A few years ago I had access to an antenna test range and designed this for repeater use. It tested a few db gain over the DB Products folded bay that we used at the time.
It is a 5/8wl over a 3/8wl and provides a very narrow beamwidth at the horizon due to the phasing of the elements.
The top section is roughly 142" long, bottom section is 86" long. The phase correction between the two elements is a loop 10" in diameter. (Think Ringo)
Use a fiberglass rod as an insulator and space the two elements approx 4". Four horizontal ground radials 58" long are required.
The feed poind is 50 ohms and I used a qtr wave stub of RG58 to provide DC ground.
In most cases this antenna can be built from a salvaged 11 meter half wave GP. There should be enough aluminum tubing.