Maximum-Gain Narrowband Yagi

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 an electrical boom length of 182″ to 226″ depending on frequency.

This shows the antenna geometry. The red dot marks the 75Ω feedpoint.

Modeling Results

I optimized for maximum 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 35 analysis segments per conductor halfwave with AO 8.57. 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   43.5 + j1.3   1.73     0.32      0.05      10.75     18.94
   97.2   47.8 + j3.4   1.58     0.22      0.05      10.92     19.17
   97.3   53.2 + j5.5   1.43     0.14      0.06      11.07     19.44
   97.4   60.8 + j7.2   1.26     0.06      0.06      11.21     19.13
   97.5   70.7 + j8.3   1.14     0.02      0.06      11.32     17.32
   97.6   82.0 + j7.6   1.14     0.02      0.07      11.36     15.81
   97.7   95.8 + j3.8   1.28     0.07      0.07      11.34     14.48
   97.8    111 - j5     1.49     0.17      0.07      11.25     13.27
   97.9    126 - j22    1.76     0.34      0.08      11.07     12.17

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 equivalent 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.

Antenna File

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

Sensitivity Analysis

The following table shows the performance change in dB at 97.5 MHz when altering a single dimension by ⅛″ (116″ for element half-lengths and 5% for c).

Symbol    Gain    F/R
    x1    0.00   0.08
    x2    0.01   0.07
    x3    0.00   0.10
    x4    0.00   0.05
    x5    0.00   0.08
    x6    0.00   0.22
    y1    0.01   0.35
    y2    0.03   0.00
    y3    0.04   0.47
    y4    0.03   0.86
    y5    0.02   0.86
    y6    0.01   0.26
     c    0.02   0.00

Gallery

Hossein Khamooshi in Gorgan, Iran, scaled the design to 103 MHz and built this vertically polarized antenna for long-distance reception across the Caspian Sea.

Markku Sollo in Tohmajärvi, Finland, built this 93-MHz Yagi for double-hop Es reception from the Middle East.


July 9, 201488–108 MHz