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 244″ and the feed impedance is 75Ω.
I optimized the design using the AO 8.50 Antenna Optimizer. The red dot is the feedpoint.
Below are calculated performance figures for a segmentation density of 28 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain including wire and mismatch losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane. The SWR reference impedance is 75Ω.
88.000 MHz: Impedance 46.0 - j5.3 ohms
SWR 1.64
Mismatch Loss 0.26 dB
Wire Loss 0.04 dB
Mismatched Gain 10.06 dBd
F/R 34.64 dB
89.000 MHz: Impedance 58.2 - j1.4 ohms
SWR 1.29
Mismatch Loss 0.07 dB
Wire Loss 0.05 dB
Mismatched Gain 10.57 dBd
F/R 35.81 dB
90.000 MHz: Impedance 73.2 - j1.6 ohms
SWR 1.03
Mismatch Loss 0.00 dB
Wire Loss 0.07 dB
Mismatched Gain 10.91 dBd
F/R 36.21 dB
91.000 MHz: Impedance 86.4 - j1.9 ohms
SWR 1.15
Mismatch Loss 0.02 dB
Wire Loss 0.09 dB
Mismatched Gain 11.02 dBd
F/R 35.27 dB
92.000 MHz: Impedance 97.9 + j20.4 ohms
SWR 1.43
Mismatch Loss 0.14 dB
Wire Loss 0.17 dB
Mismatched Gain 10.78 dBd
F/R 34.71 dB
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 x2 = 20.37588 x3 = 24.19926 x4 = 31.14301 x5 = 46.43945 x6 = 80.42815 x7 = 121.0225 x8 = 162.8651 x9 = 203.6821 x10 = 243.1635 y1 = 33.83294 y2 = 34.49509 y3 = 31.18144 y4 = 30.1337 y5 = 29.32495 y6 = 28.75998 y7 = 28.62208 y8 = 28.32187 y9 = 27.85086 y10 = 24.9975 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 = 28.85343 Wire 2, center 1 0 c pF 28 segments/halfwave Trade-offs: 25% gain, 75% F/B Worst/avg 50% for F/B
Use ⅜″ tubing. The x symbols are element positions and the y symbols are element half-lengths. The dimensions are valid only for isolated elements (insulated mounts above the boom). The matching network is the lowpass version of a hairpin match. Split the driven element leaving a gap no larger than ¼″, solder a 30-pF capacitor across the feedpoint, and feed with 75Ω coax. Use a current balun at the feedpoint.
88–108 MHz