EE434 Yagi Antenna Design Example Spring 2016

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EE434
Yagi Antenna Design Example
Spring 2016
The following is taken from Antenna Theory and Design by Stutzman and Thiele
Given: Design a 12-Element Yagi Antenna for TV Channel 12 (205.25 MHz) using 1.0 cm diameter
elements and a non-metallic boom. To maximize gain, the boom length is set to 2.2λ.
Step 1: From Table 5.4, we see that the spacing between elements should be 0.2λ. At 205.25 MHz, the
wavelength is 29.2 cm. The table provides us the nominal element lengths assuming that the
ratio of element diameter to wavelength (d/λ) is 0.0085; in our case, the ratio is:
d
=
λ
1 cm
= 0.0068
1.46 m
Thus, we will need to apply a correction factor.
EE434
Yagi Antenna Design Example
Spring 2016
Step 2: Because the d/λ ratio is low, we need to increase the element lengths slightly. To determine the
increase, we compute the intersection of the actual d/λ = 0.0068 on the design curve B on Figure
5.37. This intersection gives us the compensated lengths of the reflector and first director:
=
=
λ 0.792m
LR 0.483
=
=
λ 0.639m
LD1 0.4375
Step 3: To compute the length adjustment to the other directors, we need calculate the amount of
increase between the 0.0065 and nominal 0.0085 points for D1 – in this case the element
increased by:
D
=
LD 0.4375λ − 0.432
=
λ 0.0055λ
EE434
Yagi Antenna Design Example
Spring 2016
Thus, we need to increase the lengths of all remaining drivers by an identical amount:
LD2 0.421
=
=
λ 0.615m
LD
LD
=
=
=
λ 0.604m
0.414
3
10
LD
LD
=
=
=
λ 0.591m
0.405
4
9
LD
LD
LD
LD
=
=
=
=
=
λ 0.581m
0.398
5
6
7
8
With some minor tweaking, the resulting pattern is:
The computed input impedance (again, after some tweaking) is Z in =49.1 + j17.6Ω .
Note that both of these differ significantly from what Stutzman claims, illustrating the art (and not just
science) component of antenna design.
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