TITLE:
On the Effects of Driven Element L/D Ratio and Length in VHF-SHF Yagi-Uda Arrays
AUTHORS:
Richard A. Formato
KEYWORDS:
Yagi, Array, Driven Element, Impedance Bandwidth
JOURNAL NAME:
Wireless Engineering and Technology,
Vol.14 No.1,
January
31,
2023
ABSTRACT: While the Yagi-Uda array has been studied for decades, one issue appears
to have received less attention than perhaps it should, namely, the effects on
performance of the array’s driven element length and its length-to-diameter
ratio. This paper looks at that question. It shows that decreasing the L/D ratio increases impedance bandwidth, but it may shift the IBW band sufficiently
far from the design frequency that other parameters such as gain and
front-to-back ratio probably are adversely affected. It also shows that array
performance is not relatively independent of element diameters. This paper also
investigates the effect of lengthening the driven element, which can
substantially improve IBW. Several iterations of a 3-element prototype and
improved arrays are modeled with the Method of Moments and discussed in detail.
A five step design procedure is recommended and applied to a Genetic
Algorithm-optimized 3-element Yagi at 146 MHz. This array exhibits excellent
performance in terms of gain, front-to-back ratio, and especially impedance
bandwidth (nearly 14% for voltage standing wave ratio ≤ 2:1 with two
frequencies at which 50 ? is almost perfectly matched). While the analysis and recommended design
steps are applied to cylindrical array elements, which commonly are aluminum tubing for stand-alone VHF-SHF Yagis, they can be applied to
other element geometries as well using equivalent cylindrical radii, for
example, Printed Circuit Board traces for planar arrays. One consequence of
lengthening the driven element while reducing its L/D ratio is that some
reactance is introduced at the array feedpoint which must be tuned out, and two
approaches for doing so are suggested.