A long while ago, I thought it would be fun and useful to try creating long-distance wireless links. I started out with a biquad, then onwards to cantennas and other interesting designs. At one point I decided to build a wifi antenna out of a satellite dish. For my dish I used a sky dish and the feed was built according to Trevor Marshalls design, found on his site here.
The dish antenna was very good, but I wanted to see if I could improve upon it. One thing that was annoying was the issue of polarisation. By nature the dish and feed are horizontally polarised, so if you want to have your link vertically polarised, you would have to either turn the feed on it's own, resulting in a loss of SNR, or turn both the feed and the dish, which (for me at least) resulted in difficulty positioning and securing the dish, problems with natural elements and general unsightliness.
After doing some research on antenna designs and feeds, I decided to make a helical antenna and use it as a feed for the dish. The reason? Helical antennas are circulary polarised. Circular polarisation is normally used for space based communication because satellites tend to rotate, hence constnatly having different polarisation. Circular polarisation solves this by having essentially two linearly polarized waves of equal amplitude, 90 degrees apart and with their planes of polarization at right angles to each other.
The result of this is that the antenna will be able to respond to signals of different linear polarisation equally well without having to constantly alter the rotation of the dish.
The ARRL Antenna book is an excellent source, in addition to a lot of useful theory and information regarding antennas, they also give you sample antenna designs. It is here that I found out another nice thing about helical antennas: they are very forgiving of mechanical inaccuracies, which means that you don't have to strive to getting it perfect if you don't have the means to do it easily.