A "ZL special" is a 2-element phased array. I have two of them
made commercially by VK4BRG for transmitter hunting, in addition
to 3-element yagis and 2-element quads.
Unless the short boomlength is important, I would generally prefer
the 3-element yagi, even though the longer boom is somewhat
more awkward when running through the woods. The measured
F/B on the ZL specials was only 6 to 8dB, while the yagis were
around 20dB and the quads at least 12 to 15dB. (However, I have
seen other ZL specials which had a very deep null off the back.)
I use the quads for hunting in my car, since they do not stick up as
high above the roof when vertically polarized. However, my wife
prefers the ZL special because it is smaller, while still providing
adequate bearing accuracy. (Quads are a poor choice for running
through the woods, because the loops snag easily on branches.)
The common construction techniques for the ZL special use an
unbalanced feed to both elements, which takes great care to
prevent distortion in the pattern, at least when used with horizontal
polarization. However, I have not noticed any consistent errors
in the bearings I have taken. A balanced feed system is
recommended for any of the antenna types when bearing accuracy
is important - particularly with horizontal polarization.
You probably have different applications in mind, and different
performance criteria, but this should give you an idea of the
tradeoffs in the different designs.
The truth is, the performance differences among the 2-element
beams are rather small, and the specific dimensions chosen (and
the construction techniques) will probably make more of a difference
in the final performance than the design type. The decision usually
comes down to what is the easiest mechanical construction with
the materials and equipment available to you.
W4RNL has some excellent simulation data comparing the ZL
special to 2- and 3-element yagis on his web site at www.cebik.com
. He also has design data for quads and yagis,
and many other antennas.