While real world construction can modify results, and there are
well-known noise performance and ease of construction differences
between quads and yagis, comparing the two in idealized circumstances
in models does give a sense of some interesting playoffs between the
two.
These have not thus far been mentioned in this thread. Speaking only
of horizontally polarized configurations,
The optimal two and three element quad runs ahead of an optimal yagi
with an equal number of elements. A *wide-spaced* optimal four element
yagi (does not include most commercial 4 element yagis which have
shorter booms) and an optimal four element quad are nearly equal to
one another. This is slightly in favor of the yagi at optimal five
wide-spaced elements, and the yagi is clearly and increasingly in the
lead after that.
There appear to be two factors involved in this.
1) As the feed impedance of the quad and yagi go down with the
additional elements, the losses in the quad's #12 copper wire
increase, whereas the typical tubing elements of the yagi stand the
higher current better. Remember the loss is I squared R. This
difference if not present if the quad loop conductor is 3/8" copper
tubing.
2) In the low element count configurations, the quad loops have a
natural reduction of radiation to the vertical due to effectively a
pair of radiating conductors per quad loop, whereas the dipole
elements of the yagi do not. As the antennas lengthen with more
elements, the vertical radiation is significantly reduced in either by
the horizontal gain. Reducing the vertical radiation from -15 to -17
db in a long quad or yagi doesn't do much to increase horizontal gain.
Reducing it from 0 to -2db DOES make a notable difference, as in the
single element comparison.
The additional power gain is less and less a result of the loop's
reduction of vertical radiation as elements are added, and at a point
in the lengthening, you can actually increase the gain of a long quad
by substituting a yagi element for a loop director (5 or 6 element
configurations, depending on height).
This last is the derivation of the quagi. Given the spacings of long
beams, though, I would put THREE quad elements (ref, de, dir1) in a
quagi before switching to yagi elements.
The highest gain configurations I have derived for 5+ element beams
seem to have Ref, dir1 through second to last dir in near equal
spacings, the DE approx halfway between Ref and dir 1, with the dir
spacing slightly reduced between the forward most directors.
73, Guy.
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