that will only make a little difference. What aerial do you have already?
Have a look round this place:
http://www.solwise.co.uk/
They have lots of good information.
I have experience of their NET-WL-ANT-015ON (or something very similar) on the end of ~10m of their very fat cable. This allows coverage of a whole factory.
I also have experience of one of these on a mag-mount and a short piece of very thin cable.
http://cpc.farnell.com/jsp/Computer+...sp?sku=CS12466
Frankly, it makes very little difference to the reception.
If you want something that works every time then I'd look for something like the first solution.