If you have a favorite FM station you'd like to better receive and there's no interference from the rear to contend with, consider a dedicated, gain-optimized Yagi. This six-element design has a boom length of 182″ to 226″ depending on frequency.
This shows the antenna geometry. The red dot marks the 75Ω feedpoint.
I used the AO 9.51 Antenna Optimizer to maximize forward gain at 97.5 MHz, the geometric center of the FM band. To slightly broadband the response and reduce sensitivity to modeling and construction errors, I optimized over 97.1, 97.5, and 97.9 MHz. This reduced forward gain 0.23 dB at 97.5 MHz.
The following results are for 30 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 97.1 44.7 + j1.9 1.68 0.29 0.05 10.80 18.96 97.2 49.3 + j4.1 1.53 0.19 0.05 10.97 19.20 97.3 55.1 + j6.2 1.38 0.11 0.06 11.11 19.47 97.4 62.4 + j8.0 1.24 0.05 0.06 11.24 18.69 97.5 71.7 + j8.9 1.14 0.02 0.06 11.32 17.01 97.6 83.4 + j8.1 1.16 0.02 0.07 11.36 15.54 97.7 97.7 + j4.0 1.31 0.08 0.07 11.33 14.23 97.8 114 - j6 1.52 0.19 0.08 11.23 13.04 97.9 128 - j24 1.80 0.37 0.08 11.05 11.95
Use the frequency scaler to obtain dimensions in inches or millimeters for any frequency between 87.5 and 108 MHz. Dimensions are valid only for isolated ⅜″ or 10mm 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 the capacitor across the feedpoint, and feed with 75Ω coax. Coil the feedline into a current balun at the feedpoint.
Max-Gain Narrowband Yagi Free Space Symmetric 97.1 97.5 97.9 MHz 6 6063-T832 wires, inches x1 = 0 ; element positions x2 = 29.65504 x3 = 62.6821 x4 = 111.2026 x5 = 159.681 x6 = 202.0254 y1 = 29.0955 ; element half-lengths y2 = 28.57604 y3 = 26.81602 y4 = 26.17355 y5 = 26.05091 y6 = 26.40563 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 source c = 37.26171 ; matching capacitance Wire 2, center 1 0 c pF
The following table shows the performance degradation in dB at 97.5 MHz when changing a single dimension by ±⅛″ (±1⁄16″ for element half-lengths and 5% for c).
Symbol Gain F/R
x1 0.00 0.07
x2 0.01 0.07
x3 0.00 0.10
x4 0.00 0.05
x5 0.00 0.07
x6 0.00 0.21
y1 0.01 0.34
y2 0.04 0.01
y3 0.06 0.48
y4 0.03 0.82
y5 0.02 0.82
y6 0.01 0.28
c 0.03 0.00
Hossein Khamooshi in Gorgan, Iran, scaled an earlier version of the design to 103 MHz and built this vertically polarized antenna for long-distance reception across the Caspian Sea.
88–108 MHz