There are a few sites that take you through making your own. I tried the following link
They were cheap and easy to make. Most of my flying since has been of limited range . I did try a fairly limited test and got about 1.2klm away with and without the antenna. I just wasn't happy having the I1 so far away and losing signal so gave up the testing. There are posted videos that appear to show a significant increase in range. Good luck