High-Performance Yagis for 8892 MHz

It's difficult to make a Yagi perform well over the entire 88108 MHz band. Multiple driven elements or multiple reflectors are necessary for really good performance. However, if bandwidth is restricted, a simple design with excellent performance is possible. I optimized this ten-element Yagi for 8892 MHz. The boom length is 240″.

I optimized the design with the AO 8.51 Antenna Optimizer. The red dot marks the 75Ω feedpoint.

Modeling Results

Calculated performance is for 28 analysis segments per conductor halfwave. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane.

Frequency   Impedance    SWR   Mismatch  Conductor  Forward     F/R 
   MHz         ohms             Loss dB   Loss dB   Gain dBd     dB  
  88.0    58.9 - j1.1   1.27     0.06      0.04      10.27     34.54
  88.5    66.1 + j0.9   1.14     0.02      0.05      10.47     35.10
  89.0    74.7 + j1.7   1.02     0.00      0.06      10.63     36.02
  89.5    83.8 - j0.1   1.12     0.01      0.06      10.75     36.84
  90.0    91.8 - j3.7   1.23     0.05      0.07      10.82     37.01
  90.5    97.2 - j7.3   1.31     0.08      0.09      10.84     35.38
  91.0     101 - j8     1.36     0.10      0.11      10.82     34.56
  91.5    99.4 - j3.9   1.33     0.09      0.14      10.77     37.17
  92.0    83.2 + j26.6  1.42     0.13      0.24      10.51     35.90

Antenna File

High-Performance Yagi for 88-92 MHz
Free Space Symmetric
88 88.5 89 89.5 90 90.5 91 91.5 92 MHz
10 6061-T6 wires, inches
x1 = 0				; element positions
x2 = 21.40401
x3 = 25.09015
x4 = 31.62594
x5 = 42.994
x6 = 74.3959
x7 = 114.2762
x8 = 156.6696
x9 = 197.9624
x10 = 239
y1 = 33.7048			; element half-lengths
y2 = 34.96238
y3 = 31.2886
y4 = 30.14052
y5 = 29.58937
y6 = 29.08475
y7 = 28.74854
y8 = 28.4368
y9 = 27.88064
y10 = 24.82702
1  x1  -y1  0   x1  y1  0    0.375
1  x2  -y2  0   x2  y2  0    0.375
1  x3  -y3  0   x3  y3  0    0.375
1  x4  -y4  0   x4  y4  0    0.375
1  x5  -y5  0   x5  y5  0    0.375
1  x6  -y6  0   x6  y6  0    0.375
1  x7  -y7  0   x7  y7  0    0.375
1  x8  -y8  0   x8  y8  0    0.375
1  x9  -y9  0   x9  y9  0    0.375
1 x10 -y10  0  x10 y10  0    0.375
1 source
c = 26.02646			; matching capacitance
Wire 2, center 1 0 c pF

Use ⅜″ tubing. Dimensions are valid only for isolated elements (nonconductive boom or insulated mounts). The matching network is the lowpass equivalent of a hairpin match. Split the driven element leaving a gap no larger than ¼″, solder a 27-pF capacitor across the feedpoint, and feed with 75Ω coax. Use a current balun at the feedpoint.

Sensitivity Analysis

The following table shows the degradation of average performance over 88, 89, 90, 91, and 92 MHz in dB when changing a single dimension by ⅛″ (116″ for element half-lengths and 5% for c).

Symbol    Gain    F/R
    x1    0.00   0.03
    x2    0.02   0.29
    x3    0.02   0.22
    x4    0.00   0.10
    x5    0.00   0.14
    x6    0.00   0.23
    x7    0.00   0.17
    x8    0.00   0.43
    x9    0.00   0.20
   x10    0.00   0.08
    y1    0.00   0.13
    y2    0.00   0.05
    y3    0.07   0.47
    y4    0.01   0.13
    y5    0.01   0.27
    y6    0.02   0.84
    y7    0.00   1.46
    y8    0.01   1.11
    y9    0.01   1.08
   y10    0.00   0.27
     c    0.01   0.00

With an ordinary tape measure it should be easy to measure tip-to-tip element lengths to the nearest 116″. F/R degradation is 0.39 dB maximum for this case.

Shorter Antenna

This design uses seven elements on a 120″ boom.

Frequency   Impedance    SWR   Mismatch  Conductor  Forward    F/R 
   MHz         ohms             Loss dB   Loss dB   Gain dBd    dB  
  88.0    57.2 - j5.3   1.33     0.09      0.03      7.97     28.42
  88.5    62.0 - j3.0   1.22     0.04      0.04      8.11     28.95
  89.0    68.2 - j1.0   1.10     0.01      0.04      8.24     29.41
  89.5    75.5 + j0.5   1.01     0.00      0.04      8.35     30.27
  90.0    82.7 + j0.1   1.10     0.01      0.05      8.42     31.19
  90.5    88.8 - j1.1   1.18     0.03      0.06      8.48     31.74
  91.0    91.2 - j1.3   1.22     0.04      0.07      8.52     32.08
  91.5    89.2 + j6.4   1.21     0.04      0.08      8.55     31.34
  92.0    86.8 + j32.4  1.53     0.19      0.12      8.37     28.26

Antenna file:

Shorter Yagi for 88-92 MHz
Free Space Symmetric
88 88.5 89 89.5 90 90.5 91 91.5 92 MHz
7 6061-T6 wires, inches
x1 = 0				; element positions
x2 = 19.79921
x3 = 24.45148
x4 = 31.39832
x5 = 46.79759
x6 = 81.04308
x7 = 119
y1 = 33.4243			; element half-lengths
y2 = 34.52807
y3 = 30.91553
y4 = 29.57341
y5 = 29.33822
y6 = 28.60242
y7 = 26.61786
1  x1  -y1  0   x1  y1  0    0.375
1  x2  -y2  0   x2  y2  0    0.375
1  x3  -y3  0   x3  y3  0    0.375
1  x4  -y4  0   x4  y4  0    0.375
1  x5  -y5  0   x5  y5  0    0.375
1  x6  -y6  0   x6  y6  0    0.375
1  x7  -y7  0   x7  y7  0    0.375
1 source
c = 23.96186			; matching capacitance
Wire 2, center 1 0 c pF

Sensitivity analysis:

Symbol    Gain    F/R
    x1    0.00   0.01
    x2    0.01   0.06
    x3    0.01   0.08
    x4    0.01   0.07
    x5    0.00   0.08
    x6    0.00   0.11
    x7    0.00   0.06
    y1    0.00   0.04
    y2    0.00   0.01
    y3    0.07   0.34
    y4    0.00   0.09
    y5    0.01   0.50
    y6    0.02   0.50
    y7    0.01   0.12
     c    0.01   0.00

Gallery

Joe Hageli built this ten-footer in Spring Grove, Illinois.


February 10, 201488108 MHz