Good morning.
Am finally beginning to catch up a little on phone calls and e-mail and this
one was towards the front of the stack. My apology for delayed responses to
many of you, as things have been backed up due to a death on the family.
"Zagi" is a term that came out of "unequal Z-axis Yagi." The Zagi has
several advantages. One is that the effective height of the Yagi antenna
(any Yagi) is essentially where the driver is located, so the effective
height of the antenna is higher, without markedly increasing the windload
(not having to raise the whole antenna). A second reason is that the
effective boom length is longer, as the dimension from the driver to the
reflector/director is the diagonal from the elevated driver to the
parasitics. This also can position the driver for a "clearer view" of the
relevant parasitics if the Yagi is a composite of two or more monobanders.
Our common term "boom" has probably caused most thinking to move in the
direction that the elements all must reside in a straight, horizontal plane.
Such is not the case.
The effect of the Zagi configuration is obviously more pronounced when the
antenna height is not very high. The Zagi can be modeled using any software
that allows for the driver to separately elevated. The highest elevation
tested for a 40 mtr driver has been just over seven (7) feet for a 30' boom
3 element. This antenna is often placed on a mast with other antennas, so
the practical elevation is usually less than this, residing 4-5' above the
boom. The driver tuning and match were readjusted slightly, as the coupling
is not the same. On 80/75 mtrs, the highest elevation tested is the photo on
the web page: the driver 14' higher and using a very short 34' boom for the
remaining two elements. This antenna was run for almost a year and the
increased effective height was quite noticeable, The effective boom length
was really too narrow for a 3 element 80, but the primary testing was for
the effect of the increased height.
The most practical use seems to be for a 3 element, simply because the
driver can be raised on the mast. However, other configurations are
obviously possible, especially using wire Yagis (for 80) where elevating the
entire array is difficult. Installations utilizing roof mounts that desire
more effective height without moving the entire Yagi to the top of the mount
might fint the Zagi configuration attractive.
Have a good day and 73,
Tom, N6BT
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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