It's difficult to make a Yagi perform well over the entire 88–108 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 88–92 MHz. The boom length is 20′ and the feed impedance is 75Ω.
I optimized the design using the AO 8.51 Antenna Optimizer. The red dot marks the feedpoint.
Calculated performance figures are for 28 analysis segments per 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
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 version 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.
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″ (±1⁄16″ 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 1⁄16″. F/R degradation is 0.39 dB maximum for this case.
This design uses seven elements on a 10′ 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
Joe Hageli built this ten-footer in Spring Grove, Illinois.
88–108 MHz