[Antennas] Information on Phased Verticals?

[Niel - W0VLZ]

Does anyone have a favorite source of information on phased verticals? 
See http://w0vlz.blogspot.com/ for a report on my latest antenna 
project, a self supporting C-Pole antenna for 20 mtrs.

While this version is self supporting, the crappie poles and most of the 
support structure could be removed and the whole antenna suspended by 
the top of the wire loop. It then follows that two might be suspended 
16' to 33' apart and fed properly to make a directional antenna. For 
about the same space as a 20 meter dipole and a height requirement of 
less than 20' I could have a low radiation angle antenna with some gain 
and directivity.

This gets me back to needing more information about phased verticals. 
How do I make the phasing harness and what spacing to I need?

73,
Niel - W0VLZ