There are some good descriptions of this antenna (and some
of the variations you may encounter) on W4RNL's web site:
http://www.cebik.com/phase/hb.htmlhttp://www.cebik.com/phase/ph.html(and browse any other articles on phased arrays you find there)
One of the common designs uses a metal boom with a wire
running along it as the phasing line between the two
elements - in this case you either need to use a metal
boom or replace this with a standard coax phasing line.
Balanced versions need not use a metal boom.
Note that the HB9CV is the same basic antennas as the
"ZL special", though the latter is generally built with
direct feed in the center of the elements and the HB9CV
generally uses gamma or "T" matching to the elements,
which gives more flexibility in setting the impedance
to match the available phasing line.
I have two commercial versions from VK4BRG at home for
2m DF use. These actually feed the antenna at the rear
element rather than at the front, and have a rather poor
front/back ratio. I modified one of them to feed it at
the front element, and it only changed things a little
bit. Based on the W4RNL articles, I think this antenna
was designed for maximum gain rather than good F/B, but
I've seen some other designs around that might be better.
I know many of the European foxhunters use the HB9CV
because it gives a reasonably good pattern in a small
antenna, but remember that they use horizontal polarization
which will have a much sharper pattern than hunting a
vertical signal with the same antenna. (The same is
true of the Moxon: I use it for horizontal signals but
it is not sharp enough for good bearings on vertically
polarized signals unless you know the characteristics of
the rear null at the frequency you are hunting.) But
they also stress the importance of using a balanced
version, as the unbalanced ones may have a skew of several
degrees in the pattern.
I don't know if these notes plus the linked articles have
answered all of your questions - if not, feel free to
ask some more.