I like to try a new antenna every year for one of my favorite contests. A year or two ago I had great hopes for a 20 meter Colinear wire antenna in the ARRL CW Sweepstakes. I ran out of time and threw one together quickly. It did not do much for me, but I think the implementation was really poor.
So I am going to give it another try. This time I am modeling it with EZNEC. I started a model last go-round, but never really got to the point that the dimensions were very good. I suspect I was not getting nearly the gain it promises.
There is a good write-up in the ARRL Antenna Handbook about this antenna. If you have any interest in HF antennas, and you do not have this book, what are you waiting for? This is the definitive practical reference book for HF (and VHF/UHF) antennas. My other favorite antenna reference is the LB Cebick (Silent Key) Web Site. LB was a prolific author and antenna enthusiast. I have purchased several of his antenna models and some of his books. This is very practical stuff, and extremely easy to read.
My 20 meter colinear will use four half-wave elements. The goal is to get a decent signal into the far Northwest of the US and Canada. That has always been my biggest difficulty in trying to work all Sections in the Sweepstakes.