YAGI GAIN de N4KG
As has been shown and known for some time now, YAGI GAIN is
more dependent on BOOM LENGTH than the number of elements.
Figure 2-12 on page 2-23 of the W2PV "Yagi Antenna Design" book
depicts several curves of gain versus boom length for 3,4,5,6, and 7
element Yagi's. The curves track very closely as long as there are
sufficient elements to maintain coupling (3 elements drop off after
0.4 WL, 4 elements after 0.75 WL, etc.)
Additional elements for a given boom length are used primarily to
extend bandwidth of one or more desired parameters such as SWR
(dual driven elements), or Front-to-Rear rejection, or broaden /
flatten the gain response. This is why most commercial antennas
have more elements than required simply to provide a desired gain
in a given boom length.
It should also be noted that gain does not increase LINEARLY with
boom length. It rises rapidly between 0.15 WL to 0.40 WL, most
commonly represented by 2 to 4 element Yagi's, then gradually
from 0.4 to 0.6 WL where there is virtually NO difference between
4, 5, 6, or even 7 elements. Gain then rises rapidly from 0.6 to
0.8 WL where 4 (and even 5) elements start to lose sufficient
coupling. This is the range used by the BIG and effective
monobanders in the HF range. From 0.8 to 1.1 WL, gain is
fairly flat, even dipping slightly before rising again at 1.2 WL
These lengths are more common in long boom VHF antennas.
de Tom N4KG
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