Check this outBefore you buy a DDWRT device give this a look.
I just bought two of these. One to install at the RV park that had an old G model that was about useless and one for the RV that has a DDWRT repeater unit,.
This router is also a repeater and is the easiest thing I have ever found to set up. It has removable antennas so connecting an outside antenna is easy,. Just buy a pre-made cable and nice Omni gain outside antenna. Setup takes less than ten minutes. You can even get it in a dual band model to make it more versatile.
Amazingly inexpensive. As a techno geek I was stunned at how well it works for the price.
If you order a coax and antenna be sure your connectors are compatible. Those sma connectors on the router can surprise you.